/**
 * @fileoverview Implements the PCx86 Video component.
 * @author <a href="mailto:Jeff@pcjs.org">Jeff Parsons</a>
 * @copyright © Jeff Parsons 2012-2017
 *
 * This file is part of PCjs, a computer emulation software project at <http://pcjs.org/>.
 *
 * PCjs is free software: you can redistribute it and/or modify it under the terms of the
 * GNU General Public License as published by the Free Software Foundation, either version 3
 * of the License, or (at your option) any later version.
 *
 * PCjs is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without
 * even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License along with PCjs.  If not,
 * see <http://www.gnu.org/licenses/gpl.html>.
 *
 * You are required to include the above copyright notice in every modified copy of this work
 * and to display that copyright notice when the software starts running; see COPYRIGHT in
 * <http://pcjs.org/modules/shared/lib/defines.js>.
 *
 * Some PCjs files also attempt to load external resource files, such as character-image files,
 * ROM files, and disk image files. Those external resource files are not considered part of PCjs
 * for purposes of the GNU General Public License, and the author does not claim any copyright
 * as to their contents.
 */

"use strict";

if (NODE) {
    var Str         = require("../../shared/lib/strlib");
    var Web         = require("../../shared/lib/weblib");
    var DumpAPI     = require("../../shared/lib/dumpapi");
    var Component   = require("../../shared/lib/component");
    var State       = require("../../shared/lib/state");
    var PCX86       = require("./defines");
    var Memory      = require("./memory");
    var Messages    = require("./messages");
    var ChipSet     = require("./chipset");
    var Keyboard    = require("./keyboard");
    var Mouse       = require("./mouse");
}

/*
 * MDA/CGA Support
 *
 * Since there's a lot of similarity between the MDA and CGA (eg, their text-mode video buffer
 * format, and their use of the 6845 CRT controller), since the MDA ROM contains the fonts used
 * by both devices, and since the same ROM BIOS supports both (in fact, the BIOS indiscriminately
 * initializes both, regardless which is actually installed), this same component emulates both
 * devices.
 *
 * When no model is specified, this component supports the ability to dynamically switch between
 * MDA and CGA emulation, by simply toggling the SW1 motherboard "monitor type" switch settings
 * and resetting the machine.  In that model-less configuration, we install I/O port handlers for
 * both MDA and CGA cards, regardless which monitor type is initially selected.
 *
 * To simulate an IBM PC containing both an MDA and CGA (ie, a "dual display" system), the machine
 * configuration simply defines two video components, one with model "mda" and the other with model
 * "cga", resulting in two displays; setting a specific model forces each instance of this component
 * to register only those I/O ports belonging to that model.
 *
 * In a single-display system, dynamically switching cards (ie, between MDA and CGA) creates some
 * visual challenges.  For one, the MDA prefers a native screen size of 720x350, as it supports only
 * one video mode, 80x25, with a 9x14 cell size.  The CGA, on the other hand, has an 8x8 cell size,
 * so when using an MDA-size screen, an 80x25 CGA screen will end up with 40-pixel borders on the
 * left and right, and 75-pixel borders on the top and bottom.  The result is a rather tiny CGA font
 * surrounded by lots of wasted space, so it's best to turn on font scaling (see the "scale" property)
 * and go with a larger screen size of, say, 960x400 (50% larger in width, 100% larger in height).
 *
 * I've also added support for font-doubling in createFont().  We use the 8x8 font for 80-column
 * modes and the "doubled" 16x16 font for 40-column modes OR whenever the screen is large enough
 * to use the 16x16 font, since font rendering without scaling provides the sharpest results.
 * In fact, there's special logic in setDimensions() to ignore fScaleFont in certain cases (eg,
 * 40-column modes, to improve sharpness and avoid stretching the font beyond readability).
 *
 * Graphics modes, on the other hand, are always scaled to the screen size.  Pixels are captured
 * in an off-screen buffer, which is then drawn to match the size of the virtual screen.
 *
 * TODO: Whenever there are borders, they should be filled with the CGA's overscan colors.  However,
 * in the case of graphics modes (and text modes whenever font scaling is enabled), we don't reserve
 * any space for borders, so if borders are important, explicit border support will be required.
 */

/*
 * EGA Support
 *
 * EGA support piggy-backs on the existing MDA/CGA support.  All the existing MDA/CGA port handlers
 * now refer to either cardMono or cardColor (instead of directly to cardMDA or cardCGA), enabling
 * the handlers to be redirected to cardMDA, cardCGA or cardEGA as appropriate.
 *
 * Note that an MDA card supported only a Monochrome Display and a CGA card supported only a Color
 * Display (well, OK, *or* a TV monitor, which we don't currently support), but the EGA is much
 * more flexible: the Enhanced Color Display was the preferred display, but the EGA also supported
 * older displays; a Color Display on EGA wasn't ideal (same low resolutions but with more colors),
 * but the EGA also brought high-resolution graphics to Monochrome displays, which was nice.  Anyway,
 * while all those EGA/monitor combinations will be nice to support, our virtual display support
 * will focus initially on the Enhanced Color Display.
 *
 * TODO: Add support for jumpers P1 and P3 (see EGA TechRef p.85).  P1 selects either 5-color-output
 * for a CGA monitor or 6-color-output for an EGA monitor; we would presumably use this only to
 * control certain assumptions about the virtual display's capabilities (ie, Color Display vs. Enhanced
 * Color Display).  P3 can switch all the I/O ports from 0x3nn to 0x2nn; the default is 0x3nn, and
 * that's the only port range the EGA ROM supports as well.
 */

/*
 * VGA Support
 *
 * More will be said here about PCjs VGA support later.  But first, a word from IBM: "Video Graphics Array [VGA]
 * Programming Considerations":
 *
 *      Certain internal timings must be guaranteed by the user, in order to have the CRTC perform properly.
 *      This is due to the physical design of the chip. These timings can be guaranteed by ensuring that the
 *      rules listed below are followed when programming the CRTC.
 *
 *           1. The Horizontal Total [HTOTAL] register (R0) must be greater than or equal to a value of
 *              25 decimal.
 *
 *           2. The minimum positive pulse width of the HSYNC output must be four character clock units.
 *
 *           3. Register R5, Horizontal Sync End [HRETRACE_END], must be programmed such that the HSYNC
 *              output goes to a logic 0 a minimum of one character clock time before the 'horizontal display enable'
 *              signal goes to a logical 1.
 *
 *           4. Register R16, Vsync Start [VRETRACE_START], must be a minimum of one horizontal scan line greater
 *              than register R18 [VDISP_END].  Register R18 defines where the 'vertical display enable' signal ends.
 *
 *     When bit 5 of the Attribute Mode Control register equals 1, a successful line compare (see Line Compare
 *     [LINE_COMPARE] register) in the CRT Controller forces the output of the PEL Panning register to 0's until Vsync
 *     occurs.  When Vsync occurs, the output returns to the programmed value.  This allows the portion of the screen
 *     indicated by the Line Compare register to be operated on by the PEL Panning register.
 *
 *     A write to the Character Map Select register becomes valid on the next whole character line.  No deformed
 *     characters are displayed by changing character generators in the middle of a character scan line.
 *
 *     For 256-color 320 x 200 graphics mode hex 13, the attribute controller is configured so that the 8-bit attribute
 *     stored in video memory for each PEL becomes the 8-bit address (P0 - P7) into the integrated DAC.  The user should
 *     not modify the contents of the internal Palette registers when using this mode.
 *
 *     The following sequence should be followed when accessing any of the Attribute Data registers pointed to by the
 *     Attribute Index register:
 *
 *           1. Disable interrupts
 *           2. Reset read/write flip/flop
 *           3. Write to Index register
 *           4. Read from or write to a data register
 *           5. Enable interrupts
 *
 *      The Color Select register in the Attribute Controller section may be used to rapidly switch between sets of colors
 *      in the video DAC.  When bit 7 of the Attribute Mode Control register equals 0, the 8-bit color value presented to the
 *      video DAC is composed of 6 bits from the internal Palette registers and bits 2 and 3 from the Color Select register.
 *      When bit 7 of the Attribute Mode Control register equals 1, the 8-bit color value presented to the video DAC is
 *      composed of the lower four bits from the internal Palette registers and the four bits in the Color Select register.
 *      By changing the value in the Color Select register, software rapidly switches between sets of colors in the video DAC.
 *      Note that BIOS does not support multiple sets of colors in the video DAC.  The user must load these colors if this
 *      function is to be used.  Also see the Attribute Controller block diagram on page 4-26.  Note that the above discussion
 *      applies to all modes except 256 Color Graphics mode.  In this mode the Color Select register is not used to switch
 *      between sets of colors.
 *
 *      An application that saves the "Video State" must store the 4 bytes of information contained in the system microprocessor
 *      latches in the graphics controller subsection. These latches are loaded with 32 bits from video memory (8 bits per map)
 *      each time the system microprocessor does a read from video memory.  The application needs to:
 *
 *           1. Use write mode 1 to write the values in the latches to a location in video memory that is not part of
 *              the display buffer.  The last location in the address range is a good choice.
 *
 *           2. Save the values of the latches by reading them back from video memory.
 *
 *           Note: If in a chain 4 or odd/even mode, it will be necessary to reconfigure the memory organization as four
 *           sequential maps prior to performing the sequence above.  BIOS provides support for completely saving and
 *           restoring video state.  See the IBM Personal System/2 and Personal Computer BIOS Interface Technical Reference
 *           for more information.
 *
 *      The description of the Horizontal PEL Panning register includes a figure showing the number of PELs shifted left
 *      for each valid value of the PEL Panning register and each valid video mode.  Further panning beyond that shown in
 *      the figure may be accomplished by changing the start address in the CRT Controller registers, Start Address High
 *      and Start Address Low.  The sequence involved in further panning would be as follows:
 *
 *           1. Use the PEL Panning register to shift the maximum number of bits to the left. See Figure 4-103 on page
 *              4-106 for the appropriate values.
 *
 *           2. Increment the start address.
 *
 *           3. If you are not using Modes 0 + , 1 + , 2 + , 3 + ,7, or7 + , set the PEL Panning register to 0.  If you
 *              are using these modes, set the PEL Panning register to 8.  The screen will now be shifted one PEL left
 *              of the position it was in at the end of step 1.  Step 1 through Step 3 may be repeated as desired.
 *
 *      The Line Compare register (CRTC register hex 18) should be programmed with even values in 200 line modes when
 *      used in split screen applications that scroll a second screen on top of a first screen.  This is a requirement
 *      imposed by the scan doubling logic in the CRTC.
 *
 *      If the Cursor Start register (CRTC register hex 0A) is programmed with a value greater than that in the Cursor End
 *      register (CRTC register hex 0B), then no cursor is displayed.  A split cursor is not possible.
 *
 *      In 8-dot character modes, the underline attribute produces a solid line across adjacent characters, as in the IBM
 *      Color/Graphics Monitor Adapter, Monochrome Display Adapter and the Enhanced Graphics Adapter.  In 9-dot modes, the
 *      underline across adjacent characters is dashed, as in the IBM 327X display terminals.  In 9-dot modes, the line
 *      graphics characters (C0 - DF character codes) have solid underlines.
 *
 *      For compatibility with the IBM Enhanced Graphics Adapter (EGA), the internal VGA palette is programmed the same
 *      as the EGA.  The video DAC is programmed by BIOS so that the compatible values in the internal VGA palette produce
 *      a color compatible with what was produced by EGA.  Mode hex 13 (256 colors) is programmed so that the first 16
 *      locations in the DAC produce compatible colors.
 *
 *      Summing: When BIOS is used to load the video DAC palette for a color mode and a monochrome display is connected
 *      to the system unit, the color palette is changed.  The colors are summed to produce shades of gray that allow
 *      color applications to produce a readable screen.
 *
 *      There are 4 bits that should not be modified unless the sequencer is reset by setting bit 1 of the Reset register
 *      to 0.  These bits are:
 *
 *           • Bit 3, or bit 0 of the Clocking Mode register
 *           • Bit 3, or bit 2 of the Miscellaneous Output register
 *
 * Also, for quick reference, IBM VGA register values for the standard VGA modes (from http://www.pcjs.org/blog/2015/06/01/):
 *
 *      INT 0x10 Mode Requested:    0x00 0x01 0x02 0x03 0x04 0x05 0x06 0x0D 0x0E 0x10 0x12 0x13
 *
 *      BIOSMODE:                   0x01 0x01 0x03 0x03 0x04 0x04 0x06 0x0D 0x0E 0x10 0x12 0x13
 *      CRTC[0x00]: HTOTAL          0x2D 0x2D 0x5F 0x5F 0x2D 0x2D 0x5F 0x2D 0x5F 0x5F 0x5F 0x5F
 *      CRTC[0x01]: HDISP_END       0x27 0x27 0x4F 0x4F 0x27 0x27 0x4F 0x27 0x4F 0x4F 0x4F 0x4F
 *      CRTC[0x02]: HBLANK_START    0x28 0x28 0x50 0x50 0x28 0x28 0x50 0x28 0x50 0x50 0x50 0x50
 *      CRTC[0x03]: HBLANK_END      0x90 0x90 0x82 0x82 0x90 0x90 0x82 0x90 0x82 0x82 0x82 0x82
 *      CRTC[0x04]: HRETRACE_START  0x2B 0x2B 0x55 0x55 0x2B 0x2B 0x54 0x2B 0x54 0x54 0x54 0x54
 *      CRTC[0x05]: HRETRACE_END    0xA0 0xA0 0x81 0x81 0x80 0x80 0x80 0x80 0x80 0x80 0x80 0x80
 *      CRTC[0x06]: VTOTAL          0xBF 0xBF 0xBF 0xBF 0xBF 0xBF 0xBF 0xBF 0xBF 0xBF 0x0B 0xBF
 *      CRTC[0x07]: OVERFLOW        0x1F 0x1F 0x1F 0x1F 0x1F 0x1F 0x1F 0x1F 0x1F 0x1F 0x3E 0x1F
 *      CRTC[0x08]: PRESET_ROW      0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
 *      CRTC[0x09]: MAX_SCAN        0x4F 0x4F 0x4F 0x4F 0xC1 0xC1 0xC1 0xC0 0xC0 0x40 0x40 0x41
 *      CRTC[0x0A]: CURSOR_START    0x0D 0x0D 0x0D 0x0D 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
 *      CRTC[0x0B]: CURSOR_END      0x0E 0x0E 0x0E 0x0E 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
 *      CRTC[0x0C]: START_ADDR_HI   0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
 *      CRTC[0x0D]: START_ADDR_LO   0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
 *      CRTC[0x0E]: CURSOR_ADDR_HI  0x01 0x01 0x01 0x01 0x01 0x01 0x01 0x01 0x01 0x01 0x01 0x00
 *      CRTC[0x0F]: CURSOR_ADDR_LO  0x19 0x19 0x41 0x41 0x19 0x19 0x41 0x19 0x41 0x41 0xE1 0xA2
 *      CRTC[0x10]: VRETRACE_START  0x9C 0x9C 0x9C 0x9C 0x9C 0x9C 0x9C 0x9C 0x9C 0x83 0xEA 0x9C
 *      CRTC[0x11]: VRETRACE_END    0x8E 0x8E 0x8E 0x8E 0x8E 0x8E 0x8E 0x8E 0x8E 0x85 0x8C 0x8E
 *      CRTC[0x12]: VDISP_END       0x8F 0x8F 0x8F 0x8F 0x8F 0x8F 0x8F 0x8F 0x8F 0x5D 0xDF 0x8F
 *      CRTC[0x13]: OFFSET          0x14 0x14 0x28 0x28 0x14 0x14 0x28 0x14 0x28 0x28 0x28 0x28
 *      CRTC[0x14]: UNDERLINE       0x1F 0x1F 0x1F 0x1F 0x00 0x00 0x00 0x00 0x00 0x0F 0x00 0x40
 *      CRTC[0x15]: VBLANK_START    0x96 0x96 0x96 0x96 0x96 0x96 0x96 0x96 0x96 0x63 0xE7 0x96
 *      CRTC[0x16]: VBLANK_END      0xB9 0xB9 0xB9 0xB9 0xB9 0xB9 0xB9 0xB9 0xB9 0xBA 0x04 0xB9
 *      CRTC[0x17]: MODE_CTRL       0xA3 0xA3 0xA3 0xA3 0xA2 0xA2 0xC2 0xE3 0xE3 0xE3 0xE3 0xA3
 *      CRTC[0x18]: LINE_COMPARE    0xFF 0xFF 0xFF 0xFF 0xFF 0xFF 0xFF 0xFF 0xFF 0xFF 0xFF 0xFF
 *       GRC[0x00]: SRESET          0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
 *       GRC[0x01]: ESRESET         0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
 *       GRC[0x02]: COLORCMP        0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
 *       GRC[0x03]: DATAROT         0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
 *       GRC[0x04]: READMAP         0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
 *       GRC[0x05]: MODE            0x10 0x10 0x10 0x10 0x30 0x30 0x00 0x00 0x00 0x00 0x00 0x40
 *       GRC[0x06]: MISC            0x0E 0x0E 0x0E 0x0E 0x0F 0x0F 0x0D 0x05 0x05 0x05 0x05 0x05
 *       GRC[0x07]: COLORDC         0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x0F 0x0F 0x0F 0x0F 0x0F
 *       GRC[0x08]: BITMASK         0xFF 0xFF 0xFF 0xFF 0xFF 0xFF 0xFF 0xFF 0xFF 0xFF 0xFF 0xFF
 *       SEQ[0x00]: RESET           0x03 0x03 0x03 0x03 0x03 0x03 0x03 0x03 0x03 0x03 0x03 0x03
 *       SEQ[0x01]: CLOCKING        0x08 0x08 0x00 0x00 0x09 0x09 0x01 0x09 0x01 0x01 0x01 0x01
 *       SEQ[0x02]: MAPMASK         0x03 0x03 0x03 0x03 0x03 0x03 0x01 0x0F 0x0F 0x0F 0x0F 0x0F
 *       SEQ[0x03]: CHARMAP         0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
 *       SEQ[0x04]: MEMMODE         0x03 0x03 0x03 0x03 0x02 0x02 0x06 0x06 0x06 0x06 0x06 0x0E
 *       ATC[0x00]: PAL00           0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
 *       ATC[0x01]: PAL01           0x01 0x01 0x01 0x01 0x13 0x13 0x17 0x01 0x01 0x01 0x01 0x01
 *       ATC[0x02]: PAL02           0x02 0x02 0x02 0x02 0x15 0x15 0x17 0x02 0x02 0x02 0x02 0x02
 *       ATC[0x03]: PAL03           0x03 0x03 0x03 0x03 0x17 0x17 0x17 0x03 0x03 0x03 0x03 0x03
 *       ATC[0x04]: PAL04           0x04 0x04 0x04 0x04 0x02 0x02 0x17 0x04 0x04 0x04 0x04 0x04
 *       ATC[0x05]: PAL05           0x05 0x05 0x05 0x05 0x04 0x04 0x17 0x05 0x05 0x05 0x05 0x05
 *       ATC[0x06]: PAL06           0x14 0x14 0x14 0x14 0x06 0x06 0x17 0x06 0x06 0x14 0x14 0x06
 *       ATC[0x07]: PAL07           0x07 0x07 0x07 0x07 0x07 0x07 0x17 0x07 0x07 0x07 0x07 0x07
 *       ATC[0x08]: PAL08           0x38 0x38 0x38 0x38 0x10 0x10 0x17 0x10 0x10 0x38 0x38 0x08
 *       ATC[0x09]: PAL09           0x39 0x39 0x39 0x39 0x11 0x11 0x17 0x11 0x11 0x39 0x39 0x09
 *       ATC[0x0A]: PAL0A           0x3A 0x3A 0x3A 0x3A 0x12 0x12 0x17 0x12 0x12 0x3A 0x3A 0x0A
 *       ATC[0x0B]: PAL0B           0x3B 0x3B 0x3B 0x3B 0x13 0x13 0x17 0x13 0x13 0x3B 0x3B 0x0B
 *       ATC[0x0C]: PAL0C           0x3C 0x3C 0x3C 0x3C 0x14 0x14 0x17 0x14 0x14 0x3C 0x3C 0x0C
 *       ATC[0x0D]: PAL0D           0x3D 0x3D 0x3D 0x3D 0x15 0x15 0x17 0x15 0x15 0x3D 0x3D 0x0D
 *       ATC[0x0E]: PAL0E           0x3E 0x3E 0x3E 0x3E 0x16 0x16 0x17 0x16 0x16 0x3E 0x3E 0x0E
 *       ATC[0x0F]: PAL0F           0x3F 0x3F 0x3F 0x3F 0x17 0x17 0x17 0x17 0x17 0x3F 0x3F 0x0F
 *       ATC[0x10]: MODE            0x0C 0x0C 0x0C 0x0C 0x01 0x01 0x01 0x01 0x01 0x01 0x01 0x41
 *       ATC[0x11]: OVERSCAN        0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
 *       ATC[0x12]: PLANES          0x0F 0x0F 0x0F 0x0F 0x03 0x03 0x01 0x0F 0x0F 0x0F 0x0F 0x0F
 *       ATC[0x13]: HPAN            0x08 0x08 0x08 0x08 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
 *
 * TODO: Build a similar table for the IBM EGA, and then work on rationalizing the mode detection logic in checkMode().
 */

/**
 * TODO: The Closure Compiler treats ES6 classes as 'struct' rather than 'dict' by default,
 * which would force us to declare all class properties in the constructor, as well as prevent
 * us from defining any named properties.  So, for now, we mark all our classes as 'unrestricted'.
 *
 * @unrestricted
 */
class Card {
    /**
     * Card(video, nCard, data, cbMemory)
     *
     * Creates an object representing an initial video card state;
     * can also restore a video card from state data created by saveCard().
     *
     * WARNING: Since Card objects are low-level objects that have no UI requirements,
     * they do not inherit from the Component class, so you should only use class methods
     * of Component, such as Component.assert(), or methods of the parent (video) object.
     *
     * @this {Card}
     * @param {Video} [video]
     * @param {number} [nCard] (see Video.CARD.*)
     * @param {Array|null} [data]
     * @param {number} [cbMemory] is specified if the card must allocate its own memory buffer
     */
    constructor(video, nCard, data, cbMemory)
    {
        /*
         * If a card was originally not present (eg, EGA), then the state will be empty,
         * so we need to detect that case and continue indicating that the card is not present.
         */
        if (nCard !== undefined && (!data || data.length)) {

            this.video = video;

            var specs = Video.cardSpecs[nCard];
            var nMonitorType = video.nMonitorType || specs[5];

            if (!data || data.length < 6) {
                data = [false, 0, null, null, 0, new Array(nCard < Video.CARD.EGA? Card.CRTC.TOTAL_REGS : Card.CRTC.EGA.TOTAL_REGS)];
            }

            /*
             * If a Debugger is present, we want to stash a bit more info in each Card.
             */
            if (DEBUGGER) {
                this.dbg = video.dbg;
                this.type = specs[0];
                this.port = specs[1];
            }

            this.nCard = nCard;
            this.addrBuffer = specs[2];     // default (physical) video buffer address
            this.sizeBuffer = specs[3];     // default video buffer length (this is the total size, not the current visible size; this.cbScreen is calculated on the fly to reflect the latter)

            /*
             * If no memory size is specified, then setMode() will use addMemory() to automatically add enough
             * memory blocks to cover the video buffer specified above; otherwise, it instructs addMemory() to call
             * getMemoryBuffer(), which will return a portion of the buffer (adwMemory) allocated below.  This allows
             * a card like the EGA to move/resize its video buffer as needed, as well as giving it total control over
             * the underlying memory.
             */
            this.cbMemory = cbMemory || specs[4];

            /*
             * All of our cardSpec video buffer sizes are based on the default text mode (eg, 4Kb for an MDA, 16Kb for
             * a CGA), but for a card with 64Kb or more of memory (ie, any EGA card), the default text mode video buffer
             * size should be dynamically recalculated as the smaller of: cbMemory divided by 4, or 32Kb.
             */
            if (this.cbMemory >= 0x10000 && this.addrBuffer >= 0xB0000) {
                this.sizeBuffer = Math.min(this.cbMemory >> 2, 0x8000);
            }

            this.fActive    = data[0];
            this.regMode    = data[1];      // see MDA.MODE* or CGA.MODE_* (use (MDA.MODE.HIRES | MDA.MODE.VIDEO_ENABLE | MDA.MODE.BLINK_ENABLE) if you want to test blinking immediately after the initial power-on reset)
            this.regColor   = data[2];      // see CGA.COLOR.* (undefined on MDA)
            this.regStatus  = data[3];      // see MDA.STATUS.* or CGA.STATUS.*
            this.regCRTIndx = data[4] & 0xff;
            this.regCRTPrev = (data[4] >> 8) & 0xff;
            this.regCRTData = data[5];
            this.nCRTCRegs  = Card.CRTC.TOTAL_REGS;
            this.asCRTCRegs = DEBUGGER? Card.CRTC.REGS : [];

            if (nCard >= Video.CARD.EGA) {
                this.nCRTCRegs = Card.CRTC.EGA.TOTAL_REGS;
                this.asCRTCRegs = DEBUGGER? Card.CRTC.EGA_REGS : [];
                this.initEGA(data[6], nMonitorType);
            }

            var monitorSpecs = Video.monitorSpecs[nMonitorType] || Video.monitorSpecs[ChipSet.MONITOR.MONO];

            var nCyclesPerSecond = video.cpu.getCyclesPerSecond();      // eg, 4772727
            this.nCyclesHorzPeriod = (nCyclesPerSecond / monitorSpecs.nHorzPeriodsPerSec)|0;
            this.nCyclesHorzActive = (this.nCyclesHorzPeriod * monitorSpecs.percentHorzActive / 100)|0;
            this.nCyclesVertPeriod = (this.nCyclesHorzPeriod * monitorSpecs.nHorzPeriodsPerFrame)|0;
            this.nCyclesVertActive = (this.nCyclesVertPeriod * monitorSpecs.percentVertActive / 100)|0;
            this.nInitCycles = (data[7] || 0);
        }
    }

    /**
     * initEGA(data)
     *
     * Another one of my frustrations with JSON is that it encodes empty arrays with non-zero lengths as
     * arrays of nulls, which means that any uninitialized register arrays whose elements were all originally
     * undefined come back via the JSON round-trip as *initialized* arrays whose elements are now all null.
     *
     * I'm a bit surprised, because JavaScript purists tell us to always use the '===' operator (eg, use
     * 'aReg[i] === undefined' to determine if an element is initialized), but because of this JSON stupidity,
     * that would require all such tests to become 'aReg[i] === undefined || aReg[i] === null'.  I'm puzzled
     * why the coercion of '==' is considered evil but JSON's coercion of undefined to null is perfectly fine.
     *
     * The simple solution is to change such comparisons to 'aReg[i] == null', because undefined is coerced
     * to null, whereas numeric values are not.
     *
     * [What do I mean by "another" frustration?  Let me talk to you some day about disallowing hex constants,
     * or insisting that property names be quoted, or refusing to allow comments.  I think it's fine for
     * JSON.stringify() to produce output that adheres to rules like that -- although some parameters to control
     * the output would be nice -- but it's completely unnecessary for JSON.parse() to refuse to parse objects
     * that are perfectly valid.]
     *
     * @this {Card}
     * @param {Array|undefined} data
     * @param {number} nMonitorType
     */
    initEGA(data, nMonitorType)
    {
        if (data === undefined) {
            data = [
                /* 0*/  false,
                /* 1*/  0,
                /* 2*/  new Array(Card.ATC.TOTAL_REGS),
                /* 3*/  0,
                /* 4*/  (nMonitorType == ChipSet.MONITOR.MONO? 0: Card.MISC.IO_SELECT),
                /* 5*/  0,
                /* 6*/  0,
                /* 7*/  new Array(Card.SEQ.TOTAL_REGS),
                /* 8*/  0,
                /* 9*/  0,
                /*10*/  0,
                /*11*/  new Array(Card.GRC.TOTAL_REGS),
                /*12*/  0,
                /*13*/  [this.addrBuffer, this.sizeBuffer, this.cbMemory],
                /*14*/  new Array(this.cbMemory >> 2),      // divide cbMemory by 4 since this is an array of DWORDs (8 bits for each of 4 planes)
                /*
                 * Card.ACCESS.WRITE.MODE0 by itself is a pretty good default, but if we choose to "randomize" the screen with
                 * text characters prior to starting the machine, defaulting to Card.ACCESS.WRITE.EVENODD is more faithful to how
                 * characters and attributes are typically stored (ie, in planes 0 and 1, respectively).  As soon as the machine
                 * starts up and initializes the hardware itself, these defaults won't matter.
                 */
                /*15*/  Card.ACCESS.READ.MODE0 | Card.ACCESS.READ.EVENODD | Card.ACCESS.WRITE.MODE0 | Card.ACCESS.WRITE.EVENODD | Card.ACCESS.V2,
                /*16*/  0,
                /*17*/  0xffffffff|0,
                /*18*/  0,
                /*19*/  0xffffffff|0,
                /*20*/  0,
                /*21*/  0xffffffff|0,
                /*22*/  0,
                /*23*/  0,
                /*24*/  0,
                /*25*/  0,
                /*26*/  Card.VGA_ENABLE.ENABLED,
                /*27*/  Card.DAC.MASK.DEFAULT,
                /*28*/  0,
                /*29*/  0,
                /*30*/  Card.DAC.STATE.MODE_WRITE,
                /*31*/  new Array(Card.DAC.TOTAL_REGS)
            ];
        }

        this.fATCData   = data[0];
        this.regATCIndx = data[1];
        this.regATCData = data[2];
        this.asATCRegs  = DEBUGGER? Card.ATC.REGS : [];
        this.regStatus0 = data[3];      // aka STATUS0 (not to be confused with this.regStatus, which the EGA refers to as STATUS1)
        this.regMisc    = data[4];
        this.regFeat    = data[5];      // for feature control bits, see Card.FEAT_CTRL.BITS; for feature status bits, see Card.STATUS0.FEAT
        this.regSEQIndx = data[6];
        this.regSEQData = data[7];
        this.asSEQRegs  = DEBUGGER? Card.SEQ.REGS : [];
        this.regGRCPos1 = data[8];
        this.regGRCPos2 = data[9];
        this.regGRCIndx = data[10];
        this.regGRCData = data[11];
        this.asGRCRegs  = DEBUGGER? Card.GRC.REGS : [];
        this.latches    = data[12];

        /*
         * Since we originally neglected to save/restore the card's active video buffer address and length,
         * we're now stashing all that information in data[13].  So if we're presented with an old data entry
         * that contains only the card's memory size, fix it up.
         *
         * TODO: This code just creates the required array; the correct video buffer address and length would
         * still need to be calculated from the current GRC registers; checkMode() knows how to do that, but I'm
         * not prepared to shoehorn in a call to checkMode() here, and potentially create more issues, for an
         * old problem that will eventually disappear anyway.
         */
        var a = data[13];
        if (typeof a == "number") {
            a = [this.addrBuffer, this.sizeBuffer, a];
        }
        this.addrBuffer = a[0];
        this.sizeBuffer = a[1];
        this.video.assert(this.cbMemory === a[2]);

        var cdw = this.cbMemory >> 2;
        this.adwMemory  = data[14];
        if (this.adwMemory && this.adwMemory.length < cdw) {
            this.adwMemory = State.decompressEvenOdd(this.adwMemory, cdw);
        }

        var nAccess = data[15];
        if (nAccess) {
            if (nAccess & Card.ACCESS.V2) {
                nAccess &= ~Card.ACCESS.V2;
            } else {
                this.video.assert(Card.ACCESS.V1[nAccess & 0xff00] !== undefined && Card.ACCESS.V1[nAccess & 0xff] !== undefined);
                nAccess = Card.ACCESS.V1[nAccess & 0xff00] | Card.ACCESS.V1[nAccess & 0xff];
            }
        }
        this.setMemoryAccess(nAccess);

        /*
         * nReadMapShift must perfectly track how the GRC.READMAP register is programmed, so that Card.ACCESS.READ.MODE0
         * memory read functions read the appropriate plane.  This default is not terribly critical, unless Card.ACCESS.WRITE.MODE0
         * is chosen as our default AND you want the screen randomizer to work.
         */
        this.nReadMapShift  = data[16];

        /*
         * Similarly, nSeqMapMask must perfectly track how the SEQ.MAPMASK register is programmed, so that memory write
         * functions write the appropriate plane(s).  Again, this default is not terribly critical, unless Card.ACCESS.WRITE.MODE0
         * is chosen as our default AND you want the screen randomizer to work.
         */
        this.nSeqMapMask    = data[17];
        this.nDataRotate    = data[18];
        this.nBitMapMask    = data[19];
        this.nSetMapData    = data[20];
        this.nSetMapMask    = data[21];
        this.nSetMapBits    = data[22];
        this.nColorCompare  = data[23];
        this.nColorDontCare = data[24];
        this.offStartAddr   = data[25];     // this is the last CRTC start address latched from CRTC.START_ADDR_HI,CRTC.START_ADDR_LO

        this.nVertPeriods = this.nVertPeriodsStartAddr = 0;

        if (this.nCard == Video.CARD.VGA) {
            this.regVGAEnable   = data[26];
            this.regDACMask     = data[27];
            this.regDACAddr     = data[28];
            this.regDACShift    = data[29];
            this.regDACState    = data[30];
            this.regDACData     = data[31];
        }
    }

    /**
     * saveCard()
     *
     * @this {Card}
     * @return {Array}
     */
    saveCard()
    {
        var data = [];
        if (this.nCard !== undefined) {
            data[0] = this.fActive;
            data[1] = this.regMode;
            data[2] = this.regColor;
            data[3] = this.regStatus;
            data[4] = this.regCRTIndx | (this.regCRTPrev << 8);
            data[5] = this.regCRTData;
            if (this.nCard >= Video.CARD.EGA) {
                data[6] = this.saveEGA();
            }
            data[7] = this.nInitCycles;
        }
        return data;
    }

    /**
     * saveEGA()
     *
     * @this {Card}
     * @return {Array}
     */
    saveEGA()
    {
        var data = [];
        data[0]  = this.fATCData;
        data[1]  = this.regATCIndx;
        data[2]  = this.regATCData;
        data[3]  = this.regStatus0;
        data[4]  = this.regMisc;
        data[5]  = this.regFeat;
        data[6]  = this.regSEQIndx;
        data[7]  = this.regSEQData;
        data[8]  = this.regGRCPos1;
        data[9]  = this.regGRCPos2;
        data[10] = this.regGRCIndx;
        data[11] = this.regGRCData;
        data[12] = this.latches;
        data[13] = [this.addrBuffer, this.sizeBuffer, this.cbMemory];
        data[14] = State.compressEvenOdd(this.adwMemory);
        data[15] = this.nAccess | Card.ACCESS.V2;
        data[16] = this.nReadMapShift;
        data[17] = this.nSeqMapMask;
        data[18] = this.nDataRotate;
        data[19] = this.nBitMapMask;
        data[20] = this.nSetMapData;
        data[21] = this.nSetMapMask;
        data[22] = this.nSetMapBits;
        data[23] = this.nColorCompare;
        data[24] = this.nColorDontCare;
        data[25] = this.offStartAddr;

        if (this.nCard == Video.CARD.VGA) {
            data[26] = this.regVGAEnable;
            data[27] = this.regDACMask;
            data[28] = this.regDACAddr;
            data[29] = this.regDACShift;
            data[30] = this.regDACState;
            data[31] = this.regDACData;
        }
        return data;
    }

    /**
     * dumpRegs()
     *
     * Since we don't pre-allocate the register arrays (eg, ATC, CRTC, GRC, etc) on a Card, we can't
     * rely on their array length, so we instead rely on the number of register names supplied in asRegs.
     *
     * @this {Card}
     * @param {string} sName
     * @param {number} iReg
     * @param {Array} [aRegs]
     * @param {Array} [asRegs]
     */
    dumpRegs(sName, iReg, aRegs, asRegs)
    {
        if (DEBUGGER) {
            if (!aRegs) {
                this.dbg.println(sName + ": " + Str.toHex(iReg, 2));
                return;
            }
            var i, cchMax = 18, s = "";
            for (i = 0; i < asRegs.length; i++) {
                var reg = (aRegs === this.regCRTData)? this.getCRTCReg(i) : aRegs[i];
                if (s) s += '\n';
                s += sName + "[" + Str.toHex(i, 2) + "]: " + Str.pad(asRegs[i], cchMax) + (i === iReg? '*' : ' ') + Str.toHex(reg, reg > 0xff? 4 : 2);
                if (reg != null) s += " (" + reg + ".)"
            }
            this.dbg.println(s);
        }
    }

    /**
     * dumpVideoCard()
     *
     * @this {Card}
     */
    dumpVideoCard()
    {
        if (DEBUGGER) {
            /*
             * Start with registers that are common to all cards....
             */
            this.dumpRegs("CRTC", this.regCRTIndx, this.regCRTData, this.asCRTCRegs);

            if (this.nCard >= Video.CARD.EGA) {
                this.dumpRegs(" GRC", this.regGRCIndx, this.regGRCData, this.asGRCRegs);
                this.dumpRegs(" SEQ", this.regSEQIndx, this.regSEQData, this.asSEQRegs);
                this.dumpRegs(" ATC", this.regATCIndx, this.regATCData, this.asATCRegs);
                this.dumpRegs(" ATCINDX", this.regATCIndx);
                this.dbg.println(" ATCDATA: " + this.fATCData);
                this.dumpRegs("    FEAT", this.regFeat);
                this.dumpRegs("    MISC", this.regMisc);
                this.dumpRegs(" STATUS0", this.regStatus0);
                /*
                 * There are few more EGA regs we could dump, like GRCPos1, GRCPos2, but does anyone care?
                 */
            }

            /*
             * TODO: This simply dumps the last value read from the STATUS1 register, not necessarily
             * its current state; consider dumping getRetraceBits() instead of (or in addition to) this.
             */
            this.dumpRegs(" STATUS1", this.regStatus);

            if (this.nCard == Video.CARD.MDA || this.nCard == Video.CARD.CGA) {
                this.dumpRegs(" MODEREG", this.regMode);
            }

            if (this.nCard == Video.CARD.CGA) {
                this.dumpRegs("   COLOR", this.regColor);
            }

            if (this.nCard >= Video.CARD.EGA) {
                this.dbg.println(" LATCHES: " + Str.toHex(this.latches));
                this.dbg.println("  ACCESS: " + Str.toHex(this.nAccess, 4));
                this.dbg.println("Use 'dump video [addr]' to dump video memory");
                /*
                 * There are few more EGA regs we could dump, like GRCPos1, GRCPos2, but does anyone care?
                 */
            }
        }
    }

    /**
     * dumpVideoBuffer(asArgs)
     *
     * Rather than requiring the first parameter to ALWAYS be a frame buffer address OR a frame buffer
     * offset, we'll just make a guess as to what the user intended and support BOTH; basically, if the
     * value is less than the frame buffer address, we'll assume it's an offset.
     *
     * Also, we allow some special options to be encoded in asArgs: 'l' followed by a number means
     * print that many rows of data.  'n' followed by a number (1-8) means print only that number of
     * memory locations per row, and then adjust the starting address of the next row by the number
     * of bytes per row (or whatever is specified by the 'w' option) so that the dump reflects a
     * rectangular chunk of video data.  Finally, if asArgs contains 'p' followed by a number (0-3),
     * we display only the bits from that plane for each memory location, in binary instead of hex.
     *
     * For example, assuming a standard VGA frame buffer with 640x480 pixels across 38400 (0x9600) memory
     * locations, the following command will dump a vertical swath of bits from plane 0 that is 32 (0x20)
     * rows tall and 8 columns wide, from roughly the center of the screen (0x4B00 + 0x28 - 2 = 0x4B26).
     *
     *      d video 4b26 l20 n8 p0
     *
     * Subsequent commands that omit a starting address or offset will continue where the last dump
     * left off; eg:
     *
     *      d video n8 p0
     *
     * To dump a chunk of off-screen memory starting at 0x9600, where the Windows VGA driver typically
     * stores a copy of the video memory containing the current mouse pointer:
     *
     *      d video 9600 l20 n5 w5 p0
     *
     * Alternatively, you could use decimal values:
     *
     *      d video 9600 l32. n5. w5. p0.
     *
     * NOTE: If these commands look suspiciously like weird Hayes modem command strings, trust me,
     * that is ENTIRELY coincidental (but mildly amusing).
     *
     * TODO: Make these options more general-purpose (it currently assumes a conventional VGA planar layout).
     *
     * @this {Card}
     * @param {Array.<string>} asArgs (all numeric arguments default to base 16 unless otherwise specified)
     */
    dumpVideoBuffer(asArgs)
    {
        if (DEBUGGER) {
            if (!this.adwMemory) {
                this.dbg.println("no buffer");
                return;
            }

            var i, j, idw, fColAdjust = false;
            var l = 8, n = 8, p = -1, w = this.video.nCols >> 3;

            for (i = 0; i < asArgs.length; i++) {

                var s = asArgs[i];
                if (!i) {
                    idw = Str.parseInt(s, 16);
                    continue;
                }

                var ch = s.charAt(0);
                j = Str.parseInt(s.substr(1), 16);

                switch(ch) {
                case 'l':
                    l = j;
                    break;
                case 'n':
                    if (j >= 1 && j <= 8) {
                        n = j;
                        fColAdjust = true;
                    }
                    break;
                case 'p':
                    if (j >= 0 && j <= 3) p = j;
                    break;
                case 'w':
                    if (j < w) w = j;
                    break;
                default:
                    this.dbg.println("unrecognized argument: " + s);
                    break;
                }
            }

            if (idw === undefined) {
                idw = this.prevDump || 0;
            } else if (idw >= this.addrBuffer) {
                idw -= this.addrBuffer;
            }

            var sDump = "";
            for (i = 0; i < l; i++) {
                var sData = Str.toHex(this.addrBuffer + idw) + ":";
                for (j = 0; j < n && idw < this.adwMemory.length; j++) {
                    var dw = this.adwMemory[idw++];
                    sData += ' ' + ((p < 0)? Str.toHex(dw) : Str.toBin((dw >> (p << 3)), 8));
                }
                if (fColAdjust) idw += w - n;
                if (sDump) sDump += "\n";
                sDump += sData;
            }

            if (sDump) this.dbg.println(sDump);
            this.prevDump = idw;
        }
    }

    /**
     * getMemoryBuffer(addr)
     *
     * If we passed a controller object (ie, this card) to addMemory(), then each allocated Memory block
     * will call this function to obtain a buffer.
     *
     * @this {Card}
     * @param {number} addr
     * @return {Array} containing the buffer (and the offset within that buffer that corresponds to the requested block)
     */
    getMemoryBuffer(addr)
    {
        return [this.adwMemory, addr - this.addrBuffer];
    }

    /**
     * getMemoryAccess()
     *
     * WARNING: This is a public method, whereas most Card methods are private to the Video component;
     * because a Card also acts as a Memory controller, it must provide getMemoryAccess() to the Memory component.
     *
     * Return the last set of memory access functions recorded by setMemoryAccess().
     *
     * @this {Card}
     * @return {Array.<function()>}
     */
    getMemoryAccess()
    {
        return this.afnAccess;
    }

    /**
     * setMemoryAccess(nAccess)
     *
     * This transforms the memory access value that getCardAccess() returns into the best available set of
     * memory access functions, which are then returned via getMemoryAccess() to any memory blocks we allocate
     * or modify.
     *
     * @this {Card}
     * @param {number|undefined} nAccess
     */
    setMemoryAccess(nAccess)
    {
        if (nAccess != null && nAccess != this.nAccess) {

            var nReadAccess = nAccess & Card.ACCESS.READ.MASK;
            var fnReadByte = Card.ACCESS.afn[nReadAccess];
            if (!fnReadByte) {
                if (DEBUG && this.dbg && this.dbg.messageEnabled(Messages.VIDEO)) {
                    this.dbg.message("Card.setMemoryAccess(" + Str.toHexWord(nAccess) + "): missing readByte handler");
                    /*
                     * I've taken a look, and the cases I've seen so far stem from the order in which the IBM VGA BIOS
                     * reprograms registers during a mode change: it reprograms the Sequencer registers BEFORE the Graphics
                     * Controller registers, so if GRC.MODE was set to READ.MODE1 prior to the mode change and the new mode
                     * clears SEQ.MEMMODE.SEQUENTIAL, we will briefly be in an "odd" (unsupported) state.
                     *
                     * This didn't used to occur when we relied on the GRC.MODE register instead of the SEQ.MEMMODE for
                     * determining the EVENODD state.  But, as explained in getCardAccess(), we've run into inconsistencies in
                     * how GRC.MODE.EVENODD is programmed, so we must live with this warning.
                     *
                     * The ultimate solution is to provide a EVENODD handler for READ.MODE1, since there is the remote
                     * possibility of third-party software that relies on that "odd" combination.
                     *
                     *      this.dbg.stopCPU();     // let's take a look
                     */
                }
                if (nReadAccess & Card.ACCESS.READ.EVENODD) {
                    fnReadByte = Card.ACCESS.afn[Card.ACCESS.READ.EVENODD];
                }
            }
            var nWriteAccess = nAccess & Card.ACCESS.WRITE.MASK;
            var fnWriteByte = Card.ACCESS.afn[nWriteAccess];
            if (!fnWriteByte) {
                if (DEBUG && this.dbg && this.dbg.messageEnabled(Messages.VIDEO)) {
                    this.dbg.message("Card.setMemoryAccess(" + Str.toHexWord(nAccess) + "): missing writeByte handler");
                    /*
                     * I've taken a look, and the cases I've seen so far stem from the order in which the IBM VGA BIOS
                     * reprograms registers during a mode change: it reprograms the Sequencer registers BEFORE the Graphics
                     * Controller registers, so if GRC.MODE was set to WRITE.MODE2 prior to the mode change and the new mode
                     * clears SEQ.MEMMODE.SEQUENTIAL, we will briefly be in an "odd" (unsupported) state.
                     *
                     * This didn't used to occur when we relied on the GRC.MODE register instead of the SEQ.MEMMODE for
                     * determining the EVENODD state.  But, as explained in getCardAccess(), we've run into inconsistencies in
                     * how GRC.MODE.EVENODD is programmed, so we must live with this warning.
                     *
                     * The ultimate solution is to provide EVENODD handlers for all modes other than WRITE.MODE0, since there
                     * is the remote possibility of third-party software that relies on one of those "odd" combinations.
                     *
                     *      this.dbg.stopCPU();     // let's take a look
                     */
                }
                if (nWriteAccess & Card.ACCESS.WRITE.EVENODD) {
                    fnWriteByte = Card.ACCESS.afn[Card.ACCESS.WRITE.EVENODD];
                }
            }
            if (!this.afnAccess) this.afnAccess = new Array(6);
            this.afnAccess[0] = fnReadByte;
            this.afnAccess[1] = fnWriteByte;
            this.nAccess = nAccess;
        }
    }

    /**
     * getCRTCReg()
     *
     * @this {Card}
     * @param {number} iReg
     * @return {number}
     */
    getCRTCReg(iReg)
    {
        var reg = this.regCRTData[iReg];
        if (reg != null && this.nCard >= Video.CARD.EGA) {
            var bOvrflowBit8 = 0, bOvrflowBit9 = 0, bMaxScanBit9 = 0;
            switch(iReg) {
            case Card.CRTC.EGA.VTOTAL:              // 0x06
                bOvrflowBit8 = Card.CRTC.EGA.OVERFLOW.VTOTAL_BIT8;          // 0x01
                if (this.nCard == Video.CARD.VGA) bOvrflowBit9 = Card.CRTC.EGA.OVERFLOW.VTOTAL_BIT9;
                break;
            case Card.CRTC.EGA.CURSOR_START.INDX:   // 0x0A
                if (this.nCard == Video.CARD.EGA) bOvrflowBit8 = Card.CRTC.EGA.OVERFLOW.CURSOR_START_BIT8;
                break;
            case Card.CRTC.EGA.VRETRACE_START:      // 0x10
                bOvrflowBit8 = Card.CRTC.EGA.OVERFLOW.VRETRACE_START_BIT8;  // 0x04
                if (this.nCard == Video.CARD.VGA) bOvrflowBit9 = Card.CRTC.EGA.OVERFLOW.VRETRACE_START_BIT9;
                break;
            case Card.CRTC.EGA.VDISP_END:           // 0x12
                bOvrflowBit8 = Card.CRTC.EGA.OVERFLOW.VDISP_END_BIT8;       // 0x02
                if (this.nCard == Video.CARD.VGA) bOvrflowBit9 = Card.CRTC.EGA.OVERFLOW.VDISP_END_BIT9;
                break;
            case Card.CRTC.EGA.VBLANK_START:        // 0x15
                bOvrflowBit8 = Card.CRTC.EGA.OVERFLOW.VBLANK_START_BIT8;    // 0x08
                if (this.nCard == Video.CARD.VGA) bMaxScanBit9 = Card.CRTC.EGA.MAX_SCAN.VBLANK_START_BIT9;
                break;
            case Card.CRTC.EGA.LINE_COMPARE:        // 0x18
                bOvrflowBit8 = Card.CRTC.EGA.OVERFLOW.LINE_COMPARE_BIT8;    // 0x10
                if (this.nCard == Video.CARD.VGA) bMaxScanBit9 = Card.CRTC.EGA.MAX_SCAN.LINE_COMPARE_BIT9;
                break;
            }
            if (bOvrflowBit8) {
                reg |= ((this.regCRTData[Card.CRTC.EGA.OVERFLOW.INDX] & bOvrflowBit8)? 0x100 : 0);
                reg |= ((this.regCRTData[Card.CRTC.EGA.OVERFLOW.INDX] & bOvrflowBit9)? 0x200 : 0);
                reg |= ((this.regCRTData[Card.CRTC.EGA.MAX_SCAN.INDX] & bMaxScanBit9)? 0x200 : 0);
            }
        }
        return reg;
    }
}

/*
 * MDA Registers (ports 0x3B4, 0x3B5, 0x3B8, and 0x3BA)
 */
Card.MDA = {
    CRTC: {
        INDX: {
            PORT:           0x3B4,      // NOTE: the low byte of this port address (0xB4) is mirrored at 40:0063 (0x0463)
            MASK:           0x1F
        },
        DATA: {
            PORT:           0x3B5
        }
    },
    MODE: {
        PORT:               0x3B8,      // Mode Select Register, aka CRT Control Port 1 (write-only); the BIOS mirrors this register at 40:0065 (0x0465)
        HIRES:              0x01,
        VIDEO_ENABLE:       0x08,
        BLINK_ENABLE:       0x20
    },
    STATUS: {
        PORT:               0x3BA,
        HDRIVE:             0x01,
        BWVIDEO:            0x08
    },
    /*
     * TODO: Add support for parallel port(s) someday....
     */
    PRT_DATA: {
        PORT:               0x3BC
    },
    PRT_STATUS: {
        PORT:               0x3BD
    },
    PRT_CTRL: {
        PORT:               0x3BE
    }
};

/*
 * CGA Registers (ports 0x3D4, 0x3D5, 0x3D8, 0x3D9, and 0x3DA)
 */
Card.CGA = {
    CRTC: {
        INDX: {
            PORT:           0x3D4,      // NOTE: the low byte of this port address (0xB4) is mirrored at 40:0063 (0x0463)
            MASK:           0x1F
        },
        DATA: {
            PORT:           0x3D5
        }
    },
    MODE: {
        PORT:               0x3D8,      // Mode Select Register (write-only); the BIOS mirrors this register at 40:0065 (0x0465)
        _80X25:             0x01,
        GRAPHIC_SEL:        0x02,
        BW_SEL:             0x04,
        VIDEO_ENABLE:       0x08,       // same as MDA.MODE.VIDEO_ENABLE
        HIRES_BW:           0x10,
        BLINK_ENABLE:       0x20        // same as MDA.MODE.BLINK_ENABLE
    },
    COLOR: {
        PORT:               0x3D9,      // write-only
        BORDER:             0x07,
        BRIGHT:             0x08,
        BGND_ALT:           0x10,       // alternate, intensified background colors in text mode
        COLORSET2:          0x20        // selects aCGAColorSet2 colors for 320x200 graphics mode; aCGAColorSet1 otherwise
    },
    STATUS: {
        PORT:               0x3DA,      // read-only; same for EGA (although the EGA calls this STATUS1, to distinguish it from STATUS0)
        RETRACE:            0x01,
        PEN_TRIGGER:        0x02,
        PEN_ON:             0x04,
        VRETRACE:           0x08        // when set, this indicates the CGA is performing a vertical retrace
    },
    /*
     * TODO: Add support for light pen port(s) someday....
     */
    CLEAR_PEN: {
        PORT:               0x3DB
    },
    PRESET_PEN: {
        PORT:               0x3DC
    }
};

/*
 * Common CRT hardware registers (ports 0x3B4/0x3B5 or 0x3D4/0x3D5)
 *
 * NOTE: In this implementation, because we have to make at least two of the registers readable (CURSOR_ADDR_HI and CURSOR_ADDR_LO),
 * we end up making ALL the registers readable, otherwise we would have to explicitly block any register marked write-only.  I don't
 * think making the CRT registers fully readable presents any serious compatibility issues, and it actually offers some benefits
 * (eg, improved debugging).
 *
 * However, some things are broken: the (readable) light pen registers on the EGA are overloaded as (writable) vertical retrace
 * registers, so the vertical retrace registers cannot actually be read that way.  I'm sure the VGA solved that problem, but I haven't
 * looked into it yet.
 */
Card.CRTC = {
    HTOTAL:                 0x00,
    HDISP:                  0x01,
    HSYNC_POS:              0x02,
    HSYNC_WIDTH:            0x03,
    VTOTAL:                 0x04,
    VTOTAL_ADJ:             0x05,
    VDISP_TOTAL:            0x06,
    VSYNC_POS:              0x07,
    INTERLACE_POS:          0x08,
    MAX_SCAN: {
        INDX:               0x09,
        MASK:               0x1F
    },
    CURSOR_START: {
        INDX:               0x0A,
        MASK:               0x1F,
        /*
         * I don't entirely understand these cursor blink control bits.  Here's what the MC6845 datasheet says:
         *
         *      Bit 5 is the blink timing control.  When bit 5 is low, the blink frequency is 1/16 of the vertical field rate,
         *      and when bit 5 is high, the blink frequency is 1/32 of the vertical field rate.  Bit 6 is used to enable a blink.
         */
        BLINKON:            0x00,       // (supposedly, 0x04 has the same effect as 0x00)
        BLINKOFF:           0x20,       // if blinking is disabled, the cursor is effectively hidden
        BLINKFAST:          0x60        // default is 1/16 of the frame rate; this switches to 1/32 of the frame rate
    },
    CURSOR_END: {
        INDX:               0x0B,
        MASK:               0x1F
    },
    START_ADDR_HI:          0x0C,
    START_ADDR_LO:          0x0D,
    CURSOR_ADDR_HI:         0x0E,
    CURSOR_ADDR_LO:         0x0F,
    LIGHT_PEN_HI:           0x10,
    LIGHT_PEN_LO:           0x11,
    TOTAL_REGS:             0x12,       // total CRT registers on MDA/CGA
    EGA: {
        HDISP_END:          0x01,
        HBLANK_START:       0x02,
        HBLANK_END:         0x03,
        HRETRACE_START:     0x04,
        HRETRACE_END:       0x05,
        VTOTAL:             0x06,
        OVERFLOW: {
            INDX:               0x07,
            VTOTAL_BIT8:        0x01,   // bit 8 of register 0x06
            VDISP_END_BIT8:     0x02,   // bit 8 of register 0x12
            VRETRACE_START_BIT8:0x04,   // bit 8 of register 0x10
            VBLANK_START_BIT8:  0x08,   // bit 8 of register 0x15
            LINE_COMPARE_BIT8:  0x10,   // bit 8 of register 0x18
            CURSOR_START_BIT8:  0x20,   // bit 8 of register 0x0A (EGA only)
            VTOTAL_BIT9:        0x20,   // bit 9 of register 0x06 (VGA only)
            VDISP_END_BIT9:     0x40,   // bit 9 of register 0x12 (VGA only, unused on EGA)
            VRETRACE_START_BIT9:0x80    // bit 9 of register 0x10 (VGA only, unused on EGA)
        },
        PRESET_SCAN:        0x08,
        /*
         * NOTE: EGA/VGA CRTC registers 0x09-0x0F are the same as the MDA/CGA CRTC registers defined above
         */
        MAX_SCAN: {
            INDX:               0x09,
            SCAN_LINE:          0x1f,
            VBLANK_START_BIT9:  0x20,   // (VGA only)
            LINE_COMPARE_BIT9:  0x40,   // (VGA only)
            CONVERT400:         0x80    // 200-to-400 scan-line conversion is in effect (VGA only)
        },
        CURSOR_START: {
            INDX:           0x0A,
            MASK:           0x1F,
            BLINKON:        0x00,       // (VGA only; supposedly, 0x04 has the same effect as 0x00)
            BLINKOFF:       0x20,       // if blinking is disabled, the cursor is effectively hidden (VGA only)
            BLINKFAST:      0x60        // default is 1/16 of the frame rate; this switches to 1/32 of the frame rate (VGA only)
        },
        CURSOR_END: {
            INDX:           0x0B,
            MASK:           0x1F
        },
        START_ADDR_HI:      0x0C,
        START_ADDR_LO:      0x0D,
        CURSOR_ADDR_HI:     0x0E,
        CURSOR_ADDR_LO:     0x0F,
        VRETRACE_START:     0x10,
        VRETRACE_END:       0x11,
        VDISP_END:          0x12,
        /*
         * The OFFSET register (bits 0-7) specifies the logical line width of the screen.  The starting memory address
         * for the next character row is larger than the current character row by two or four times this amount.
         * The OFFSET register is programmed with a word address.  Depending on the method of clocking the CRT Controller,
         * this word address is [effectively] either a word or double-word address. #IBMVGATechRef
         */
        OFFSET:             0x13,
        UNDERLINE: {
            INDX:           0x14,
            ROWSCAN:        0x1f,
            COUNTBY4:       0x20,       // (VGA only)
            DWORD:          0x40        // (VGA only)
        },
        VBLANK_START:       0x15,
        VBLANK_END:         0x16,
        MODE_CTRL: {
            INDX:           0x17,
            COMPAT_MODE:    0x01,       // Compatibility Mode Support (CGA A13 control)
            SEL_ROW_SCAN:   0x02,       // Select Row Scan Counter
            SEL_HRETRACE:   0x04,       // Horizontal Retrace Select
            COUNTBY2:       0x08,       // Count By Two
            OUTPUT_CTRL:    0x10,       // Output Control
            ADDR_WRAP:      0x20,       // Address Wrap (in Word mode, 1 maps A15 to A0 and 0 maps A13; use the latter when only 64Kb is installed)
            BYTE_MODE:      0x40,       // Byte Mode (1 selects Byte Mode; 0 selects Word Mode)
            HARD_RESET:     0x80        // Hardware Reset
        },
        LINE_COMPARE:       0x18,
        TOTAL_REGS:         0x19        // total CRT registers on EGA/VGA
    },
    ADDR_HI_MASK:           0x3F
};

if (DEBUGGER) {
    Card.CRTC.REGS      = ["HTOTAL","HDISP","HSYNC_POS","HSYNC_WIDTH","VTOTAL","VTOTAL_ADJ",
        "VDISP","VSYNC_POS","INTERLACE_POS","MAX_SCAN","CURSOR_START","CURSOR_END",
        "START_ADDR_HI","START_ADDR_LO","CURSOR_ADDR_HI","CURSOR_ADDR_LO","LIGHT_PEN_HI","LIGHT_PEN_LO"];

    Card.CRTC.EGA_REGS  = ["HTOTAL","HDISP_END","HBLANK_START","HBLANK_END","HRETRACE_START","HRETRACE_END",
        "VTOTAL","OVERFLOW","PRESET_SCAN","MAX_SCAN","CURSOR_START","CURSOR_END",
        "START_ADDR_HI","START_ADDR_LO","CURSOR_ADDR_HI","CURSOR_ADDR_LO","VRETRACE_START","VRETRACE_END",
        "VDISP_END","OFFSET","UNDERLINE","VBLANK_START","VBLANK_END","MODE_CTRL","LINE_COMPARE"];
}

/*
 * EGA/VGA Input Status 1 Register (port 0x3DA)
 *
 * STATUS1 bit 0 has confusing documentation: the EGA Tech Ref says "Logical 0 indicates the CRT raster is in a
 * horizontal or vertical retrace interval", whereas the VGA Tech Ref says "Logical 1 indicates a horizontal or
 * vertical retrace interval," but then clarifies: "This bit is the real-time status of the INVERTED display enable
 * signal".  So, instead of calling bit 0 DISP_ENABLE (or more precisely, DISP_ENABLE_INVERTED), it's simply RETRACE.
 *
 * STATUS1 diagnostic bits 5 and 4 are set according to the Card.ATC.PLANES.MUX bits:
 *
 *      MUX     Bit 5   Bit 4
 *      ---     ----    ----
 *      00:     Red     Blue
 *      01:     SecBlue Green
 *      10:     SecRed  SecGreen
 *      11:     unused  unused
 */
Card.STATUS1 = {
    PORT:                   0x3DA,
    RETRACE:                0x01,       // bit 0: logical OR of horizontal and vertical retrace
    VRETRACE:               0x08,       // bit 3: set during vertical retrace interval
    DIAGNOSTIC:             0x30,       // bits 5,4 are controlled by the Card.ATC.PLANES.MUX bits
    RESERVED:               0xC6
};

/*
 * EGA/VGA Attribute Controller Registers (port 0x3C0: regATCIndx and regATCData)
 *
 * The current ATC INDX value is stored in cardEGA.regATCIndx (including the Card.ATC.INDX_ENABLE bit), and the
 * ATC DATA values are stored in cardEGA.regATCData.  The state of the ATC INDX/DATA flip-flop is stored in fATCData.
 *
 * Note that the ATC palette registers (0x0-0xf) all use the following 6 bit assignments, with bits 6 and 7 unused:
 *
 *      0: Blue
 *      1: Green
 *      2: Red
 *      3: SecBlue (or mono video)
 *      4: SecGreen (or intensity)
 *      5: SecRed
 */
Card.ATC = {
    PORT:                   0x3C0,      // ATC Index/Data Port
    INDX_MASK:              0x1F,
    INDX_PAL_ENABLE:        0x20,       // must be clear when loading palette registers
    PALETTE: {
        INDX:               0x00,       // 16 registers: 0x00 - 0x0F
        MASK:               0x3f,
        BLUE:               0x01,
        GREEN:              0x02,
        RED:                0x04,
        SECBLUE:            0x08,
        BRIGHT:             0x10,       // NOTE: The IBM EGA manual (p.56) also calls this the "intensity" bit
        SECGREEN:           0x10,
        SECRED:             0x20
    },
    PALETTE_REGS:           0x10,       // 16 total palette registers
    MODE: {
        INDX:               0x10,       // ATC Mode Control Register
        GRAPHICS:           0x01,       // bit 0: set for graphics mode, clear for alphanumeric mode
        MONOEM:             0x02,       // bit 1: set for monochrome emulation mode, clear for color emulation
        TEXT_9DOT:          0x04,       // bit 2: set for 9-dot replication in character codes 0xC0-0xDF
        BLINK_ENABLE:       0x08,       // bit 3: set for text/graphics blink, clear for background intensity
        RESERVED:           0x10,       // bit 4: reserved
        PANCOMPAT:          0x20,       // bit 5: set for pixel-panning compatibility
        PELWIDTH:           0x40,       // bit 6: set for 256-color modes, clear for all other modes
        COLORSEL_ALL:       0x80        // bit 7: set to enable all COLORSEL bits (ie, COLORSEL.DAC_BIT5 and COLORSEL.DAC_BIT4)
    },
    OVERSCAN: {
        INDX:               0x11        // ATC Overscan Color Register
    },
    PLANES: {
        INDX:               0x12,       // ATC Color Plane Enable Register
        MASK:               0x0F,
        MUX:                0x30,
        RESERVED:           0xC0
    },
    HPAN: {
        INDX:               0x13,       // ATC Horizontal PEL Panning Register
        SHIFT_LEFT:         0x0F        // bits 0-3 indicate # of pixels to shift left
    },
    COLORSEL: {
        INDX:               0x14,       // ATC Color Select Register (VGA only)
        DAC_BIT7:           0x08,       // specifies bit 7 of DAC values (ignored in 256-color modes)
        DAC_BIT6:           0x04,       // specifies bit 6 of DAC values (ignored in 256-color modes)
        DAC_BIT5:           0x02,       // specifies bit 5 of DAC values (if ATC.MODE.COLORSEL_ALL is set; ignored in 256-color modes)
        DAC_BIT4:           0x01        // specifies bit 4 of DAC values (if ATC.MODE.COLORSEL_ALL is set; ignored in 256-color modes)
    },
    TOTAL_REGS:             0x14
};

if (DEBUGGER) {
    Card.ATC.REGS = ["PAL00","PAL01","PAL02","PAL03","PAL04","PAL05","PAL06","PAL07",
        "PAL08","PAL09","PAL0A","PAL0B","PAL0C","PAL0D","PAL0E","PAL0F", "MODE","OVERSCAN","PLANES","HPAN"];
}

/*
 * EGA/VGA Feature Control Register (port 0x3BA or 0x3DA: regFeat)
 *
 * The EGA BIOS writes 0x1 to Card.FEAT_CTRL.BITS and reads Card.STATUS0.FEAT, then writes 0x2 to
 * Card.FEAT_CTRL.BITS and reads Card.STATUS0.FEAT.  The bits from the first and second reads are shifted
 * into the high nibble of the byte at 40:88h.
 */
Card.FEAT_CTRL = {
    PORT_MONO:              0x3BA,      // write port address (other than the two bits below, the rest are reserved and/or unused)
    PORT_COLOR:             0x3DA,      // write port address (other than the two bits below, the rest are reserved and/or unused)
    PORT_READ:              0x3CA,      // read port address (VGA only)
    BITS:                   0x03        // feature control bits
};

/*
 * EGA/VGA Miscellaneous Output Register (port 0x3C2: regMisc)
 */
Card.MISC = {
    PORT_WRITE:             0x3C2,      // write port address (EGA and VGA)
    PORT_READ:              0x3CC,      // read port addresss (VGA only)
    IO_SELECT:              0x01,       // 0 sets CRT ports to 0x3Bn, 1 sets CRT ports to 0x3Dn
    ENABLE_RAM:             0x02,       // 0 disables video RAM, 1 enables
    CLOCK_SELECT:           0x0C,       // 0x0: 14Mhz I/O clock, 0x4: 16Mhz on-board clock, 0x8: external clock, 0xC: unused
    DISABLE_DRV:            0x10,       // 0 activates internal video drivers, 1 activates feature connector direct drive outputs
    PAGE_ODD_EVEN:          0x20,       // 0 selects the low 64Kb page of video RAM for text modes, 1 selects the high page
    HPOLARITY:              0x40,       // 0 selects positive horizontal retrace
    VPOLARITY:              0x80        // 0 selects positive vertical retrace
};

/*
 * EGA/VGA Input Status 0 Register (port 0x3C2: regStatus0)
 */
Card.STATUS0 = {
    PORT:                   0x3C2,      // read-only (aka STATUS0, to distinguish it from PORT_CGA_STATUS)
    RESERVED:               0x0F,
    SWSENSE:                0x10,
    SWSENSE_SHIFT:          4,
    FEAT:                   0x60,       // VGA: reserved
    INTERRUPT:              0x80        // 1: video is being displayed; 0: vertical retrace is occurring
};

/*
 * VGA Subsystem Enable Register (port 0x3C3: regVGAEnable)
 */
Card.VGA_ENABLE = {
    PORT:                   0x3C3,
    ENABLED:                0x01,       // when set, all VGA I/O and memory decoding is enabled; otherwise disabled (TODO: Implement)
    RESERVED:               0xFE
};

/*
 * EGA/VGA Sequencer Registers (ports 0x3C4/0x3C5: regSEQIndx and regSEQData)
 */
Card.SEQ = {
    INDX: {
        PORT:               0x3C4,      // Sequencer Index Port
        MASK:               0x07
    },
    DATA: {
        PORT:               0x3C5       // Sequencer Data Port
    },
    RESET: {
        INDX:               0x00,       // Sequencer Reset Register
        ASYNC:              0x01,
        SYNC:               0x02
    },
    CLOCKING: {
        INDX:               0x01,       // Sequencer Clocking Mode Register
        DOTS8:              0x01,       // 1: 8 dots; 0: 9 dots
        BANDWIDTH:          0x02,       // 0: CRTC has access 4 out of every 5 cycles (for high-res modes); 1: CRTC has access 2 out of 5 (VGA: reserved)
        SHIFTLOAD:          0x04,
        DOTCLOCK:           0x08,       // 0: normal dot clock; 1: master clock divided by two (used for 320x200 modes: 0, 1, 4, 5, and D)
        SHIFT4:             0x10,       // VGA only
        SCREEN_OFF:         0x20,       // VGA only
        RESERVED:           0xC0
    },
    MAPMASK: {
        INDX:               0x02,       // Sequencer Map Mask Register
        PL0:                0x01,
        PL1:                0x02,
        PL2:                0x04,
        PL3:                0x08,
        MAPS:               0x0F,
        RESERVED:           0xF0
    },
    CHARMAP: {
        INDX:               0x03,       // Sequencer Character Map Select Register
        SELB:               0x03,       // 0x0: 1st 8Kb of plane 2; 0x1: 2nd 8Kb; 0x2: 3rd 8Kb; 0x3: 4th 8Kb
        SELA:               0x0C,       // 0x0: 1st 8Kb of plane 2; 0x4: 2nd 8Kb; 0x8: 3rd 8Kb; 0xC: 4th 8Kb
        SELB_HIGH:          0x10,       // VGA only
        SELA_HIGH:          0x20        // VGA only
    },
    MEMMODE: {
        INDX:               0x04,       // Sequencer Memory Mode Register
        ALPHA:              0x01,       // set for alphanumeric (A/N) mode, clear for graphics (APA or "All Points Addressable") mode (EGA only)
        EXT:                0x02,       // set if memory expansion installed, clear if not installed
        SEQUENTIAL:         0x04,       // set for sequential memory access, clear for mapping even addresses to planes 0/2, odd addresses to planes 1/3
        CHAIN4:             0x08        // VGA only: set to select memory map (plane) based on low 2 bits of address
    },
    TOTAL_REGS:             0x05
};

if (DEBUGGER) Card.SEQ.REGS = ["RESET","CLOCKING","MAPMASK","CHARMAP","MEMMODE"];

/*
 * VGA Digital-to-Analog Converter (DAC) Registers (regDACMask, regDACState, regDACAddr, and regDACData)
 *
 * To write DAC data, write an address to DAC.ADDR.PORT_WRITE, then write 3 bytes to DAC.DATA.PORT; the low 6 bits
 * of each byte will be concatenated to form an 18-bit DAC value (red is least significant, followed by green, then blue).
 * When the final byte is received, the 18-bit DAC value is updated and regDACAddr is auto-incremented.
 *
 * To read DAC data, the process is similar, but the initial address is written to DAC.ADDR.PORT_READ instead.
 *
 * DAC.STATE.PORT and DAC.ADDR.PORT_WRITE can be read at any time and will not interfere with a read or write operation
 * in progress.  To prevent "snow", reading or writing DAC values should be limited to retrace intervals (see regStatus1),
 * or by using the SCREEN_OFF bit in the SEQ.CLOCKING register.
 */
Card.DAC = {
    MASK: {
        PORT:               0x3C6,      // initialized to 0xFF and should not be changed
        DEFAULT:            0xFF
    },
    STATE: {
        PORT:               0x3C7,
        MODE_WRITE:         0x00,       // the DAC is in write mode if bits 0 and 1 are clear
        MODE_READ:          0x03        // the DAC is in read mode if bits 0 and 1 are set
    },
    ADDR: {
        PORT_READ:          0x3C7,      // write to initiate a read
        PORT_WRITE:         0x3C8       // write to initiate a write; read to determine the current ADDR
    },
    DATA: {
        PORT:               0x3C9
    },
    TOTAL_REGS:             0x100
};

/*
 * EGA/VGA Graphics Controller Registers (ports 0x3CE/0x3CF: regGRCIndx and regGRCData)
 *
 * The VGA added Write Mode 3, which is described as follows:
 *
 *      "Each map is written with 8 bits of the value contained in the Set/Reset register for that map
 *      (the Enable Set/Reset register has no effect). Rotated system microprocessor data is ANDed with the
 *      Bit Mask register data to form an 8-bit value that performs the same function as the Bit Mask register
 *      does in write modes 0 and 2."
 */
Card.GRC = {
    POS1_PORT:              0x3CC,      // EGA only, write-only
    POS2_PORT:              0x3CA,      // EGA only, write-only
    INDX: {
        PORT:               0x3CE,      // GRC Index Port
        MASK:               0x0F
    },
    DATA: {
        PORT:               0x3CF       // GRC Data Port
    },
    SRESET: {
        INDX:               0x00        // GRC Set/Reset Register (write-only; each bit used only if WRITE.MODE0 and corresponding ESR bit set)
    },
    ESRESET: {
        INDX:               0x01        // GRC Enable Set/Reset Register
    },
    COLORCMP: {
        INDX:               0x02        // GRC Color Compare Register
    },
    DATAROT: {
        INDX:               0x03,       // GRC Data Rotate Register
        COUNT:              0x07,
        AND:                0x08,
        OR:                 0x10,
        XOR:                0x18,
        FUNC:               0x18,
        MASK:               0x1F
    },
    READMAP: {
        INDX:               0x04,       // GRC Read Map Select Register
        NUM:                0x03
    },
    MODE: {
        INDX:               0x05,       // GRC Mode Register
        WRITE: {
            MODE0:          0x00,       // write mode 0: each plane written with CPU data, rotated as needed, unless SR enabled
            MODE1:          0x01,       // write mode 1: each plane written with contents of the processor latches (loaded by a read)
            MODE2:          0x02,       // write mode 2: memory plane N is written with 8 bits matching data bit N
            MODE3:          0x03,       // write mode 3: VGA only
            MASK:           0x03
        },
        TEST:               0x04,
        READ: {
            MODE0:          0x00,       // read mode 0: read map mode
            MODE1:          0x08,       // read mode 1: color compare mode
            MASK:           0x08
        },
        EVENODD:            0x10,
        SHIFT:              0x20,
        COLOR256:           0x40        // VGA only
    },
    MISC: {
        INDX:               0x06,       // GRC Miscellaneous Register
        GRAPHICS:           0x01,       // set for graphics mode addressing, clear for text mode addressing
        CHAIN:              0x02,       // set for odd/even planes selected with odd/even values of the processor AO bit
        MAPMEM:             0x0C,       //
        MAPA0128:           0x00,       //
        MAPA064:            0x04,       //
        MAPB032:            0x08,       //
        MAPB832:            0x0C        //
    },
    COLORDC: {
        INDX:               0x07        // GRC Color "Don't Care" Register
    },
    BITMASK: {
        INDX:               0x08        // GRC Bit Mask Register
    },
    TOTAL_REGS:             0x09
};

if (DEBUGGER) Card.GRC.REGS = ["SRESET","ESRESET","COLORCMP","DATAROT","READMAP","MODE","MISC","COLORDC","BITMASK"];

/*
 * EGA Memory Access Functions
 *
 * Here's where we define all the getMemoryAccess() functions that know how to deal with "planar" EGA memory,
 * which consists of 32-bit values for every byte of address space, allowing us to internally store plane 0
 * bytes in bits 0-7, plane 1 bytes in bits 8-15, plane 2 bytes in bits 16-23, and plane 3 bytes in bits 24-31.
 *
 * All our functions have slightly more overhead than the standard Bus memory access functions, because the
 * offset (off) parameter is block-relative, which we must transform into a buffer-relative offset.  Fortunately,
 * all our Memory objects know this and have already recorded their buffer-relative offset in "this.offset".
 *
 * Also, the EGA includes a set of latches, one for each plane, which must be updated on most reads/writes;
 * we rely on the Memory object's "this.controller" property to give us access to the Card's state.
 *
 * And we take a little extra time to conditionally set fDirty on writes, meaning if a write did not actually
 * change the value of the memory, we will not set fDirty.  The default write functions in memory.js don't take
 * that performance hit, but here, it may be worthwhile, because if it results in fewer dirty blocks, display
 * updates may be faster.
 *
 * Note that we don't have to worry about dealing with word accesses that straddle block boundaries, because
 * the Bus component automatically breaks those accesses into separate byte requests.  Similarly, byte and word
 * values for the write functions have already been pre-masked by the Bus component to 8 and 16 bits, respectively.
 *
 * My motto: Be paranoid, but also be careful not to do any more work than you absolutely have to.
 *
 *
 * CGA Emulation on the EGA
 *
 * Modes 4/5 (320x200 low-res graphics) emulate the same buffer format that the CGA uses.  To recap: 1 byte contains
 * 4 pixels (pixel 0 in bits 7-6, pixel 1 in bits 5-4, etc), and thus one row of pixels is 80 (0x50) bytes long.
 * Moreover, all even rows are stored in the first 8K of the video buffer (at 0xB8000), and all odd rows are stored
 * in the second 8K (at 0xBA000).  Of each 8K, only 8000 (0x1F40) bytes are used (80 bytes X 100 rows); the remaining
 * 192 bytes of each 8K are unused.
 *
 * For these modes, the EGA's GRC.MODE is programmed with 0x30: Card.GRC.MODE.EVENODD and Card.GRC.MODE.SHIFT.
 * The latter claims to work by forming each 2-bit pixel with even bits from plane 0 and odd bits from plane 1;
 * however, I'm unclear how that works if even bytes are only written to plane 0 and odd bytes are only written to
 * plane 1, as Card.GRC.MODE.EVENODD implies, because plane 0 would never have any bits for the odd bytes, and
 * plane 1 would never have any bits for the even bytes.  TODO: Figure this out.
 *
 *
 * Even/Odd Memory Access Functions
 *
 * The "EVENODD" functions deal with the EGA's default text-mode addressing, where EVEN addresses are mapped to
 * plane 0 (and 2) and ODD addresses are mapped to plane 1 (and 3).  This occurs when SEQ.MEMMODE.SEQUENTIAL is
 * clear (and GRC.MODE.EVENODD is set), turning address bit 0 (A0) into a "plane select" bit.  Whether A0 is also
 * used as a memory address bit depends on CRTC.MODE_CTRL.BYTE_MODE: if it's set, then we're in "Byte Mode" and A0 is
 * used as-is; if it's clear, then we're in "Word Mode", and either A15 (when CRTC.MODE_CTRL.ADDR_WRAP is set) or A13
 * (when CRTC.MODE_CTRL.ADDR_WRAP is clear, typically when only 64Kb of EGA memory is installed) is substituted for A0.
 *
 * Note that A13 remains clear until addresses reach 8K, at which point we've spanned 32Kb of EGA memory, so it makes
 * sense to propagate A13 to A0 at that point, so that the next 8K of addresses start using ODD instead of EVEN bytes,
 * and no memory is wasted on a 64Kb EGA card.
 *
 * These functions, however, don't yet deal with all those subtleties: A0 is currently used only as a "plane select"
 * bit and set to zero for addressing purposes, meaning that only the EVEN bytes in EGA memory will ever be used.
 * TODO: Implement the subtleties.
 */

/*
 * Values returned by getCardAccess(); the high byte describes the read mode, and the low byte describes the write mode.
 *
 * V2 should never appear in any values used by getCardAccess() or setCardAccess(); the sole purpose of V2 is to
 * distinguish newer (V2) access values from older (V1) access values in saved contexts.  It's set when the context
 * is saved, and cleared when the context is restored.  Thus, if V2 is not set on restore, we assume we're dealing with
 * a V1 value, so we run it through the V1 table (below) to produce a V2 value.  Hopefully at some point V1 contexts
 * can be deprecated, and the V2 bit can be eliminated/repurposed.
 */
Card.ACCESS = {
    READ: {                             // READ values are designed to be OR'ed with WRITE values
        MODE0:              0x0400,
        MODE1:              0x0500,
        EVENODD:            0x1000,
        CHAIN4:             0x4000,
        MASK:               0xFF00
    },
    WRITE: {                            // and WRITE values are designed to be OR'ed with READ values
        MODE0:              0x0000,
        MODE1:              0x0001,
        MODE2:              0x0002,
        MODE3:              0x0003,     // VGA only
        CHAIN4:             0x0004,
        EVENODD:            0x0010,
        ROT:                0x0020,
        AND:                0x0060,
        OR:                 0x00A0,
        XOR:                0x00E0,
        MASK:               0x00FF
    },
    V2:             (0x80000000|0)      // this is a signature bit used ONLY to differentiate V2 access values from V1
};

/*
 * Table of older (V1) access values and their corresponding new values; the new values are similar but more orthogonal
 */
Card.ACCESS.V1 = [];
Card.ACCESS.V1[0x0002] = Card.ACCESS.READ.MODE0;
Card.ACCESS.V1[0x0003] = Card.ACCESS.READ.MODE0  | Card.ACCESS.READ.EVENODD;
Card.ACCESS.V1[0x0010] = Card.ACCESS.READ.MODE1;
Card.ACCESS.V1[0x0200] = Card.ACCESS.WRITE.MODE0;
Card.ACCESS.V1[0x0400] = Card.ACCESS.WRITE.MODE0 | Card.ACCESS.WRITE.ROT;
Card.ACCESS.V1[0x0600] = Card.ACCESS.WRITE.MODE0 | Card.ACCESS.WRITE.AND;
Card.ACCESS.V1[0x0A00] = Card.ACCESS.WRITE.MODE0 | Card.ACCESS.WRITE.OR;
Card.ACCESS.V1[0x0E00] = Card.ACCESS.WRITE.MODE0 | Card.ACCESS.WRITE.XOR;
Card.ACCESS.V1[0x0300] = Card.ACCESS.WRITE.MODE0 | Card.ACCESS.WRITE.EVENODD;
Card.ACCESS.V1[0x1000] = Card.ACCESS.WRITE.MODE1;
Card.ACCESS.V1[0x2000] = Card.ACCESS.WRITE.MODE2;
Card.ACCESS.V1[0x6000] = Card.ACCESS.WRITE.MODE2 | Card.ACCESS.WRITE.AND;
Card.ACCESS.V1[0xA000] = Card.ACCESS.WRITE.MODE2 | Card.ACCESS.WRITE.OR;
Card.ACCESS.V1[0xE000] = Card.ACCESS.WRITE.MODE2 | Card.ACCESS.WRITE.XOR;

/**
 * readByteMode0(off, addr)
 *
 * @this {Memory}
 * @param {number} off
 * @param {number} [addr]
 * @return {number}
 */
Card.ACCESS.readByteMode0 = function readByteMode0(off, addr)
{
    off += this.offset;
    var dw = this.controller.latches = this.adw[off];
    return (dw >> this.controller.nReadMapShift) & 0xff;
};

/**
 * readByteMode0Chain4(off, addr)
 *
 * See writeByteMode0Chain4 for a description of how writes are distributed across planes.
 *
 * @this {Memory}
 * @param {number} off
 * @param {number} [addr]
 * @return {number}
 */
Card.ACCESS.readByteMode0Chain4 = function readByteMode0Chain4(off, addr)
{
    var idw = (off & ~0x3) + this.offset;
    var shift = (off & 0x3) << 3;
    return ((this.controller.latches = this.adw[idw]) >> shift) & 0xff;
};

/**
 * readByteMode0EvenOdd(off, addr)
 *
 * @this {Memory}
 * @param {number} off
 * @param {number} [addr]
 * @return {number}
 */
Card.ACCESS.readByteMode0EvenOdd = function readByteMode0EvenOdd(off, addr)
{
    /*
     * TODO: As discussed in getCardAccess(), we need to run some tests on real EGA/VGA hardware to determine
     * exactly what gets latched (ie, from which address) when EVENODD is in effect.  Whatever we learn may
     * also dictate a special EVENODD function for READ.MODE1 as well.
     */
    off += this.offset;
    var idw = off & ~0x1;
    var dw = this.controller.latches = this.adw[idw];
    return (!(off & 1)? dw : (dw >> 8)) & 0xff;
};

/**
 * readByteMode1(off, addr)
 *
 * This mode requires us to step through each of the 8 sets of 4 bits in the specified DWORD of video memory,
 * returning a 1 wherever all 4 match the Color Compare (COLORCMP) Register and a 0 otherwise.  An added wrinkle
 * is that the Color Don't Care (COLORDC) Register can specify that any/all/none of the 4 bits must be ignored.
 *
 * We perform the comparison from most to least significant bit, because that matches how the nColorCompare and
 * nColorDontCare masks are initialized; we could have gone either way, but this is more consistent with the rest
 * of the component (eg, pixels are drawn across the screen from left to right, starting with the most significant
 * bit of each byte).
 *
 * Also note that, while not well-documented, this mode also affects the internal latches, so we make sure those
 * are updated as well.
 *
 * @this {Memory}
 * @param {number} off
 * @param {number} [addr]
 * @return {number}
 */
Card.ACCESS.readByteMode1 = function readByteMode1(off, addr)
{
    off += this.offset;
    var dw = this.controller.latches = this.adw[off];
    /*
     * Minor optimization: we could pre-mask nColorCompare with nColorDontCare, whenever either register
     * is updated, but that's a drop in the bucket compared to all the other work this function must do.
     */
    var mask = this.controller.nColorDontCare;
    var color = this.controller.nColorCompare & mask;
    var b = 0, bit = 0x80;
    while (bit) {
        if ((dw & mask) == color) b |= bit;
        color >>>= 1;  mask >>>= 1;  bit >>= 1;
    }
    return b;
};

/**
 * writeByteMode0(off, b, addr)
 *
 * Supporting Set/Reset means that for every plane for which Set/Reset is enabled, we must
 * replace the corresponding byte in dw with a byte of zeros or ones.  This is accomplished with
 * nSetMapMask, nSetMapData, and nSetMapBits.  nSetMapMask is the inverse of the ESRESET bits,
 * because we use it to mask the processor data, nSetMapData records the desired SRESET bits,
 * and nSetMapBits contains the bits to replace those that we masked in the processor data.
 *
 * We could have done this:
 *
 *      dw = (dw & this.controller.nSetMapMask) | (this.controller.nSetMapData & ~this.controller.nSetMapMask)
 *
 * but by maintaining nSetMapBits equal to (nSetMapData & ~nSetMapMask), we are able to make
 * the writes slightly more efficient.
 *
 * @this {Memory}
 * @param {number} off
 * @param {number} b (which should already be pre-masked to 8 bits; see cpu.setByte())
 * @param {number} [addr]
 */
Card.ACCESS.writeByteMode0 = function writeByteMode0(off, b, addr)
{
    var idw = off + this.offset;
    var dw = b | (b << 8) | (b << 16) | (b << 24);
    dw = (dw & this.controller.nSetMapMask) | this.controller.nSetMapBits;
    dw = (dw & this.controller.nBitMapMask) | (this.controller.latches & ~this.controller.nBitMapMask);
    dw = (dw & this.controller.nSeqMapMask) | (this.adw[idw] & ~this.controller.nSeqMapMask);
    if (this.adw[idw] != dw) {
        this.adw[idw] = dw;
        this.fDirty = true;
    }
    if (DEBUG && this.controller.video.messageEnabled(Messages.MEM | Messages.VIDEO)) {
        this.controller.video.printMessage("writeByteMode0(" + Str.toHexLong(addr) + "): " + Str.toHexByte(b) + " -> " + Str.toHexLong(dw));
    }
};

/**
 * writeByteMode0Chain4(off, b, addr)
 *
 * This is how we distribute writes of 0xff across the address space to the planes (assuming that all
 * planes are enabled by the Sequencer's MAPMASK register):
 *
 *      off     idw     adw[idw]
 *      ------  ------  ----------
 *      0x0000: 0x0000  0x000000ff
 *      0x0001: 0x0000  0x0000ff00
 *      0x0002: 0x0000  0x00ff0000
 *      0x0003: 0x0000  0xff000000
 *      0x0004: 0x0004  0x000000ff
 *      0x0005: 0x0004  0x0000ff00
 *      0x0006: 0x0004  0x00ff0000
 *      0x0007: 0x0004  0xff000000
 *      ...
 *
 * Some VGA emulations calculate the video buffer index (idw) by shifting the offset (off) right 2 bits,
 * instead of simply masking off the low 2 bits, as we do here.  That would be a more "pleasing" arrangement,
 * because we would be using sequential video buffer locations, instead of multiples of 4, and would match how
 * pixels are stored in "Mode X".  However, I don't think that's how CHAIN4 modes operate (although that still
 * needs to be confirmed, because multiple sources conflict on this point).  TODO: Confirm CHAIN4 operation on
 * actual VGA hardware, including the extent to which ALU and other writeByteMode0() functionality needs to
 * be folded into this.
 *
 * Address decoding may not matter that much, as long as both the read and write CHAIN4 functions decode their
 * addresses in exactly the same manner; we'd only get into trouble with software that "unchained" or otherwise
 * reconfigured the planes and then made assumptions about existing data in the video buffer.
 *
 * @this {Memory}
 * @param {number} off
 * @param {number} b (which should already be pre-masked to 8 bits; see cpu.setByte())
 * @param {number} [addr]
 */
Card.ACCESS.writeByteMode0Chain4 = function writeByteMode0Chain4(off, b, addr)
{
    var idw = (off & ~0x3) + this.offset;
    var shift = (off & 0x3) << 3;
    /*
     * TODO: Consider adding a separate "unmasked" version of this CHAIN4 write function when nSeqMapMask is -1
     * (or removing nSeqMapMask from the equation altogether, if CHAIN4 is never used with any planes disabled).
     */
    var dw = ((b << shift) & this.controller.nSeqMapMask) | (this.adw[idw] & ~((0xff << shift) & this.controller.nSeqMapMask));
    if (this.adw[idw] != dw) {
        this.adw[idw] = dw;
        this.fDirty = true;
    }
    if (DEBUG && this.controller.video.messageEnabled(Messages.MEM | Messages.VIDEO)) {
        this.controller.video.printMessage("writeByteMode0Chain4(" + Str.toHexLong(addr) + "): " + Str.toHexByte(b) + " -> " + Str.toHexLong(dw));
    }
};

/**
 * writeByteMode0EvenOdd(off, b, addr)
 *
 * @this {Memory}
 * @param {number} off
 * @param {number} b (which should already be pre-masked to 8 bits; see cpu.setByte())
 * @param {number} [addr]
 */
Card.ACCESS.writeByteMode0EvenOdd = function writeByteMode0EvenOdd(off, b, addr)
{
    off += this.offset;
    var dw = b | (b << 8) | (b << 16) | (b << 24);
    /*
     * When even/odd addressing is enabled, nSeqMapMask must be cleared for planes 1
     * and 3 if the address is even, and cleared for planes 0 and 2 if the address is odd.
     */
    var idw = off & ~0x1;
    dw = (dw & this.controller.nBitMapMask) | (this.controller.latches & ~this.controller.nBitMapMask);
    var maskMaps = this.controller.nSeqMapMask & (idw == off? 0x00ff00ff : (0xff00ff00|0));
    dw = (dw & maskMaps) | (this.adw[idw] & ~maskMaps);
    if (this.adw[idw] != dw) {
        this.adw[idw] = dw;
        this.fDirty = true;
    }
    if (DEBUG && this.controller.video.messageEnabled(Messages.MEM | Messages.VIDEO)) {
        this.controller.video.printMessage("writeByteMode0EvenOdd(" + Str.toHexLong(addr) + "): " + Str.toHexByte(b) + " -> " + Str.toHexLong(dw));
    }
};

/**
 * writeByteMode0Rot(off, b, addr)
 *
 * @this {Memory}
 * @param {number} off
 * @param {number} b (which should already be pre-masked to 8 bits; see cpu.setByte())
 * @param {number} [addr]
 */
Card.ACCESS.writeByteMode0Rot = function writeByteMode0Rot(off, b, addr)
{
    var idw = off + this.offset;
    b = ((b >> this.controller.nDataRotate) | (b << (8 - this.controller.nDataRotate)) & 0xff);
    var dw = b | (b << 8) | (b << 16) | (b << 24);
    dw = (dw & this.controller.nSetMapMask) | this.controller.nSetMapBits;
    dw = (dw & this.controller.nBitMapMask) | (this.controller.latches & ~this.controller.nBitMapMask);
    dw = (dw & this.controller.nSeqMapMask) | (this.adw[idw] & ~this.controller.nSeqMapMask);
    if (this.adw[idw] != dw) {
        this.adw[idw] = dw;
        this.fDirty = true;
    }
    if (DEBUG && this.controller.video.messageEnabled(Messages.MEM | Messages.VIDEO)) {
        this.controller.video.printMessage("writeByteMode0Rot(" + Str.toHexLong(addr) + "): " + Str.toHexByte(b) + " -> " + Str.toHexLong(dw));
    }
};

/**
 * writeByteMode0And(off, b, addr)
 *
 * @this {Memory}
 * @param {number} off
 * @param {number} b (which should already be pre-masked to 8 bits; see cpu.setByte())
 * @param {number} [addr]
 */
Card.ACCESS.writeByteMode0And = function writeByteMode0And(off, b, addr)
{
    var idw = off + this.offset;
    b = ((b >> this.controller.nDataRotate) | (b << (8 - this.controller.nDataRotate)) & 0xff);
    var dw = b | (b << 8) | (b << 16) | (b << 24);
    dw = (dw & this.controller.nSetMapMask) | this.controller.nSetMapBits;
    dw &= this.controller.latches;
    dw = (dw & this.controller.nBitMapMask) | (this.controller.latches & ~this.controller.nBitMapMask);
    dw = (dw & this.controller.nSeqMapMask) | (this.adw[idw] & ~this.controller.nSeqMapMask);
    if (this.adw[idw] != dw) {
        this.adw[idw] = dw;
        this.fDirty = true;
    }
    if (DEBUG && this.controller.video.messageEnabled(Messages.MEM | Messages.VIDEO)) {
        this.controller.video.printMessage("writeByteMode0And(" + Str.toHexLong(addr) + "): " + Str.toHexByte(b) + " -> " + Str.toHexLong(dw));
    }
};

/**
 * writeByteMode0Or(off, b, addr)
 *
 * @this {Memory}
 * @param {number} off
 * @param {number} b (which should already be pre-masked to 8 bits; see cpu.setByte())
 * @param {number} [addr]
 */
Card.ACCESS.writeByteMode0Or = function writeByteMode0Or(off, b, addr)
{
    var idw = off + this.offset;
    b = ((b >> this.controller.nDataRotate) | (b << (8 - this.controller.nDataRotate)) & 0xff);
    var dw = b | (b << 8) | (b << 16) | (b << 24);
    dw = (dw & this.controller.nSetMapMask) | this.controller.nSetMapBits;
    dw |= this.controller.latches;
    dw = (dw & this.controller.nBitMapMask) | (this.controller.latches & ~this.controller.nBitMapMask);
    dw = (dw & this.controller.nSeqMapMask) | (this.adw[idw] & ~this.controller.nSeqMapMask);
    if (this.adw[idw] != dw) {
        this.adw[idw] = dw;
        this.fDirty = true;
    }
    if (DEBUG && this.controller.video.messageEnabled(Messages.MEM | Messages.VIDEO)) {
        this.controller.video.printMessage("writeByteMode0Or(" + Str.toHexLong(addr) + "): " + Str.toHexByte(b) + " -> " + Str.toHexLong(dw));
    }
};

/**
 * writeByteMode0Xor(off, b, addr)
 *
 * @this {Memory}
 * @param {number} off
 * @param {number} b (which should already be pre-masked to 8 bits; see cpu.setByte())
 * @param {number} [addr]
 */
Card.ACCESS.writeByteMode0Xor = function writeByteMode0Xor(off, b, addr)
{
    var idw = off + this.offset;
    b = ((b >> this.controller.nDataRotate) | (b << (8 - this.controller.nDataRotate)) & 0xff);
    var dw = b | (b << 8) | (b << 16) | (b << 24);
    dw = (dw & this.controller.nSetMapMask) | this.controller.nSetMapBits;
    dw ^= this.controller.latches;
    dw = (dw & this.controller.nBitMapMask) | (this.controller.latches & ~this.controller.nBitMapMask);
    dw = (dw & this.controller.nSeqMapMask) | (this.adw[idw] & ~this.controller.nSeqMapMask);
    if (this.adw[idw] != dw) {
        this.adw[idw] = dw;
        this.fDirty = true;
    }
    if (DEBUG && this.controller.video.messageEnabled(Messages.MEM | Messages.VIDEO)) {
        this.controller.video.printMessage("writeByteMode0Xor(" + Str.toHexLong(addr) + "): " + Str.toHexByte(b) + " -> " + Str.toHexLong(dw));
    }
};

/**
 * writeByteMode1(off, b, addr)
 *
 * @this {Memory}
 * @param {number} off
 * @param {number} b (ignored; the EGA latches provide the source data)
 * @param {number} [addr]
 */
Card.ACCESS.writeByteMode1 = function writeByteMode1(off, b, addr)
{
    var idw = off + this.offset;
    var dw = (this.adw[idw] & ~this.controller.nSeqMapMask) | (this.controller.latches & this.controller.nSeqMapMask);
    if (this.adw[idw] != dw) {
        this.adw[idw] = dw;
        this.fDirty = true;
    }
    if (DEBUG && this.controller.video.messageEnabled(Messages.MEM | Messages.VIDEO)) {
        this.controller.video.printMessage("writeByteMode1(" + Str.toHexLong(addr) + "): " + Str.toHexLong(dw));
    }
};

/**
 * writeByteMode1EvenOdd(off, b, addr)
 *
 * @this {Memory}
 * @param {number} off
 * @param {number} b (ignored; the EGA latches provide the source data)
 * @param {number} [addr]
 */
Card.ACCESS.writeByteMode1EvenOdd = function writeByteMode1EvenOdd(off, b, addr)
{
    /*
     * TODO: As discussed in getCardAccess(), we need to run some tests on real EGA/VGA hardware to
     * determine exactly where latches are written (ie, to which address) when EVENODD is in effect.
     */
    off += this.offset;
    //
    // When even/odd addressing is enabled, nSeqMapMask must be cleared for planes 1 and 3 if
    // the address is even, and cleared for planes 0 and 2 if the address is odd.
    //
    var idw = off & ~0x1;
    var maskMaps = this.controller.nSeqMapMask & (idw == off? 0x00ff00ff : (0xff00ff00|0));
    var dw = (this.adw[idw] & ~maskMaps) | (this.controller.latches & maskMaps);
    if (this.adw[idw] != dw) {
        this.adw[idw] = dw;
        this.fDirty = true;
    }
    if (DEBUG && this.controller.video.messageEnabled(Messages.MEM | Messages.VIDEO)) {
        this.controller.video.printMessage("writeByteMode1EvenOdd(" + Str.toHexLong(addr) + "): " + Str.toHexByte(dw));
    }
};

/**
 * writeByteMode2(off, b, addr)
 *
 * @this {Memory}
 * @param {number} off
 * @param {number} b (which should already be pre-masked to 8 bits; see cpu.setByte())
 * @param {number} [addr]
 */
Card.ACCESS.writeByteMode2 = function writeByteMode2(off, b, addr)
{
    var idw = off + this.offset;
    var dw = Video.aEGAByteToDW[b & 0xf];
    dw = (dw & this.controller.nBitMapMask) | (this.controller.latches & ~this.controller.nBitMapMask);
    dw = (dw & this.controller.nSeqMapMask) | (this.adw[idw] & ~this.controller.nSeqMapMask);
    if (this.adw[idw] != dw) {
        this.adw[idw] = dw;
        this.fDirty = true;
    }
    if (DEBUG && this.controller.video.messageEnabled(Messages.MEM | Messages.VIDEO)) {
        this.controller.video.printMessage("writeByteMode2(" + Str.toHexLong(addr) + "): " + Str.toHexByte(b) + " -> " + Str.toHexLong(dw));
    }
};

/**
 * writeByteMode2And(off, b, addr)
 *
 * @this {Memory}
 * @param {number} off
 * @param {number} b (which should already be pre-masked to 8 bits; see cpu.setByte())
 * @param {number} [addr]
 */
Card.ACCESS.writeByteMode2And = function writeByteMode2And(off, b, addr)
{
    var idw = off + this.offset;
    var dw = Video.aEGAByteToDW[b & 0xf];
    dw &= this.controller.latches;
    dw = (dw & this.controller.nBitMapMask) | (this.controller.latches & ~this.controller.nBitMapMask);
    dw = (dw & this.controller.nSeqMapMask) | (this.adw[idw] & ~this.controller.nSeqMapMask);
    if (this.adw[idw] != dw) {
        this.adw[idw] = dw;
        this.fDirty = true;
    }
    if (DEBUG && this.controller.video.messageEnabled(Messages.MEM | Messages.VIDEO)) {
        this.controller.video.printMessage("writeByteMode2And(" + Str.toHexLong(addr) + "): " + Str.toHexByte(b) + " -> " + Str.toHexLong(dw));
    }
};

/**
 * writeByteMode2Or(off, b, addr)
 *
 * @this {Memory}
 * @param {number} off
 * @param {number} b (which should already be pre-masked to 8 bits; see cpu.setByte())
 * @param {number} [addr]
 */
Card.ACCESS.writeByteMode2Or = function writeByteMode2Or(off, b, addr)
{
    var idw = off + this.offset;
    var dw = Video.aEGAByteToDW[b & 0xf];
    dw |= this.controller.latches;
    dw = (dw & this.controller.nBitMapMask) | (this.controller.latches & ~this.controller.nBitMapMask);
    dw = (dw & this.controller.nSeqMapMask) | (this.adw[idw] & ~this.controller.nSeqMapMask);
    if (this.adw[idw] != dw) {
        this.adw[idw] = dw;
        this.fDirty = true;
    }
    if (DEBUG && this.controller.video.messageEnabled(Messages.MEM | Messages.VIDEO)) {
        this.controller.video.printMessage("writeByteMode2Or(" + Str.toHexLong(addr) + "): " + Str.toHexByte(b) + " -> " + Str.toHexLong(dw));
    }
};

/**
 * writeByteMode2Xor(off, b, addr)
 *
 * @this {Memory}
 * @param {number} off
 * @param {number} b (which should already be pre-masked to 8 bits; see cpu.setByte())
 * @param {number} [addr]
 */
Card.ACCESS.writeByteMode2Xor = function writeByteMode2Xor(off, b, addr)
{
    var idw = off + this.offset;
    var dw = Video.aEGAByteToDW[b & 0xf];
    dw ^= this.controller.latches;
    dw = (dw & this.controller.nBitMapMask) | (this.controller.latches & ~this.controller.nBitMapMask);
    dw = (dw & this.controller.nSeqMapMask) | (this.adw[idw] & ~this.controller.nSeqMapMask);
    if (this.adw[idw] != dw) {
        this.adw[idw] = dw;
        this.fDirty = true;
    }
    if (DEBUG && this.controller.video.messageEnabled(Messages.MEM | Messages.VIDEO)) {
        this.controller.video.printMessage("writeByteMode2Xor(" + Str.toHexLong(addr) + "): " + Str.toHexByte(b) + " -> " + Str.toHexLong(dw));
    }
};

/**
 * writeByteMode3(off, b, addr)
 *
 * In MODE3, Set/Reset is always enabled, so the ESRESET bits (and therefore nSetMapMask and nSetMapBits)
 * are ignored; we look only at the SRESET bits, which are stored in nSetMapData.
 *
 * Unlike MODE0, we currently have no non-rotate function for MODE3.  If performance dictates, we can add one;
 * ditto for other features like the Sequencer's MAPMASK register (nSeqMapMask).
 *
 * @this {Memory}
 * @param {number} off
 * @param {number} b (which should already be pre-masked to 8 bits; see cpu.setByte())
 * @param {number} [addr]
 */
Card.ACCESS.writeByteMode3 = function writeByteMode3(off, b, addr)
{
    var idw = off + this.offset;
    b = ((b >> this.controller.nDataRotate) | (b << (8 - this.controller.nDataRotate)) & 0xff);
    var dw = b | (b << 8) | (b << 16) | (b << 24);
    var dwMask = (dw & this.controller.nBitMapMask);
    dw = (this.controller.nSetMapData & dwMask) | (this.controller.latches & ~dwMask);
    dw = (dw & this.controller.nSeqMapMask) | (this.adw[idw] & ~this.controller.nSeqMapMask);
    if (this.adw[idw] != dw) {
        this.adw[idw] = dw;
        this.fDirty = true;
    }
    if (DEBUG && this.controller.video.messageEnabled(Messages.MEM | Messages.VIDEO)) {
        this.controller.video.printMessage("writeByteMode3(" + Str.toHexLong(addr) + "): " + Str.toHexByte(b) + " -> " + Str.toHexLong(dw));
    }
};

/*
 * Mappings from getCardAccess() values to access functions above
 */
Card.ACCESS.afn = [];

Card.ACCESS.afn[Card.ACCESS.READ.MODE0]  = Card.ACCESS.readByteMode0;
Card.ACCESS.afn[Card.ACCESS.READ.MODE0  |  Card.ACCESS.READ.CHAIN4]  = Card.ACCESS.readByteMode0Chain4;
Card.ACCESS.afn[Card.ACCESS.READ.MODE0  |  Card.ACCESS.READ.EVENODD] = Card.ACCESS.readByteMode0EvenOdd;
Card.ACCESS.afn[Card.ACCESS.READ.MODE1]  = Card.ACCESS.readByteMode1;

Card.ACCESS.afn[Card.ACCESS.WRITE.MODE0] = Card.ACCESS.writeByteMode0;
Card.ACCESS.afn[Card.ACCESS.WRITE.MODE0 |  Card.ACCESS.WRITE.ROT] = Card.ACCESS.writeByteMode0Rot;
Card.ACCESS.afn[Card.ACCESS.WRITE.MODE0 |  Card.ACCESS.WRITE.AND] = Card.ACCESS.writeByteMode0And;
Card.ACCESS.afn[Card.ACCESS.WRITE.MODE0 |  Card.ACCESS.WRITE.OR]  = Card.ACCESS.writeByteMode0Or;
Card.ACCESS.afn[Card.ACCESS.WRITE.MODE0 |  Card.ACCESS.WRITE.XOR] = Card.ACCESS.writeByteMode0Xor;
Card.ACCESS.afn[Card.ACCESS.WRITE.MODE0 |  Card.ACCESS.WRITE.CHAIN4]  = Card.ACCESS.writeByteMode0Chain4;
Card.ACCESS.afn[Card.ACCESS.WRITE.MODE0 |  Card.ACCESS.WRITE.EVENODD] = Card.ACCESS.writeByteMode0EvenOdd;
Card.ACCESS.afn[Card.ACCESS.WRITE.MODE1] = Card.ACCESS.writeByteMode1;
Card.ACCESS.afn[Card.ACCESS.WRITE.MODE1 |  Card.ACCESS.WRITE.EVENODD] = Card.ACCESS.writeByteMode1EvenOdd;
Card.ACCESS.afn[Card.ACCESS.WRITE.MODE2] = Card.ACCESS.writeByteMode2;
Card.ACCESS.afn[Card.ACCESS.WRITE.MODE2 |  Card.ACCESS.WRITE.AND] = Card.ACCESS.writeByteMode2And;
Card.ACCESS.afn[Card.ACCESS.WRITE.MODE2 |  Card.ACCESS.WRITE.OR]  = Card.ACCESS.writeByteMode2Or;
Card.ACCESS.afn[Card.ACCESS.WRITE.MODE2 |  Card.ACCESS.WRITE.XOR] = Card.ACCESS.writeByteMode2Xor;
Card.ACCESS.afn[Card.ACCESS.WRITE.MODE3] = Card.ACCESS.writeByteMode3;

/**
 * TODO: The Closure Compiler treats ES6 classes as 'struct' rather than 'dict' by default,
 * which would force us to declare all class properties in the constructor, as well as prevent
 * us from defining any named properties.  So, for now, we mark all our classes as 'unrestricted'.
 *
 * @unrestricted
 */
class Video extends Component {
    /**
     * Video(parmsVideo, canvas, context, textarea, container)
     *
     * The Video component can be configured with the following (parmsVideo) properties:
     *
     *      model: model (eg, "mda" for Monochrome Display Adapter)
     *      mode: initial video mode (default is null, which selects a mode based on model)
     *      screenWidth: width of the screen canvas, in pixels
     *      screenHeight: height of the screen canvas, in pixels
     *      screenColor: background color of the screen canvas (default is black)
     *      scale: true for font scaling, false (default) to center the display on the screen
     *      charCols: number of character columns
     *      charRows: number of character rows
     *      fontROM: path to .rom file (or a JSON representation) containing the character set
     *      touchScreen: string specifying desired touch-screen support (default is none)
     *      autoLock: true to (attempt to) auto-lock the mouse to the canvas (default is false)
     *
     * An EGA/VGA may specify the following additional properties:
     *
     *      switches: string representing EGA switches (see "SW1-SW4" documentation below)
     *      memory: the size of the EGA's on-board memory (overrides EGA's Video.cardSpecs)
     *
     * This calls the Bus to allocate a video buffer at the appropriate memory location whenever
     * a reset() or setMode() occurs; setMode() is called whenever a mode change is detected at
     * the port level, and whenever reset() is called.  setMode() also invokes updateScreen(true),
     * which forces reallocation of our internal buffer (aCellCache) that mirrors the video buffer.
     *
     * The CPU periodically calls updateVideo(), which in turn calls updateScreen() for each Video
     * instance.  These updates should occur at a rate of 60 times/second, to update any blinking
     * elements (the cursor and any cells with the blink attribute), to compare/update the contents
     * of our internal buffer with the video buffer, and to render any differences between the two
     * buffers into the associated screen canvas, via either updateChar() or setPixel().
     *
     * Thanks to the Bus' new block-based memory manager that allows us to sparse-allocate memory
     * (in 4Kb increments on 20-bit buses, 16Kb increments on 24-bit buses), updateScreen()
     * can also ask the CPU for the "dirty" state of all the blocks underlying the video buffer,
     * bypassing the update completely if the buffer is still clean.
     *
     * Sadly, that optimization is defeated if the count of active blink elements is non-zero,
     * because we must rescan the entire buffer to locate and redraw them all; I'm assuming for now
     * that, more often than not, very few (if any) blink attributes will be present, and therefore
     * they're not worth a separate caching mechanism.  If the only blinking element is the cursor,
     * that's no problem, as we redraw only the one cell containing the cursor (assuming the buffer
     * is otherwise clean).
     *
     * @this {Video}
     * @param {Object} parmsVideo
     * @param {Object} [canvas]
     * @param {Object} [context]
     * @param {Object} [textarea]
     * @param {Object} [container]
     */
    constructor(parmsVideo, canvas, context, textarea, container)
    {
        super("Video", parmsVideo, Messages.VIDEO);

        var video = this;
        this.fGecko = Web.isUserAgent("Gecko/");
        var i, sEvent, asWebPrefixes = ['', 'moz', 'ms', 'webkit'];

        /*
         * This records the model specified (eg, "mda", "cga", "ega", "vga" or "" if none specified);
         * when a model is specified, it overrides whatever model we infer from the ChipSet's switches
         * (since those motherboard switches tell us only the type of monitor, not the type of card).
         */
        this.model = parmsVideo['model'];
        var aModelDefaults = Video.MODEL[this.model] || Video.MODEL['mda'];

        this.nCard = aModelDefaults[0];
        this.cbMemory = parmsVideo['memory'] || 0;  // zero means fallback to the cardSpec's default size
        this.sSwitches = parmsVideo['switches'];

        /*
         * powerUp() uses the default mode ONLY if ChipSet doesn't give us a default.
         */
        this.nModeDefault = parmsVideo['mode'];
        if (this.nModeDefault == null || Video.aModeParms[this.nModeDefault] == null) {
            this.nModeDefault = aModelDefaults[1];
        }

        /*
         * setDimensions() uses these values ONLY if it doesn't recognize the video mode.
         */
        this.nColsDefault = parmsVideo['charCols'];
        this.nRowsDefault = parmsVideo['charRows'];
        if (this.nColsDefault === undefined || this.nRowsDefault === undefined) {
            this.nColsDefault = Video.aModeParms[this.nModeDefault][0];
            this.nRowsDefault = Video.aModeParms[this.nModeDefault][1];
        }

        /*
         * setDimensions() uses these values unconditionally, as the machine has no idea what the
         * physical screen size should be.
         */
        this.cxScreen = parmsVideo['screenWidth'];
        this.cyScreen = parmsVideo['screenHeight'];

        /*
         * We might consider another component parameter to specify the font-doubling setting.
         * For now, it's based on whether the default SCREEN cell size is sufficiently larger than
         * the default FONT cell size.
         */
        this.fScaleFont = parmsVideo['scale'];
        this.fDoubleFont = Math.round(this.cxScreen / this.nColsDefault) >= 12;

        this.canvasScreen = canvas;
        this.contextScreen = context;
        this.textareaScreen = textarea;
        this.inputScreen = textarea || canvas || null;

        /*
         * Support for disabling (or, less commonly, enabling) image smoothing, which all browsers
         * seem to support now (well, OK, I still have to test the latest MS Edge browser), despite
         * it still being labelled "experimental technology".  Let's hope the browsers standardize
         * on this.  I see other options emerging, like the CSS property "image-rendering: pixelated"
         * that's apparently been added to Chrome.  Sigh.
         */
        var fSmoothing = parmsVideo['smoothing'];
        var sSmoothing = Web.getURLParm('smoothing');
        if (sSmoothing) fSmoothing = (sSmoothing == "true");
        if (fSmoothing != null) {
            for (i = 0; i < asWebPrefixes.length; i++) {
                sEvent = asWebPrefixes[i];
                if (!sEvent) {
                    sEvent = 'imageSmoothingEnabled';
                } else {
                    sEvent += 'ImageSmoothingEnabled';
                }
                if (this.contextScreen[sEvent] !== undefined) {
                    this.contextScreen[sEvent] = fSmoothing;
                    break;
                }
            }
        }

        /*
         * initBus() will determine touch-screen support; for now, just record values and set defaults.
         */
        this.sTouchScreen = parmsVideo['touchScreen'];
        this.nTouchConfig = Video.TOUCH.NONE;

        /*
         * If a Mouse exists, we'll be notified when it requests our canvas, and we make a note of it
         * so that if lockPointer() is ever invoked, we can notify the Mouse.
         */
        this.mouse = null;
        this.fAutoLock = parmsVideo['autoLock'];

        /*
         * Originally, setMode() would map/unmap the video buffer ONLY when the active card changed,
         * because as long as an MDA or CGA remained active, its video buffer never changed.  However,
         * since the EGA can change its video buffer on the fly, setMode() must also compare the card's
         * hard-coded and/or programmed buffer address/size to the "active" address/size; the latter
         * is recorded here.
         */
        this.addrBuffer = this.sizeBuffer = 0;

        /*
         * aFonts is an array of font objects indexed by FONT ID.  Font characters are arranged
         * in 16x16 grids, with one grid per canvas object in the aCanvas array of each font object.
         *
         * Each element is a Font object that describes the font size and provides bitmaps for all the font
         * color permutations.  aFonts.length will be non-zero if ANY fonts are loaded, but do NOT assume
         * that EVERY font has been loaded; check for the existence of a font by checking for its unique ID
         * within this sparse array.
         */
        this.aFonts = [];

        /*
         * Instead of (re)allocating a new color array every time getCardColors() is called, we preallocate
         * an array and simply update the entries as needed.  Note that for an EGA (or a VGA operating in an
         * EGA-compatible mode), only the first 16 entries get used (derived from the ATC); only when a VGA
         * is operating in an 8bpp mode are 256 entries used (derived from the DAC rather than the ATC).
         */
        this.aRGB = new Array(this.nCard == Video.CARD.VGA? 256 : 16);
        this.fRGBValid = false;     // whenever this is false, it signals getCardColors() to rebuild aRGB

        /*
         * Since I've not found clear documentation on a reliable way to check whether a particular DOM element
         * (other than the BODY element) has focus at any given time, I've added onfocus() and onblur() handlers
         * to the screen to maintain my own focus state.
         */
        this.fHasFocus = false;

        /*
         * Here's the gross code to handle full-screen support across all supported browsers.  The lack of standards
         * is exasperating; browsers can't agree on 'full' or 'Full, 'request' or 'Request', 'screen' or 'Screen', and
         * while some browsers honor other browser prefixes, most browsers don't.
         */
        this.container = container;
        if (this.container) {
            this.container.doFullScreen = container['requestFullscreen'] || container['msRequestFullscreen'] || container['mozRequestFullScreen'] || container['webkitRequestFullscreen'];
            if (this.container.doFullScreen) {
                for (i = 0; i < asWebPrefixes.length; i++) {
                    sEvent = asWebPrefixes[i] + 'fullscreenchange';
                    if ('on' + sEvent in document) {
                        var onFullScreenChange = function() {
                            var fFullScreen = (document['fullscreenElement'] || document['msFullscreenElement'] || document['mozFullScreenElement'] || document['webkitFullscreenElement']);
                            video.notifyFullScreen(!!fFullScreen);
                        };
                        document.addEventListener(sEvent, onFullScreenChange, false);
                        break;
                    }
                }
                for (i = 0; i < asWebPrefixes.length; i++) {
                    sEvent = asWebPrefixes[i] + 'fullscreenerror';
                    if ('on' + sEvent in document) {
                        var onFullScreenError = function() {
                            video.notifyFullScreen(null);
                        };
                        document.addEventListener(sEvent, onFullScreenError, false);
                        break;
                    }
                }
            }
        }

        /*
         * More gross code to handle pointer-locking support across all supported browsers.
         *
         * TODO: Consider "upgrading" this code to use the same asWebPrefixes array as above, especially once Microsoft
         * finally releases a browser that supports pointer-locking (post-Windows 10?)
         */
        if (this.inputScreen) {
            this.inputScreen.onfocus = function onFocusScreen() {
                return video.onFocusChange(true);
            };
            this.inputScreen.onblur = function onBlurScreen() {
                return video.onFocusChange(false);
            };
            this.inputScreen.lockPointer = this.inputScreen['requestPointerLock'] || this.inputScreen['mozRequestPointerLock'] || this.inputScreen['webkitRequestPointerLock'];
            this.inputScreen.unlockPointer = this.inputScreen['exitPointerLock'] || this.inputScreen['mozExitPointerLock'] || this.inputScreen['webkitExitPointerLock'];
            if (this.inputScreen.lockPointer) {
                var onPointerLockChange = function() {
                    var fLocked = (
                        document['pointerLockElement'] === video.inputScreen ||
                        document['mozPointerLockElement'] === video.inputScreen ||
                        document['webkitPointerLockElement'] === video.inputScreen);
                    video.notifyPointerLocked(fLocked);
                };
                if ('onpointerlockchange' in document) {
                    document.addEventListener('pointerlockchange', onPointerLockChange, false);
                } else if ('onmozpointerlockchange' in document) {
                    document.addEventListener('mozpointerlockchange', onPointerLockChange, false);
                } else if ('onwebkitpointerlockchange' in document) {
                    document.addEventListener('webkitpointerlockchange', onPointerLockChange, false);
                }
            }
        }

        /*
         * As far as overall image quality of scaled fonts, these options don't seem necessary for Safari (and
         * don't have any discernible effect anyway). Turning 'webkitImageSmoothingEnabled' off DOES have an effect
         * on Chrome, but it's not really a positive effect overall, so I'm leaving these off for now.
         *
         *  if (this.contextScreen) {
         *      this.contextScreen['mozImageSmoothingEnabled'] = false;
         *      this.contextScreen['webkitImageSmoothingEnabled'] = false;
         *  }
         */

        var sFileURL = parmsVideo['fontROM'];
        if (sFileURL) {
            var sFileExt = Str.getExtension(sFileURL);
            if (sFileExt != "json") {
                sFileURL = Web.getHost() + DumpAPI.ENDPOINT + '?' + DumpAPI.QUERY.FILE + '=' + sFileURL + '&' + DumpAPI.QUERY.FORMAT + '=' + DumpAPI.FORMAT.BYTES;
            }
            Web.getResource(sFileURL, null, true, function(sURL, sResponse, nErrorCode) {
                video.doneLoad(sURL, sResponse, nErrorCode);
            });
        }
    }

    /**
     * initBus(cmp, bus, cpu, dbg)
     *
     * This is a notification issued by the Computer component, after all the other components (notably the CPU)
     * have had a chance to initialize.
     *
     * @this {Video}
     * @param {Computer} cmp
     * @param {Bus} bus
     * @param {X86CPU} cpu
     * @param {DebuggerX86} dbg
     */
    initBus(cmp, bus, cpu, dbg)
    {
        this.bus = bus;
        this.cpu = cpu;
        this.dbg = dbg;

        /*
         * nCard will be undefined if no model was explicitly set (whereas this.nCard is ALWAYS defined).
         */
        var aModel = Video.MODEL[this.model], nCard = aModel && aModel[0];

        /*
         * The only time we do NOT want to trap MDA ports is when the model has been explicitly set to CGA.
         */
        if (nCard !== Video.CARD.CGA) {
            bus.addPortInputTable(this, Video.aMDAPortInput);
            bus.addPortOutputTable(this, Video.aMDAPortOutput);
        }

        /*
         * Similarly, the only time we do NOT want to trap CGA ports is when the model is explicitly set to MDA.
         */
        if (nCard !== Video.CARD.MDA) {
            bus.addPortInputTable(this, Video.aCGAPortInput);
            bus.addPortOutputTable(this, Video.aCGAPortOutput);
        }

        /*
         * Note that in the case of EGA and VGA models, the above code ensures that we will trap both MDA and CGA
         * port ranges -- which is good, because both the EGA and VGA can be reprogrammed to respond to those ports,
         * but also potentially bad if you want to simulate a "dual display" system, where one of the displays is
         * driven by either an MDA or CGA.
         *
         * However, you should still be able to make that work by loading the MDA or CGA video component first, because
         * components should be initialized in the order they appear in the machine configuration file.  Any attempt
         * by another component to trap the same ports should be ignored.
         */
        if (this.nCard >= Video.CARD.EGA) {
            bus.addPortInputTable(this, Video.aEGAPortInput);
            bus.addPortOutputTable(this, Video.aEGAPortOutput);
        }

        if (this.nCard == Video.CARD.VGA) {
            bus.addPortInputTable(this, Video.aVGAPortInput);
            bus.addPortOutputTable(this, Video.aVGAPortOutput);
        }

        if (DEBUGGER && dbg) {
            var video = this;
            dbg.messageDump(Messages.VIDEO, function onDumpVideo(asArgs) {
                video.dumpVideo(asArgs);
            });
        }

        /*
         * If we have an associated keyboard, then ensure that the keyboard will be notified
         * whenever the canvas gets focus and receives input.
         */
        this.kbd = cmp.getMachineComponent("Keyboard");
        if (this.kbd && this.canvasScreen) {
            for (var s in this.bindings) {
                if (s.indexOf("lock") > 0) this.kbd.setBinding("led", s, this.bindings[s]);
            }
            this.kbd.setBinding(this.textareaScreen? "textarea" : "canvas", "screen", this.inputScreen);
        }

        this.bEGASwitches = 0x09;   // our default "switches" setting (see aEGAMonitorSwitches)
        this.chipset = cmp.getMachineComponent("ChipSet");
        if (this.chipset && this.sSwitches) {
            if (this.nCard == Video.CARD.EGA) {
                this.bEGASwitches = this.chipset.parseDIPSwitches(this.sSwitches, this.bEGASwitches);
            }
        }

        /*
         * The default value for the 'touchScreen' parameter is an empty string; machine configs must explicitly
         * select one of the following values, via the 'touchscreen' attribute in the <video> element, to enable any
         * touch-screen support.
         */
        if (this.sTouchScreen == "mouse") {
            this.mouse = cmp.getMachineComponent("Mouse");
            if (this.mouse) this.captureTouch(Video.TOUCH.MOUSE);
        }
        else if (this.sTouchScreen == "keygrid") {
            if (this.kbd) this.captureTouch(Video.TOUCH.KEYGRID);
        }
    }

    /**
     * setBinding(sHTMLType, sBinding, control, sValue)
     *
     * @this {Video}
     * @param {string|null} sHTMLType is the type of the HTML control (eg, "button", "list", "text", "submit", "textarea", "canvas")
     * @param {string} sBinding is the value of the 'binding' parameter stored in the HTML control's "data-value" attribute (eg, "refresh")
     * @param {Object} control is the HTML control DOM object (eg, HTMLButtonElement)
     * @param {string} [sValue] optional data value
     * @return {boolean} true if binding was successful, false if unrecognized binding request
     */
    setBinding(sHTMLType, sBinding, control, sValue)
    {
        var video = this;

        if (!this.bindings[sBinding]) {

            /*
             * We now save every binding that comes in, so that if there are bindings for "caps-lock' and the like,
             * we can forward them to the Keyboard.  TODO: Perhaps we should limit this to sHTMLType == "led", and collect
             * them in a separate object (eg, ledBindings), so that initBus() can safely enumerate JUST the LEDs.  This
             * is what we do in PC8080.  Be aware that's there's also sHTMLType == "rled" now, too.
             */
            this.bindings[sBinding] = control;

            switch (sBinding) {

            case "fullScreen":
                if (this.container && this.container.doFullScreen) {
                    control.onclick = function onClickFullScreen() {
                        if (DEBUG) video.printMessage("fullScreen()");
                        video.doFullScreen();
                    };
                } else {
                    if (DEBUG) this.log("FullScreen API not available");
                    control.parentNode.removeChild(/** @type {Node} */ (control));
                }
                return true;

            case "lockPointer":
                this.sLockMessage = control.textContent;
                if (this.inputScreen && this.inputScreen.lockPointer) {
                    control.onclick = function onClickLockPointer() {
                        if (DEBUG) video.printMessage("lockPointer()");
                        video.lockPointer(true);
                    };
                } else {
                    if (DEBUG) this.log("Pointer Lock API not available");
                    control.parentNode.removeChild(/** @type {Node} */ (control));
                }
                return true;

            case "refresh":
                control.onclick = function onClickRefresh() {
                    if (DEBUG) video.printMessage("refreshScreen()");
                    video.updateScreen(true);
                };
                return true;

            default:
                break;
            }
        }
        return false;
    }

    /**
     * setFocus()
     *
     * @this {Video}
     */
    setFocus()
    {
        if (this.inputScreen) this.inputScreen.focus();
    }

    /**
     * getInput()
     *
     * This is an interface used by the Mouse component, so that it can invoke capture/release mouse events from the screen element.
     *
     * @this {Video}
     * @param {Mouse} [mouse]
     * @return {Object|undefined}
     */
    getInput(mouse)
    {
        this.mouse = mouse;
        return this.inputScreen;
    }

    /**
     * doFullScreen()
     *
     * @this {Video}
     * @return {boolean} true if request successful, false if not (eg, failed OR not supported)
     */
    doFullScreen()
    {
        var fSuccess = false;
        if (this.container) {
            if (this.container.doFullScreen) {
                /*
                 * Styling the container with a width of "100%" and a height of "auto" works great when the aspect ratio
                 * of our virtual screen is at least roughly equivalent to the physical screen's aspect ratio, but now that
                 * we support virtual VGA screens with an aspect ratio of 1.33, that's very much out of step with modern
                 * wide-screen monitors, which usually have an aspect ratio of 1.6 or greater.
                 *
                 * And unfortunately, none of the browsers I've tested appear to make any attempt to scale our container to
                 * the physical screen's dimensions, so the bottom of our screen gets clipped.  To prevent that, I reduce
                 * the width from 100% to whatever percentage will accommodate the entire height of the virtual screen.
                 *
                 * NOTE: Mozilla recommends both a width and a height of "100%", but all my tests suggest that using "auto"
                 * for height works equally well, so I'm sticking with it, because "auto" is also consistent with how I've
                 * implemented a responsive canvas when the browser window is being resized.
                 */
                var sWidth = "100%";
                var sHeight = "auto";
                if (screen && screen.width && screen.height) {
                    var aspectPhys = screen.width / screen.height;
                    var aspectVirt = this.cxScreen / this.cyScreen;
                    if (aspectPhys > aspectVirt) {
                        sWidth = Math.round(aspectVirt / aspectPhys * 100) + '%';
                    }
                    // TODO: We may need to someday consider the case of a physical screen with an aspect ratio < 1.0....
                }
                if (!this.fGecko) {
                    this.container.style.width = sWidth;
                    this.container.style.height = sHeight;
                } else {
                    /*
                     * Sadly, the above code doesn't work for Firefox, because as http://developer.mozilla.org/en-US/docs/Web/Guide/API/DOM/Using_full_screen_mode
                     * explains:
                     *
                     *      'It's worth noting a key difference here between the Gecko and WebKit implementations at this time:
                     *      Gecko automatically adds CSS rules to the element to stretch it to fill the screen: "width: 100%; height: 100%".
                     *
                     * Which would be OK if Gecko did that BEFORE we're called, but apparently it does that AFTER, effectively
                     * overwriting our careful calculations.  So we style the inner element (canvasScreen) instead, which
                     * requires even more work to ensure that the canvas is properly centered.  FYI, this solution is consistent
                     * with Mozilla's recommendation for working around their automatic CSS rules:
                     *
                     *      '[I]f you're trying to emulate WebKit's behavior on Gecko, you need to place the element you want
                     *      to present inside another element, which you'll make fullscreen instead, and use CSS rules to adjust
                     *      the inner element to match the appearance you want.'
                     */
                    this.canvasScreen.style.width = sWidth;
                    this.canvasScreen.style.width = sWidth;
                    this.canvasScreen.style.display = "block";
                    this.canvasScreen.style.margin = "auto";
                }
                this.container.style.backgroundColor = "black";
                this.container.doFullScreen();
                fSuccess = true;
            }
            this.setFocus();
        }
        return fSuccess;
    }

    /**
     * notifyFullScreen(fFullScreen)
     *
     * @this {Video}
     * @param {boolean|null} fFullScreen (null if there was a full-screen error)
     */
    notifyFullScreen(fFullScreen)
    {
        if (!fFullScreen && this.container) {
            if (!this.fGecko) {
                this.container.style.width = this.container.style.height = "";
            } else {
                this.canvasScreen.style.width = this.canvasScreen.style.height = "";
            }
        }
        this.printMessage("notifyFullScreen(" + fFullScreen + ")", true);
        if (this.kbd) this.kbd.notifyEscape(fFullScreen);
    }

    /**
     * lockPointer()
     *
     * @this {Video}
     * @param {boolean} fLock
     * @return {boolean} true if request successful, false if not (eg, failed OR not supported)
     */
    lockPointer(fLock)
    {
        var fSuccess = false;
        if (this.inputScreen) {
            if (fLock) {
                if (this.inputScreen.lockPointer) {
                    this.inputScreen.lockPointer();
                    if (this.mouse) this.mouse.notifyPointerLocked(true);
                    fSuccess = true;
                }
            } else {
                if (this.inputScreen.unlockPointer) {
                    this.inputScreen.unlockPointer();
                    if (this.mouse) this.mouse.notifyPointerLocked(false);
                    fSuccess = true;
                }
            }
            this.setFocus();
        }
        return fSuccess;
    }

    /**
     * notifyPointerActive(fActive)
     *
     * @this {Video}
     * @param {boolean} fActive
     * @return {boolean} true if autolock enabled AND pointer lock supported, false if not
     */
    notifyPointerActive(fActive)
    {
        if (this.fAutoLock) {
            return this.lockPointer(fActive);
        }
        return false;
    }

    /**
     * notifyPointerLocked(fLocked)
     *
     * @this {Video}
     * @param {boolean} fLocked
     */
    notifyPointerLocked(fLocked)
    {
        if (this.mouse) {
            this.mouse.notifyPointerLocked(fLocked);
            if (this.kbd) this.kbd.notifyEscape(fLocked);
        }
        var control = this.bindings["lockPointer"];
        if (control) control.textContent = (fLocked? "Press Esc to Unlock Pointer" : this.sLockMessage);
    }

    /**
     * captureTouch(nTouchConfig)
     *
     * @this {Video}
     * @param {number} nTouchConfig (must be one of the supported Video.TOUCH values)
     */
    captureTouch(nTouchConfig)
    {
        var control = this.inputScreen;
        if (control) {
            var video = this;
            if (!this.nTouchConfig) {
                control.addEventListener(
                    'touchstart',
                    function onTouchStart(event) { video.onTouchStart(event); },
                    false                   // we'll specify false for the 'useCapture' parameter for now...
                );
                control.addEventListener(
                    'touchmove',
                    function onTouchMove(event) { video.onTouchMove(event); },
                    true
                );
                control.addEventListener(
                    'touchend',
                    function onTouchEnd(event) { video.onTouchEnd(event); },
                    false                   // we'll specify false for the 'useCapture' parameter for now...
                );

                /*
                 * Using desktop mouse events to simulate touch events should only be enabled as needed.
                 *
                if (DEBUG) {
                    control.addEventListener(
                        'mousedown',
                        function onMouseDown(event) { video.onTouchStart(event); },
                        false               // we'll specify false for the 'useCapture' parameter for now...
                    );
                    control.addEventListener(
                        'mousemove',
                        function onMouseMove(event) { video.onTouchMove(event); },
                        true
                    );
                    control.addEventListener(
                        'mouseup',
                        function onMouseUp(event) { video.onTouchEnd(event); },
                        false               // we'll specify false for the 'useCapture' parameter for now...
                    );
                }
                */

                // this.log("touch events captured");

                this.nTouchConfig = nTouchConfig;
                this.xTouch = this.yTouch = this.timeTouch = -1;

                /*
                 * As long as fTouchDefault is false, we call preventDefault() on every touch event, to prevent
                 * the page from moving/scrolling while the canvas is processing touch events.  However, there must
                 * also be exceptions to permit the soft keyboard to activate; see processTouchEvent() for details.
                 */
                this.fTouchDefault = false;

                /*
                 * I also need to come up with some rules for when the simulated mouse's primary button stays down.
                 * Let's try setting a timeout handler whenever a touchstart is received, which we'll immediately cancel
                 * as soon as a touchmove or touchend event is received, and if the timeout handler fires, we'll set
                 * fLongTouch to true.
                 */
                this.hLongTouch = null;
                this.fLongTouch = false;
                this.onLongTouch = function onLongTouch() { video.startLongTouch(); };
            }
        }
    }

    /**
     * onFocusChange(fFocus)
     *
     * @this {Video}
     * @param {boolean} fFocus is true if gaining focus, false if losing it
     */
    onFocusChange(fFocus)
    {
        /*
         * As per http://stackoverflow.com/questions/6740253/disable-scrolling-when-changing-focus-form-elements-ipad-web-app,
         * I decided to try this work-around to prevent the webpage from scrolling around whenever the canvas is given
         * focus.  That sort of scrolling-into-view sounds great in principle, but in practice, if you were reading some other
         * portion of the page, it can be irritating to be scrolled away from that portion when refreshing/returning to the page.
         *
         * However, this work-around doesn't seem to work with the latest version of Safari (or else I misunderstood something).
         *
         *  if (fFocus) {
         *      window.scrollTo(0, 0);
         *      document.body.scrollTop = 0;
         *  }
         */
        this.fHasFocus = fFocus;
        if (this.kbd) this.kbd.onFocusChange(fFocus);
    }

    /**
     * onTouchStart(event)
     *
     * @this {Video}
     * @param {Event} event object from a 'touch' event
     */
    onTouchStart(event)
    {
        if (DEBUG) this.printMessage("onTouchStart()");
        this.processTouchEvent(event, true);
    }

    /**
     * onTouchMove(event)
     *
     * @this {Video}
     * @param {Event} event object from a 'touch' event
     */
    onTouchMove(event)
    {
        if (DEBUG) this.printMessage("onTouchMove()");
        this.processTouchEvent(event);
    }

    /**
     * onTouchEnd(event)
     *
     * @this {Video}
     * @param {Event} event object from a 'touch' event
     */
    onTouchEnd(event)
    {
        if (DEBUG) this.printMessage("onTouchEnd()");
        this.processTouchEvent(event, false);
    }

    /**
     * processTouchEvent(event, fStart)
     *
     * If nTouchConfig is non-zero, touch event handlers are installed, which pass their events to this function.
     *
     * What we do with those events here depends on the value of nTouchConfig.  Originally, the only supported
     * configuration was the experimental conversion of touch events into arrow keys, based on an invisible grid
     * that divided the screen into thirds; that configuration is now identified as Video.TOUCH.KEYGRID.
     *
     * The new preferred configuration is Video.TOUCH.MOUSE, which does little more than allow you to "push" the
     * simulated mouse around.  If Video.TOUCH.MOUSE is enabled, it's already been confirmed the machine has a mouse.
     *
     * @this {Video}
     * @param {Event} event object from a 'touch' event
     * @param {boolean} [fStart] (true if 'touchstart', false if 'touchend', undefined if 'touchmove')
     */
    processTouchEvent(event, fStart)
    {
        var xTouch, yTouch;

        // if (!event) event = window.event;

        /*
         * Touch coordinates (that is, the pageX and pageY properties) are relative to the page, so to make
         * them relative to the canvas, we must subtract the canvas's left and top positions.  This Apple web page:
         *
         *      https://developer.apple.com/library/safari/documentation/AudioVideo/Conceptual/HTML-canvas-guide/AddingMouseandTouchControlstoCanvas/AddingMouseandTouchControlstoCanvas.html
         *
         * makes it sound simple, but it turns out we have to walk the canvas' entire "parentage" of DOM elements
         * to get the exact offsets.
         *
         * TODO: Determine whether the getBoundingClientRect() code used in panel.js for mouse events can also
         * be used here to simplify this annoyingly complicated code for touch events.
         */
        var xTouchOffset = 0;
        var yTouchOffset = 0;
        var eCurrent = this.canvasScreen;

        do {
            if (!isNaN(eCurrent.offsetLeft)) {
                xTouchOffset += eCurrent.offsetLeft;
                yTouchOffset += eCurrent.offsetTop;
            }
        } while ((eCurrent = eCurrent.offsetParent));

        /*
         * Due to the responsive nature of our pages, the displayed size of the canvas may be smaller than the
         * allocated size, and the coordinates we receive from touch events are based on the currently displayed size.
         */
        var xScale =  this.cxScreen / this.canvasScreen.offsetWidth;
        var yScale = this.cyScreen / this.canvasScreen.offsetHeight;

        /**
         * @name Event
         * @property {Array} targetTouches
         */
        if (!event.targetTouches || !event.targetTouches.length) {
            xTouch = event.pageX;
            yTouch = event.pageY;
        } else {
            xTouch = event.targetTouches[0].pageX;
            yTouch = event.targetTouches[0].pageY;
        }

        xTouch = ((xTouch - xTouchOffset) * xScale);
        yTouch = ((yTouch - yTouchOffset) * yScale);

        if (this.nTouchConfig == Video.TOUCH.KEYGRID) {

            var xThird = (xTouch / (this.cxScreen / 3)) | 0;
            var yThird = (yTouch / (this.cyScreen / 3)) | 0;

            /*
             * At this point, xThird and yThird should both be one of 0, 1 or 2, indicating which horizontal and vertical
             * third of the virtual screen the touch event occurred.
             */
            if (/* xThird == 1 && */ yThird != 1) {
                if (!yThird) {
                    this.kbd.addActiveKey(Keyboard.CLICKCODES.UP, true);
                } else {
                    this.kbd.addActiveKey(Keyboard.CLICKCODES.DOWN, true);
                }
            } else if (/* yThird == 1 && */ xThird != 1) {
                if (!xThird) {
                    this.kbd.addActiveKey(Keyboard.CLICKCODES.LEFT, true);
                } else {
                    this.kbd.addActiveKey(Keyboard.CLICKCODES.RIGHT, true);
                }
            }
        } else {
            if (this.mouse) {
                /*
                 * As long as fTouchDefault is false, we call preventDefault() on every touch event, to keep
                 * the page stable.  However, we must allow some touch event(s) to perform their default action,
                 * otherwise the soft keyboard can never be activated.  So if a touchstart occurs at least 1/2
                 * second (500ms) after the last touchstart, with no intervening touchmove events, fTouchDefault
                 * is allowed to become true.
                 */
                var fTouchDefault = this.fTouchDefault;
                var timeDelta = event.timeStamp - this.timeTouch;

                if (fStart === true) {
                    this.fTouchDefault = (timeDelta > 500);
                    this.timeTouch = event.timeStamp;
                    this.hLongTouch = setTimeout(this.onLongTouch, 500);
                } else {
                    if (this.hLongTouch != null) {
                        clearTimeout(this.hLongTouch);
                        this.hLongTouch = null;
                    }
                }
                if (fStart === undefined) {
                    this.fTouchDefault = false;
                }

                if (DEBUG) {
                    this.log("processTouchEvent(" + (fStart? "touchStart" : (fStart === false? "touchEnd" : "touchMove")) + "," + timeDelta + "ms," + fTouchDefault + ")");
                }

                if (!fTouchDefault) {
                    event.preventDefault();
                }

                if (fStart === false) {
                    /*
                     * NOTE: 200ms is merely my initial stab at a reasonable number of milliseconds to interpret a
                     * start/end touch sequence as a "tap"; I also make no note of any intervening move events (ie,
                     * events where fStart is undefined), and perhaps I should....
                     */
                    if (this.endLongTouch()) {
                        return;
                    }
                    if (timeDelta < 200) {
                        this.mouse.clickMouse(Mouse.BUTTON.LEFT, true);
                        this.mouse.clickMouse(Mouse.BUTTON.LEFT, false);
                        return;
                    }
                }
                /*
                 * This 'touchmove" code mimics the 'mousemove' event processing in processMouseEvent() in mouse.js, with
                 * one important difference: every time touching "restarts", we need to reset the variables used to calculate
                 * the deltas, so that the mere act of lifting and replacing your finger doesn't generate a delta by itself.
                 */
                if (fStart || this.xTouch < 0 || this.yTouch < 0) {
                    this.xTouch = xTouch;
                    this.yTouch = yTouch;
                }
                var xDelta = Math.round(xTouch - this.xTouch);
                var yDelta = Math.round(yTouch - this.yTouch);
                this.xTouch = xTouch;
                this.yTouch = yTouch;
                // this.println("moveMouse(" + xDelta + "," + yDelta + ")");
                this.mouse.moveMouse(xDelta, yDelta, this.xTouch, this.yTouch);
            }
        }
    }

    /**
     * startLongTouch()
     *
     * @this {Video}
     */
    startLongTouch()
    {
        this.fLongTouch = true;
        this.mouse.clickMouse(Mouse.BUTTON.LEFT, true);
    }

    /**
     * endLongTouch()
     *
     * @this {Video}
     * @return {boolean} true if long touch was active, false if not
     */
    endLongTouch()
    {
        if (this.fLongTouch) {
            this.mouse.clickMouse(Mouse.BUTTON.LEFT, false);
            this.fLongTouch = false;
            return true;
        }
        return false;
    }

    /**
     * powerUp(data, fRepower)
     *
     * @this {Video}
     * @param {Object|null} data
     * @param {boolean} [fRepower]
     * @return {boolean} true if successful, false if failure
     */
    powerUp(data, fRepower)
    {
        if (!fRepower) {
            if (!data || !this.restore) {
                this.reset();
            } else {
                if (!this.restore(data)) return false;
            }
        }
        return true;
    }

    /**
     * powerDown(fSave, fShutdown)
     *
     * This is where we might add some method of blanking the display, without the disturbing the video
     * buffer contents, and blocking all further updates to the display.
     *
     * @this {Video}
     * @param {boolean} [fSave]
     * @param {boolean} [fShutdown]
     * @return {Object|boolean} component state if fSave; otherwise, true if successful, false if failure
     */
    powerDown(fSave, fShutdown)
    {
        return fSave? this.save() : true;
    }

    /**
     * reset()
     *
     * @this {Video}
     */
    reset()
    {
        var fRandomize = true;
        var nMonitorType = ChipSet.MONITOR.NONE;

        /*
         * We'll ask the ChipSet what SW1 indicates for monitor type, but we may override it if a specific
         * video card model is set.  For EGA, SW1 is supposed to be set to indicate NO monitor, and we rely
         * on the EGA's own switch settings instead.
         */
        if (this.chipset) {
            nMonitorType = this.chipset.getDIPVideoMonitor();
        }

        /*
         * As we noted in the constructor, when a model is specified, that takes precedence over any monitor
         * switch settings.  Conversely, when no model is specified, the nCard setting is considered provisional,
         * so the monitor switch settings, if any, are allowed to determine the card type.
         */
        if (!this.model) {
            this.nCard = (nMonitorType == ChipSet.MONITOR.MONO? Video.CARD.MDA : Video.CARD.CGA);
        }

        this.nModeDefault = Video.MODE.CGA_80X25;

        switch (this.nCard) {
        case Video.CARD.VGA:
            nMonitorType = ChipSet.MONITOR.VGACOLOR;
            break;
        case Video.CARD.EGA:
            var aMonitors = Video.aEGAMonitorSwitches[this.bEGASwitches];
            /*
             * TODO: Figure out how to deal with aMonitors[2], the boolean which indicates
             * whether the EGA is driving the primary monitor (true) or the secondary monitor (false).
             */
            if (aMonitors) nMonitorType = aMonitors[0];
            if (!nMonitorType) nMonitorType = ChipSet.MONITOR.EGACOLOR;
            break;
        case Video.CARD.MDA:
            nMonitorType = ChipSet.MONITOR.MONO;
            this.nModeDefault = Video.MODE.MDA_80X25;
            break;
        case Video.CARD.CGA:
            /* falls through */
        default:
            nMonitorType = ChipSet.MONITOR.COLOR;
            break;
        }

        if (this.nMonitorType !== nMonitorType) {
            this.nMonitorType = nMonitorType;
            fRandomize = true;
        }

        this.cardActive = null;
        this.cardMono = this.cardMDA = new Card(this, Video.CARD.MDA);
        this.cardColor = this.cardCGA = new Card(this, Video.CARD.CGA);

        if (this.nCard < Video.CARD.EGA) {
            this.cardEGA = new Card();      // define a dummy (uninitialized) EGA card for now
        }
        else {
            this.cardEGA = new Card(this, this.nCard, null, this.cbMemory);
            this.enableEGA();
        }

        /*
         * We need to call buildFonts() *after* the card(s) are initialized but *before* setMode() is called.
         */
        this.buildFonts();

        this.nMode = null;
        this.iCellCursor = -1;  // initially, there is no visible cursor cell
        this.cBlinks = -1;      // initially, blinking is not active
        this.cBlinkVisible = 0; // no visible blinking characters (yet)

        this.setMode(this.nModeDefault);

        if (this.cardActive.addrBuffer && fRandomize) {
            /*
             * On the initial power-on, we initialize the video buffer to random characters, as a way of testing
             * whether our font(s) were successfully loaded.  It's assumed that our default display mode is a text mode,
             * and that since this is a reset, the CRTC.START_ADDR registers are zero as well.
             *
             * If this is an MDA device, then the buffer should reside at 0xB0000 through 0xB0FFF, for a total length
             * of 4Kb (0x1000), where every even byte contains a character code, and every odd byte contains an attribute
             * code.  See the ATTR bit definitions above for applicable color, intensity, and blink values.  On a CGA
             * device, the buffer resides at 0xB8000 through 0xBBFFF, for a total length of 16Kb.
             *
             * Note that the only valid MDA display mode (7) is the 80x25 text mode, which uses 4000 bytes (2000 character
             * bytes + 2000 attribute bytes), not all 4096 bytes; addrScreenLimit reflects the visible limit, not the
             * physical limit.  Also, as noted in updateScreen(), this simplistic calculation of the extent of visible
             * screen memory is valid only for text modes; in general, it's safer to use cardActive.sizeBuffer as the extent.
             */
            var addrScreenLimit = this.cardActive.addrBuffer + this.cbScreen;
            for (var addrScreen = this.cardActive.addrBuffer; addrScreen < addrScreenLimit; addrScreen += 2) {
                var dataRandom = (Math.random() * 0x10000)|0;
                var bChar, bAttr;
                if (this.nMonitorType == ChipSet.MONITOR.EGACOLOR || this.nMonitorType == ChipSet.MONITOR.VGACOLOR) {
                    /*
                     * For the EGA, we choose sequential characters; for random characters, copy the MDA/CGA code below.
                     */
                    bChar = (addrScreen >> 1) & 0xff;
                    bAttr = (dataRandom >> 8) & ~Video.ATTRS.BGND_BLINK;    // TODO: turn blink attributes off unless we can ensure blinking is initially disabled
                    if ((bAttr >> 4) == (bAttr & 0xf)) {
                        bAttr ^= 0x0f;      // if background matches foreground, invert foreground to ensure character visibility
                    }
                } else {
                    bChar = dataRandom & 0xff;
                    bAttr = ((dataRandom & 0x100)? (Video.ATTRS.FGND_WHITE | Video.ATTRS.BGND_BLACK) : (Video.ATTRS.FGND_BLACK | Video.ATTRS.BGND_WHITE)) | ((Video.ATTRS.FGND_BRIGHT /* | Video.ATTRS.BGND_BLINK */) & (dataRandom >> 8));
                }
                this.bus.setShortDirect(addrScreen, bChar | (bAttr << 8));
            }
            this.updateScreen(true);
        }
    }

    /**
     * enableEGA()
     *
     * Redirect cardMono or cardColor to cardEGA as appropriate.
     *
     * @this {Video}
     */
    enableEGA()
    {
        if (!(this.cardEGA.regMisc & Card.MISC.IO_SELECT)) {
            this.cardMono = this.cardEGA;
            this.cardColor = this.cardCGA;  // this is done mainly to siphon away any CGA I/O
        } else {
            this.cardMono = this.cardMDA;   // similarly, this is done to siphon away any MDA I/O
            this.cardColor = this.cardEGA;
        }
    }

    /**
     * save()
     *
     * This implements save support for the Video component.
     *
     * @this {Video}
     * @return {Object}
     */
    save()
    {
        var state = new State(this);
        state.set(0, this.cardMDA.saveCard());
        state.set(1, this.cardCGA.saveCard());
        state.set(2, [this.nMonitorType, this.nModeDefault, this.nMode]);
        state.set(3, this.cardEGA.saveCard());
        return state.data();
    }

    /**
     * restore(data)
     *
     * This implements restore support for the Video component.
     *
     * @this {Video}
     * @param {Object} data
     * @return {boolean} true if successful, false if failure
     */
    restore(data)
    {
        var a = data[2];
        this.nMonitorType = a[0];
        this.nModeDefault = a[1];
        this.nMode = a[2];

        this.cardActive = null;
        this.cardMono = this.cardMDA = new Card(this, Video.CARD.MDA, data[0]);
        this.cardColor = this.cardCGA = new Card(this, Video.CARD.CGA, data[1]);

        /*
         * If no EGA was originally initialized, then cardEGA will remain uninitialized.
         */
        this.cardEGA = new Card(this, this.nCard, data[3], this.cbMemory);
        if (this.cardEGA.fActive) this.enableEGA();

        /*
         * We need to call buildFonts() *after* the card(s) are initialized but *before* setMode() is called.
         */
        this.buildFonts();

        /*
         * While I could restore the active card here, it's better for setMode() to do it, because
         * setMode() will also take care of mapping the appropriate video buffer.  So, after restore() has
         * finished, we call checkMode(), because the current video mode (nMode) is determined by the
         * active card state.
         *
         * Unfortunately, that creates a chicken-and-egg problem, since I just said I didn't want to select
         * the active card here.
         *
         * So, we'll add some "cop-out" code to checkMode(): if there's no active card, then fall-back
         * to the last known video mode (nMode) and force a call to setMode().
         *
         *      this.cardActive = (this.cardMDA.fActive? this.cardMDA : (this.cardCGA.fActive? this.cardCGA : undefined));
         */
        if (!this.checkMode()) return false;

        this.checkCursor();
        return true;
    }

    /**
     * doneLoad(sURL, sFontData, nErrorCode)
     *
     * @this {Video}
     * @param {string} sURL
     * @param {string} sFontData
     * @param {number} nErrorCode (response from server if anything other than 200)
     */
    doneLoad(sURL, sFontData, nErrorCode)
    {
        if (nErrorCode) {
            this.notice("Unable to load font ROM (error " + nErrorCode + ": " + sURL + ")");
            return;
        }

        Component.addMachineResource(this.idMachine, sURL, sFontData);

        try {
            /*
             * The most likely source of any exception will be right here, where we're parsing the JSON-encoded data.
             */
            var abFontData = eval("(" + sFontData + ")");

            var ab = abFontData['bytes'] || abFontData;

            if (!ab.length) {
                Component.error("Empty font ROM: " + sURL);
                return;
            }
            else if (ab.length == 1) {
                Component.error(ab[0]);
                return;
            }
            /*
             * Translate the character data into separate "fonts", each of which will be a separate canvas object, with all
             * 256 characters arranged in a 16x16 grid.
             */
            if (ab.length == 8192) {
                /*
                 * The assumption here is that we're dealing with the original (IBM) MDA/CGA font data, which apparently
                 * was identical on both MDA and CGA cards (even though the former had no use for the latter, and vice versa).
                 *
                 * First, let's take a look at the MDA portion of the data.  Here are the first few rows of MDA font data,
                 * at the 0K and 2K boundaries:
                 *
                 *      00000000  00 00 00 00 00 00 00 00  00 00 7e 81 a5 81 81 bd  |..........~.....|
                 *      00000010  00 00 7e ff db ff ff c3  00 00 00 36 7f 7f 7f 7f  |..~........6....|
                 *      ...
                 *      00000800  00 00 00 00 00 00 00 00  99 81 7e 00 00 00 00 00  |..........~.....|
                 *      00000810  e7 ff 7e 00 00 00 00 00  3e 1c 08 00 00 00 00 00  |..~.....>.......|
                 *
                 * 8 bytes of data from a row in each of the 2K chunks are combined to form a 8-bit wide character with
                 * a maximum height of 16 bits.  Assembling the bits for character 0x01 (a happy face), we observe the following:
                 *
                 *      0 0 0 0 0 0 0 0  <== 00 from offset 0x0008
                 *      0 0 0 0 0 0 0 0  <== 00 from offset 0x0009
                 *      0 1 1 1 1 1 1 0  <== 7e from offset 0x000A
                 *      1 0 0 0 0 0 0 1  <== 81 from offset 0x000B
                 *      1 0 1 0 0 1 0 1  <== a5 from offset 0x000C
                 *      1 0 0 0 0 0 0 1  <== 81 from offset 0x000D
                 *      1 0 0 0 0 0 0 1  <== 81 from offset 0x000E
                 *      1 0 1 1 1 1 0 1  <== bd from offset 0x000F
                 *      1 0 0 1 1 0 0 1  <== 99 from offset 0x0808
                 *      1 0 0 0 0 0 0 1  <== 81 from offset 0x0809
                 *      0 1 1 1 1 1 1 0  <== 7e from offset 0x080A
                 *      0 0 0 0 0 0 0 0  <== 00 from offset 0x080B
                 *      0 0 0 0 0 0 0 0  <== 00 from offset 0x080C
                 *      0 0 0 0 0 0 0 0  <== 00 from offset 0x080D
                 *      0 0 0 0 0 0 0 0  <== 00 from offset 0x080E
                 *      0 0 0 0 0 0 0 0  <== 00 from offset 0x080F
                 *
                 * In the second 2K chunk, we observe that the last two bytes of every font cell definition are zero;
                 * this confirms our understanding that MDA font cell size is 8x14.
                 *
                 * Finally, there's the issue of screen cell size, which is actually 9x14 on the MDA.  We compensate for that
                 * by building a 9x14 font, even though there's only 8x14 bits of data. As http://www.seasip.info/VintagePC/mda.html
                 * explains:
                 *
                 *      "For characters C0h-DFh, the ninth pixel column is a duplicate of the eighth; for others, it's blank."
                 *
                 * This last point is confirmed by "The IBM Personal Computer From The Inside Out", p.295:
                 *
                 *      "Another unique feature of the monochrome adapter is a set of line-drawing and area-fill characters
                 *      that give continuous lines and filled areas. This is unusual for a display with a 9x14 character box
                 *      because the character generator provides a row only eight dots wide. On most displays, a blank 9th
                 *      dot is then inserted between characters. On the monochrome display, there is circuitry that duplicates
                 *      the 8th dot into the 9th dot position for characters whose ASCII codes are 0xB0 [sic] through 0xDF."
                 *
                 * However, the above text is mistaken about the start of the range.  While there ARE line-drawing characters
                 * in the range 0xB0-0xBF, none of them extend all the way to the right edge; IBM carefully segregated them.
                 * And in fact, characters 0xB0-0xB2 contain hash patterns that you would NOT want extended into the 9th column.
                 *
                 * The CGA font is part of the same ROM.  In fact, there are TWO CGA fonts in the ROM: a thin 5x7 "single dot"
                 * font located at offset 0x1000, and a thick 7x7 "double dot" font at offset 0x1800.  The latter is the default
                 * font, unless overridden by a jumper setting on the CGA card, so it is our default CGA font as well (although
                 * someday we may provide a virtual jumper setting that allows you to select the thinner font).
                 *
                 * The first offset we must pass to setFontData() is the offset of the CGA font; we choose the thicker "double dot"
                 * CGA font at 0x1800 (which was the PC's default font as well), instead of the thinner "single dot" font at 0x1000.
                 * The second offset is for the MDA font.
                 */
                this.setFontData(ab, [0x1800, 0x0000]);
            }
            else if (ab.length == 2048) {
                /*
                 * The assumption here is that we're dealing strictly with CGA (8x8) font data, like the font data found
                 * in the Columbia Data Products (CDP) Font ROM.
                 */
                this.setFontData(ab, [0x0000]);
            }
            else {
                this.notice("Unrecognized font data length (" + ab.length + ")");
                return;
            }

        } catch (e) {
            this.notice("Font ROM data error: " + e.message);
            return;
        }
        /*
         * If we're still here, then we're ready!
         *
         * UPDATE: Per issue #21, I'm issuing setReady() *only* if a valid contextScreen exists *or* a Debugger is attached.
         *
         * TODO: Consider a more general-purpose solution for deciding whether or not the user wants to run in a "headless" mode.
         */
        if (this.contextScreen || this.dbg) this.setReady();
    }

    /**
     * onROMLoad(abRom, aParms)
     *
     * Called by the ROM's copyROM() function whenever a ROM component with a 'notify' attribute containing
     * our component ID has been loaded.
     *
     * @this {Video}
     * @param {Array.<number>} abROM
     * @param {Array.<number>} [aParms]
     */
    onROMLoad(abROM, aParms)
    {
        if (this.nCard == Video.CARD.EGA) {
            /*
             * TODO: Unlike the MDA/CGA font data, we may want to hang onto this data, so that we can
             * regenerate the color font(s) whenever the foreground and/or background colors have changed.
             */
            if (DEBUG) this.printMessage("onROMLoad(): EGA fonts loaded");
            /*
             * For EGA cards, in the absence of any parameters, we assume that we're receiving the original
             * IBM EGA ROM, which stores its 8x14 font data at 0x2230 as one continuous sequence; the total size
             * of the 8x14 font is 0xE00 bytes.
             *
             * At 0x3030, there is an "ALPHA SUPPLEMENT" table, which contains 15 bytes per row instead of 14,
             * because each row is preceded by one byte containing the corresponding ASCII code; there are 20
             * entries in the supplemental table, for a total size of 0x12C bytes.
             *
             * Finally, at 0x3160, we have the 8x8 font data (also known as the thicker "double dot" CGA font);
             * the total size of the 8x8 font is 0x800 bytes.  No other font data is present in the EGA ROM;
             * the thin 5x7 "single dot" CGA font is notably absent, which is fine, because we never loaded it for
             * the MDA/CGA either.
             *
             * TODO: Determine how the supplemental table is used and whether we need to add some "run-time"
             * font generation to support it (as opposed to "init-time" generation, which is all we do now).
             * There's probably a similar need for user-defined fonts; for now, they're simply not supported.
             */
            this.setFontData(abROM, aParms || [0x3160, 0x2230], 8);
        }
        else if (this.nCard == Video.CARD.VGA) {
            if (DEBUG) this.printMessage("onROMLoad(): VGA fonts loaded");
            /*
             * For VGA cards, in the absence of any parameters, we assume that we're receiving the original
             * IBM VGA ROM, which contains an 8x14 font at 0x3F8D (and corresponding supplemental table at 0x4D8D)
             * and an 8x8 font at 0x378D; however, it also contains an 8x16 font at 0x4EBA (and corresponding
             * supplemental table at 0x5EBA).  See our reconstructed source code in ibm-vga.nasm.
             */
            this.setFontData(abROM, aParms || [0x378d, 0x3f8d], 8);
        }
        this.setReady();
    }

    /**
     * getCardColors(nBitsPerPixel)
     *
     * @this {Video}
     * @param {number} [nBitsPerPixel]
     * @returns {Array}
     */
    getCardColors(nBitsPerPixel)
    {
        if (nBitsPerPixel == 1) {
            /*
             * Only 2 total colors.
             */
            this.aRGB[0] = Video.aCGAColors[Video.ATTRS.FGND_BLACK];
            this.aRGB[1] = Video.aCGAColors[Video.ATTRS.FGND_WHITE];
            return this.aRGB;
        }

        if (nBitsPerPixel == 2) {
            /*
             * Of the 4 colors returned, the first color comes from regColor and the other 3 come from one of
             * the two hard-coded CGA color sets:
             *
             *      Color Set 1             Color Set 2
             *      -----------             -----------
             *      Background (0x00)       Background (0x00)
             *      Green      (0x12)       Cyan       (0x13)
             *      Red        (0x14)       Magenta    (0x15)
             *      Brown      (0x16)       White      (0x17)
             *
             * The numbers in parentheses are the EGA ATC palette register values that the EGA BIOS uses for each
             * color set; on an EGA, I synthesize a fake CGA regColor value, until I (TODO:) figure out exactly how
             * the EGA simulates the CGA color palette.
             */
            var regColor = this.cardActive.regColor;
            if (this.cardActive === this.cardEGA) {
                var bBackground = this.cardEGA.regATCData[0];
                regColor = bBackground & Card.CGA.COLOR.BORDER;
                if (bBackground & Card.ATC.PALETTE.BRIGHT) regColor |= Card.CGA.COLOR.BRIGHT;
                if (this.cardEGA.regATCData[1] != 0x12) regColor |= Card.CGA.COLOR.COLORSET2;
            }
            this.aRGB[0] = Video.aCGAColors[regColor & (Card.CGA.COLOR.BORDER | Card.CGA.COLOR.BRIGHT)];
            var aColorSet = (regColor & Card.CGA.COLOR.COLORSET2)? Video.aCGAColorSet2 : Video.aCGAColorSet1;
            for (var iColor = 0; iColor < aColorSet.length; iColor++) {
                this.aRGB[iColor+1] = Video.aCGAColors[aColorSet[iColor]];
            }
            return this.aRGB;
        }

        if (this.cardColor === this.cardCGA) {
            /*
             * There's no need to update this.aRGB if we simply want to return a hard-coded set of 16 colors.
             */
            return Video.aCGAColors;
        }

        this.assert(this.cardColor === this.cardEGA);

        if (this.fRGBValid && nBitsPerPixel && !this.aRGB[16]) {
            this.fRGBValid = false;
        }

        if (!this.fRGBValid) {

            var card = this.cardEGA;
            var aDAC = card.regDACData;
            var aRegs, i, dw, b, bRed, bGreen, bBlue;

            if (nBitsPerPixel == 8) {
                /*
                 * The card must be a VGA, and it's using an (8bpp) mode that bypasses the ATC, so we need to pull
                 * RGB data exclusively from the 256-entry DAC; each entry contains 6-bit red, green, and blue values
                 * packed into bits 0-5, 6-11, and 12-17, respectively, each of which we effectively shift left 2 bits:
                 * a crude 6-to-8-bit color conversion.
                 */
                for (i = 0; i < 256; i++) {
                    dw = aDAC[i] || 0;
                    this.assert(dw >= 0 && dw <= 0x3ffff);
                    bRed =   (dw << 2) & 0xfc;
                    bGreen = (dw >> 4) & 0xfc;
                    bBlue =  (dw >> 10) & 0xfc;
                    this.aRGB[i] = [bRed, bGreen, bBlue, 0xff];
                }
            } else {
                /*
                 * We need to pull RGB data from the ATC; moreover, if the ATC hasn't been initialized yet,
                 * we go with a default EGA-compatible 16-color palette.  We'll also use the DAC if there is one
                 * (ie, this is actually a VGA) and it appears to be initialized (ie, the VGA BIOS has been run).
                 */
                var fDAC = (aDAC && aDAC[255]);
                aRegs = (card.regATCData[15] != null? card.regATCData : Video.aEGAPalDef);
                for (i = 0; i < 16; i++) {
                    b = aRegs[i] & Card.ATC.PALETTE.MASK;
                    /*
                     * If the DAC is valid, we need to supplement the 6 bits of each ATC palette entry with the values
                     * for bits 6 and 7 from the ATC COLORSEL register (and overwrite bits 4 and 5 if ATC.MODE.COLORSEL_ALL
                     * is set as well).
                     *
                     * The only reasons the DAC wouldn't be valid are if 1) we're trying to display an image before the machine
                     * and its BIOS have had a chance to initialize the DAC (because we don't preset it to anything, although
                     * perhaps we should), or 2) this is an EGA, which doesn't have a DAC.
                     */
                    if (fDAC) {
                        b |= (card.regATCData[Card.ATC.COLORSEL.INDX] & (Card.ATC.COLORSEL.DAC_BIT7 | Card.ATC.COLORSEL.DAC_BIT6)) << 4;
                        if (card.regATCData[Card.ATC.MODE.INDX] & Card.ATC.MODE.COLORSEL_ALL) {
                            b &= ~0x30;
                            b |= (card.regATCData[Card.ATC.COLORSEL.INDX] & (Card.ATC.COLORSEL.DAC_BIT5 | Card.ATC.COLORSEL.DAC_BIT4)) << 4;
                        }
                        this.assert(b >= 0 && b <= 255);
                        dw = aDAC[b];
                        this.assert(dw >= 0 && dw <= 0x3ffff);
                        bRed =   (dw << 2) & 0xfc;
                        bGreen = (dw >> 4) & 0xfc;
                        bBlue =  (dw >> 10) & 0xfc;
                    } else {
                        bRed =   (((b & 0x04)? 0xaa : 0) | ((b & 0x20)? 0x55 : 0));
                        bGreen = (((b & 0x02)? 0xaa : 0) | ((b & 0x10)? 0x55 : 0));
                        bBlue =  (((b & 0x01)? 0xaa : 0) | ((b & 0x08)? 0x55 : 0));
                    }
                    this.aRGB[i] = [bRed, bGreen, bBlue, 0xff];
                }
            }
            this.fRGBValid = true;
        }

        return this.aRGB;
    }

    /**
     * setFontData(abFontData, aFontOffsets, cxFontChar)
     *
     * To support partial font rebuilds (required for the EGA), we now preserve the original font data (abFontData),
     * font offsets (aFontOffsets), and font character width (8 for the EGA, undefined for the MDA/CGA).
     *
     * TODO: Ultimately, we want to have exactly one dedicated font for the EGA, the data for which we'll read directly
     * from plane 2 of video memory, instead of relying on the original font data in ROM.  Relying on the ROM data was
     * originally just a crutch to help get EGA support bootstrapped.
     *
     * Also, for the MDA/CGA, we should be discarding the font data after the first buildFonts() call, because we
     * should not need to ever rebuild the fonts for those cards (both their font patterns and colors were hard-coded).
     *
     * @this {Video}
     * @param {*} abFontData is the raw font data, from the ROM font file
     * @param {Array.<number>} aFontOffsets contains offsets into abFontData: [0] for CGA, [1] for MDA
     * @param {number} [cxFontChar] is a fixed character width to use for all fonts; undefined to use MDA/CGA defaults
     */
    setFontData(abFontData, aFontOffsets, cxFontChar)
    {
        this.abFontData = abFontData;
        this.aFontOffsets = aFontOffsets;
        this.cxFontChar = cxFontChar;
    }

    /**
     * buildFonts()
     *
     * buildFonts() is called whenever the Video component is reset or restored; we used to build the fonts as soon
     * as the ROM containing them was loaded, and then throw away the underlying font data, but with the EGA's ability
     * to change the color of any font, font building must now be deferred until the reset or restore notifications,
     * ensuring we have access to all the colors the card is currently programmed to use.
     *
     * We're also called whenever EGA palette registers are modified, since one or more fonts will likely need
     * to be rebuilt (this is because our fonts contain pre-rendered images of all glyphs for all 16 active colors).
     * Calls to buildFonts() should not be expensive though: the underlying createFont() function rebuilds a font only
     * if its color has actually changed.
     *
     * TODO: Our font code is still written with the assumption that, like the MDA/CGA, the underlying font data never
     * changes.  The EGA, however, stores its fonts in plane 2, which means fonts are dynamic; this needs to be fixed.
     *
     * Supporting dynamic EGA fonts should not be hard though.  We can get rid of abFontData and simply build a
     * temporary snapshot of all the font bytes in plane 2 of the EGA's video buffer (adwMemory), and pass that on to
     * buildFont() instead.  We'll also need to either invalidate the existing font's color (to trigger a rebuild) or
     * pass a new "force rebuild" flag.
     *
     * Once that's done, an added benefit will be that we can build just the font(s) that have been loaded into plane 2,
     * instead of the multitude of fonts that we now build on a just-in-case basis (eg, the MDA font, the 8x8 CGA font
     * for 43-line mode, and so on).
     *
     * @this {Video}
     * @param {boolean} [fRebuild] (true if this is a rebuild, not an initial build)
     * @return {boolean} true if any or all fonts were (re)built, false if nothing changed
     */
    buildFonts(fRebuild)
    {
        var fChanges = false;

        /*
         * There's no point building fonts if this is a non-windowed (command-line) environment
         * OR no font data is available (OR this is a rebuild AND we're currently in a graphics mode).
         *
         * In other words, build fonts if this IS a windowed environment AND font data is available
         * AND this is not a rebuild, OR it IS a rebuild but we're in a graphics mode (ie, nFont is zero).
         */
        if (window && this.abFontData && (!fRebuild || this.nFont)) {

            var offSplit = 0x0000;
            var cxChar = this.cxFontChar? this.cxFontChar : 8;
            var aRGBColors = this.getCardColors();

            if (this.aFontOffsets[0] != null) {
                if (this.buildFont(Video.FONT.CGA, this.aFontOffsets[0], offSplit, cxChar, 8, this.abFontData, aRGBColors)) {
                    fChanges = true;
                }
            }

            offSplit = this.cxFontChar? 0 : 0x0800;
            cxChar = this.cxFontChar? this.cxFontChar : 9;

            if (this.aFontOffsets[1] != null) {
                if (this.buildFont(Video.FONT.MDA, this.aFontOffsets[1], offSplit, cxChar, 14, this.abFontData, Video.aMDAColors, Video.aMDAColorMap)) {
                    fChanges = true;
                }
                if (this.cxFontChar) {
                    if (this.buildFont(this.nCard, this.aFontOffsets[1], 0, this.cxFontChar, 14, this.abFontData, aRGBColors)) {
                        fChanges = true;
                    }
                }
            }
        }
        return fChanges;
    }

    /**
     * buildFont(nFont, offData, offSplit, cxChar, cyChar, abFontData, aRGBColors, aColorMap)
     *
     * This is a wrapper for createFont() which also takes care loading double-size fonts when fDoubleFont is set.
     *
     * @this {Video}
     * @param {number} nFont
     * @param {number|null} offData is the offset of the font data, null if none
     * @param {number} offSplit is the offset of any split font data, or zero if not split
     * @param {number} cxChar is the width of the font characters
     * @param {number} cyChar is the height of the font characters
     * @param {*} abFontData is the raw font data, from the ROM font file
     * @param {Array} aRGBColors is an array of color RGB variations, corresponding to supported FGND attribute values
     * @param {Array} [aColorMap] contains color indexes corresponding to attribute values (if not supplied, the mapping is assumed to be 1-1)
     * @return {boolean} true if any or all fonts were (re)built, false if nothing changed
     */
    buildFont(nFont, offData, offSplit, cxChar, cyChar, abFontData, aRGBColors, aColorMap)
    {
        var fChanges = false;

        if (offData != null) {
            if (DEBUG && this.messageEnabled()) {
                this.printMessage("buildFont(" + nFont + "): building " + Video.cardSpecs[nFont][0] + " font");
            }
            if (this.createFont(nFont, offData, offSplit, cxChar, cyChar, abFontData, aRGBColors, aColorMap)) fChanges = true;
            /*
             * If font-doubling is enabled, then load a double-size version of the font as well, as it provides
             * sharper rendering, especially when the screen cell size is a multiple of the above font cell size;
             * in the case of the CGA, this may also be useful for 40-column modes.
             */
            if (this.fDoubleFont) {
                nFont <<= 1;
                if (DEBUG && this.messageEnabled()) {
                    this.printMessage("buildFont(" + nFont + "): building " + Video.cardSpecs[nFont >> 1][0] + " double-size font");
                }
                if (this.createFont(nFont, offData, offSplit, cxChar, cyChar, abFontData, aRGBColors, aColorMap)) fChanges = true;
            }
        }
        return fChanges;
    }

    /**
     * createFont(nFont, offData, offSplit, cxChar, cyChar, abFontData, aRGBColors, aColorMap)
     *
     * All color variations are stored on the same font canvas, arranged vertically as a series of grids, where each
     * grid is a 16x16 character glyph array.
     *
     * Since every character must be drawn first with its background color and then with the foreground shape on top,
     * I used to include a series of empty cells at the top every font canvas containing all supported background colors
     * (ie, before the character grids).  But now createFont() also creates an aCSSColors array that is saved alongside
     * the font canvas, and updateChar() uses that array in conjunction with fillRect() to draw character backgrounds.
     *
     * @this {Video}
     * @param {number} nFont
     * @param {number} offData is the offset of the font data
     * @param {number} offSplit is the offset of any split font data, or zero if not split
     * @param {number} cxChar is the width of the font characters
     * @param {number} cyChar is the height of the font characters
     * @param {*} abFontData is the raw font data, from the ROM font file
     * @param {Array} aRGBColors is an array of color RGB variations, corresponding to supported FGND attribute values
     * @param {Array|undefined} aColorMap contains color indexes corresponding to attribute values (if not supplied, the mapping is assumed to be 1-1)
     * @return {boolean} true if any or all fonts were (re)created, false if nothing changed
     */
    createFont(nFont, offData, offSplit, cxChar, cyChar, abFontData, aRGBColors, aColorMap)
    {
        var fChanges = false;
        var nDouble = (nFont & 0x1)? 0 : 1;
        var font = this.aFonts[nFont];
        var nColors = (aRGBColors.length < 16? aRGBColors.length : 16);
        if (!font) {
            font = {
                cxCell:     cxChar << nDouble,
                cyCell:     cyChar << nDouble,
                aCSSColors: new Array(nColors),
                aRGBColors: aRGBColors.slice(0, nColors),   // using the Array slice() method to simply make a copy
                aColorMap:  aColorMap,
                aCanvas:    new Array(nColors)
            };
        }
        for (var iColor = 0; iColor < nColors; iColor++) {
            var rgbColor = aRGBColors[iColor];
            var rgbColorOrig = font.aCSSColors[iColor]? font.aRGBColors[iColor] : [];
            if (rgbColor[0] !== rgbColorOrig[0] || rgbColor[1] !== rgbColorOrig[1] || rgbColor[2] !== rgbColorOrig[2]) {
                if (DEBUG && this.messageEnabled()) {
                    this.printMessage("creating font color " + iColor + " for font " + nFont);
                }
                this.createFontColor(font, iColor, rgbColor, nDouble, offData, offSplit, cxChar, cyChar, abFontData);
                fChanges = true;
            }
        }
        this.aFonts[nFont] = font;
        return fChanges;
    }

    /**
     * createFontColor(font, iColor, rgbColor, nDouble, offData, offSplit, cxChar, cyChar, abFontData)
     *
     * @this {Video}
     * @param {Object} font
     * @param {number} iColor
     * @param {Array} rgbColor contains the RGB values for iColor
     * @param {number} nDouble is 1 to double output font dimensions, 0 to match input dimensions
     * @param {number} offData is the offset of the font data
     * @param {number} offSplit is the offset of any split font data, or zero if not split
     * @param {number} cxChar is the width of the font characters
     * @param {number} cyChar is the height of the font characters
     * @param {*} abFontData is the raw font data, from the ROM font file
     */
    createFontColor(font, iColor, rgbColor, nDouble, offData, offSplit, cxChar, cyChar, abFontData)
    {
        /*
         * Now we're ready to create a 16x16 character grid for the specified color.  Note that all
         * the character bits are opaque (alpha=0xff) while all the surrounding bits are transparent
         * (alpha=0x00, as specified in the 4th byte of rgbOff).
         *
         * Originally, I created 256 ImageData objects, using context.createImageData(cxChar,cyChar),
         * then setting its pixels to match those of an individual character, and then drawing characters
         * with contextFont.putImageData().  But putImageData() is relatively slow....
         *
         * Now I create a new canvas, with dimensions that allow me to arrange all 256 characters in an
         * 16x16 grid -- much like the "chargen.png" bitmap used in the C1Pjs version of the Video component.
         * Then drawing becomes much the same as before, because it turns out that drawImage() accepts either
         * an image object OR a canvas object.
         *
         * This also yields better performance, since drawImage() is much faster than putImageData().
         * We still have to use putImageData() to build the font canvas, but that's a one-time operation.
         */
        var rgbOff = [0x00, 0x00, 0x00, 0x00];
        var canvasFont = document.createElement("canvas");
        canvasFont.width = font.cxCell << 4;
        canvasFont.height = (font.cyCell << 4);
        var contextFont = canvasFont.getContext("2d");

        /*
         * See notes above regarding ImageSmoothingEnabled....
         *
         contextFont['mozImageSmoothingEnabled'] = false;
         contextFont['webkitImageSmoothingEnabled'] = false;
         */

        var iChar, x, y;
        var cyLimit = (cyChar < 8 || !offSplit)? cyChar : 8;
        var imageChar = contextFont.createImageData(font.cxCell, font.cyCell);

        for (iChar = 0; iChar < 256; iChar++) {
            for (y = 0; y < cyChar; y++) {
                /*
                 * fUnderline should be true only in the FONT_MDA case, and only for the odd color variations
                 * (1 and 3, out of variations 0 to 4), and only for the two bottom-most rows of the character cell
                 * (which I still need to confirm)
                 */
                var fUnderline = (font.aColorMap && (iColor & 0x1) && y >= cyChar - 2);
                var offChar = (y < cyLimit? offData + iChar * cyLimit + y : offSplit + iChar * cyLimit + y - cyLimit);
                var b = abFontData[offChar];
                for (var nRowDoubler = 0; nRowDoubler <= nDouble; nRowDoubler++) {
                    for (x = 0; x < cxChar; x++) {
                        /*
                         * This "bit" of logic takes care of those characters (0xC0-0xDF) whose 9th bit must mirror the 8th bit;
                         * in all other cases, any bit past the 8th bit is automatically zero.  It also takes care of embedding a solid
                         * row of bits whenever fUnderline is true.
                         *
                         * TODO: For EGA/VGA, replication of the 9th dot needs to be based on the TEXT_9DOT bit of the ATC.MODE
                         * register, which is particularly important for user-defined fonts that do not want that bit replicated.
                         */
                        var bit = (fUnderline? 1 : (b & (0x80 >> (x >= 8 && iChar >= 0xC0 && iChar <= 0xDF? 7 : x))));
                        var xDst = (x << nDouble);
                        var yDst = (y << nDouble) + nRowDoubler;
                        var rgb = (bit? rgbColor : rgbOff);
                        this.setPixel(imageChar, xDst, yDst, rgb);
                        if (nDouble) this.setPixel(imageChar, xDst + 1, yDst, rgb);
                    }
                }
            }
            /*
             * (iChar >> 4) performs the integer equivalent of Math.floor(iChar / 16), and (iChar & 0xf) is the equivalent of (iChar % 16).
             */
            contextFont.putImageData(imageChar, x = (iChar & 0xf) * font.cxCell, y = (iChar >> 4) * font.cyCell);
        }

        /*
         * The colors for cell backgrounds and cursor elements must be converted to CSS color strings.
         */
        font.aCSSColors[iColor] = "#" + Str.toHex(rgbColor[0], 2) + Str.toHex(rgbColor[1], 2) + Str.toHex(rgbColor[2], 2);
        font.aRGBColors[iColor] = rgbColor;

        /*
         * Enable this code if you want to see what the generated font looks like....
         *
        if (MAXDEBUG) {
            var iSrcColor = (iColor == 15? 0 : iColor + 1);
            this.contextScreen.fillStyle = aCSSColors[iSrcColor];
            this.contextScreen.fillRect(iColor*(font.cxCell<<2), 0, canvasFont.width>>2, font.cyCell<<4);
            this.contextScreen.drawImage(canvasFont, 0, iColor*(font.cyCell<<4), canvasFont.width>>2, font.cyCell<<4, iColor*(font.cxCell<<2), 0, canvasFont.width>>2, font.cyCell<<4);
        }
         */

        font.aCanvas[iColor] = canvasFont;
    }

    /**
     * checkBlink()
     *
     * Called at the end of every updateScreen(), which may have updated cBlinkVisible to a non-zero value.
     *
     * Also called at the end of every checkCursor(); ie, whenever the CRT register(s) affecting the position or shape
     * of the hardware cursor have been modified, and any of iCellCursor, yCursor or cyCursor have been modified as a result.
     *
     * Note that the cursor always blinks when it's ON; it can only be turned OFF, moved off-screen, or its rate set to half
     * the normal blink rate (by default, it blinks at the normal blink rate).  Bits 5-6 of the CRTC.CURSOR_START register can
     * be set as follows:
     *
     *    00: Cursor blinks at normal blink rate
     *    01: Cursor is off
     *    10: (Same as 00)
     *    11: Cursor blinks at half the normal blink rate
     *
     * According to documentation, the normal blink rate is 1/16 of the frame rate (8 frames on, 8 off).
     *
     * TODO: As an aside, I've observed in the "real world" that the MDA cursor cycles about 3 times per second, and by "cycle"
     * I mean one full off-and-on-again cycle.  I'm assuming that's the normal rate (00), not the slower "half rate" (11).
     * Since that's faster than our current cursor blink rate, we should look into an option to boost our rate, without adversely
     * affecting the attribute blink rate (which is currently hard-coded at half the cursor blink rate), and we should look into
     * supporting "half rate" blinking, too.
     *
     * @this {Video}
     * @return {boolean} true if there are things to blink, false if not
     */
    checkBlink()
    {
        if (this.cBlinkVisible > 0 || this.iCellCursor >= 0) {
            if (this.cBlinks < 0) {
                this.cBlinks = 0;
                /*
                 * At this point, we can either fire up our own timer (doBlink), or rely on updateScreen()
                 * being called by the CPU at regular bursts (eg, CPU.VIDEO_UPDATES_PER_SECOND = 60) and advance
                 * cBlinks at the start of updateScreen() accordingly.
                 *
                 * doBlink() wants to increment cBlinks every 266ms.  On the other hand, if updateScreen() is being
                 * called 60 times per second, that's about once every 16ms, so if every 16th updateScreen() increments
                 * cBlinks, cBlinks should advance at the same rate.
                 *
                 * The only downside to relying on the CPU driving our blink count is that whenever the CPU is halted
                 * (eg, by the PCjs debugger) all blinking stops -- all characters with the blink attribute AND the cursor.
                 *
                 * But we can simply say that when we halt, we mean "halt everything" (ie, call it a feature).
                 *
                 *      this.doBlink(true);
                 */
            }
            return true;
        }
        this.cBlinks = -1;
        return false;
    }

    /**
     * checkCursor()
     *
     * Called whenever a CRT data register is updated, since there are multiple registers that can affect the
     * visibility of the cursor (more than these, actually, but I'm going to limit my initial support to standard
     * ROM BIOS controller settings):
     *
     *      CRTC.MAX_SCAN
     *      CRTC.CURSOR_START
     *      CRTC.CURSOR_END
     *      CRTC.START_ADDR_HI
     *      CRTC.START_ADDR_LO
     *      CRTC.CURSOR_ADDR_HI
     *      CRTC.CURSOR_ADDR_LO
     *
     * @this {Video}
     * @return {boolean} true if the cursor is visible, false if not
     */
    checkCursor()
    {
        /*
         * The "hardware cursor" is never visible in graphics modes.
         */
        if (!this.nFont) return false;

        for (var i = Card.CRTC.CURSOR_START.INDX; i <= Card.CRTC.CURSOR_ADDR_LO; i++) {
            if (this.cardActive.regCRTData[i] == null)
                return false;
        }

        var bCursorFlags = this.cardActive.regCRTData[Card.CRTC.CURSOR_START.INDX];
        var bCursorStart = bCursorFlags & Card.CRTC.CURSOR_START.MASK;
        var bCursorEnd = this.cardActive.regCRTData[Card.CRTC.CURSOR_END.INDX] & Card.CRTC.CURSOR_END.MASK;
        var bCursorMax = this.cardActive.regCRTData[Card.CRTC.MAX_SCAN.INDX] & Card.CRTC.MAX_SCAN.MASK;

        /*
         * HACK: The original EGA BIOS has a cursor emulation bug when 43-line mode is enabled, so we attempt to detect
         * that particular combination of bad values and automatically fix them (we're so thoughtful!)
         */
        var fEGAHack = false;
        if (this.cardActive === this.cardEGA) {
            fEGAHack = true;
            if (bCursorMax == 7 && bCursorStart == 4 && !bCursorEnd) bCursorEnd = 7;
        }

        /*
         * One way of disabling the cursor is to set bit 5 (Card.CRTC.CURSOR_START.BLINKOFF) of the CRTC.CURSOR_START flags;
         * another way is setting bCursorStart > bCursorEnd (unless it's an EGA, in which case we must actually draw a
         * "split block" cursor instead).
         *
         * TODO: Verify whether the second test (bCursorStart > bCursorMax) should also result in a hidden cursor;
         * ThinkTank sets both start and end values to 0x0f, which doesn't make sense on a CGA, where the max is 0x07.
         */
        if ((bCursorFlags & Card.CRTC.CURSOR_START.BLINKOFF) || bCursorStart > bCursorEnd && !fEGAHack || bCursorStart > bCursorMax) {
            this.removeCursor();
            return false;
        }

        /*
         * The most compatible way of disabling the cursor is to simply move the cursor to an off-screen position.
         */
        var iCellCursor = (this.cardActive.regCRTData[Card.CRTC.CURSOR_ADDR_LO] + ((this.cardActive.regCRTData[Card.CRTC.CURSOR_ADDR_HI] & Card.CRTC.ADDR_HI_MASK) << 8));
        if (this.iCellCursor != iCellCursor) {
            if (MAXDEBUG && this.messageEnabled()) {
                this.printMessage("checkCursor(): cursor moved from " + this.iCellCursor + " to " + iCellCursor);
            }
            this.removeCursor();
            this.iCellCursor = iCellCursor;
        }

        /*
         * yCursor and cyCursor are no longer scaled at this point, because the necessary scaling will depend on whether we're
         * drawing the cursor to the on-screen or off-screen buffer, and updateChar() is in the best position to determine that.
         *
         * We also record cyCursorCell, the hardware cell height, since we'll need to know what the yCursor and cyCursor values
         * are relative to when it's time to scale them.
         */
        var bCursorSize = bCursorEnd - bCursorStart + 1;
        if (this.yCursor != bCursorStart || this.cyCursor != bCursorSize) {
            this.yCursor = bCursorStart;
            this.cyCursor = bCursorSize;
        }
        this.cyCursorCell = bCursorMax + 1;

        this.checkBlink();
        return true;
    }

    /**
     * removeCursor()
     *
     * @this {Video}
     */
    removeCursor()
    {
        if (this.iCellCursor >= 0) {
            if (this.aCellCache !== undefined) {
                var drawCursor = (Video.ATTRS.DRAW_CURSOR << 8);
                var data = this.aCellCache[this.iCellCursor];
                if (data & drawCursor) {
                    data &= ~drawCursor;
                    var col = this.iCellCursor % this.nCols;
                    var row = (this.iCellCursor / this.nCols)|0;
                    if (this.nFont && this.aFonts[this.nFont]) {
                        /*
                         * If we're using an off-screen buffer in text mode, then we need to keep it in sync with "reality".
                         */
                        if (this.contextBuffer) {
                            this.updateChar(col, row, data, this.contextBuffer);
                        }
                        /*
                         * While updating the on-screen canvas directly could open us up to potential subpixel artifacts again,
                         * I'm hopeful that won't be the case, since removeCursor() is called only during certain well-defined
                         * events.  The alternative to this simple updateChar() call is unappealing: redrawing the ENTIRE off-screen
                         * buffer to the on-screen canvas, just as updateScreen() does.
                         */
                        this.updateChar(col, row, data);
                    }
                    if (MAXDEBUG && this.messageEnabled()) {
                        this.printMessage("removeCursor(): removed from " + row + "," + col);
                    }
                    this.aCellCache[this.iCellCursor] = data;
                }
            }
            this.iCellCursor = -1;
        }
    }

    /**
     * getCardAccess()
     *
     * @this {Video}
     * @return {number|undefined} current memory access setting, or undefined if unknown
     */
    getCardAccess()
    {
        var nAccess;
        var card = this.cardActive;

        this.fColor256 = false;
        var regGRCMode = card.regGRCData[Card.GRC.MODE.INDX];
        if (regGRCMode != null) {
            var nReadAccess = Card.ACCESS.READ.MODE0;
            var nWriteAccess = Card.ACCESS.WRITE.MODE0;
            var nWriteMode = regGRCMode & Card.GRC.MODE.WRITE.MASK;
            var regDataRotate = card.regGRCData[Card.GRC.DATAROT.INDX] & Card.GRC.DATAROT.MASK;
            switch (nWriteMode) {
            case Card.GRC.MODE.WRITE.MODE0:
                if (regDataRotate) {
                    nWriteAccess = Card.ACCESS.WRITE.MODE0 | Card.ACCESS.WRITE.ROT;
                    switch (regDataRotate & Card.GRC.DATAROT.FUNC) {
                    case Card.GRC.DATAROT.AND:
                        nWriteAccess = Card.ACCESS.WRITE.MODE0 | Card.ACCESS.WRITE.AND;
                        break;
                    case Card.GRC.DATAROT.OR:
                        nWriteAccess = Card.ACCESS.WRITE.MODE0 | Card.ACCESS.WRITE.OR;
                        break;
                    case Card.GRC.DATAROT.XOR:
                        nWriteAccess = Card.ACCESS.WRITE.MODE0 | Card.ACCESS.WRITE.XOR;
                        break;
                    default:
                        break;
                    }
                    card.nDataRotate = regDataRotate & Card.GRC.DATAROT.COUNT;
                }
                break;
            case Card.GRC.MODE.WRITE.MODE1:
                nWriteAccess = Card.ACCESS.WRITE.MODE1;
                break;
            case Card.GRC.MODE.WRITE.MODE2:
                switch (regDataRotate & Card.GRC.DATAROT.FUNC) {
                default:
                    nWriteAccess = Card.ACCESS.WRITE.MODE2;
                    break;
                case Card.GRC.DATAROT.AND:
                    nWriteAccess = Card.ACCESS.WRITE.MODE2 | Card.ACCESS.WRITE.AND;
                    break;
                case Card.GRC.DATAROT.OR:
                    nWriteAccess = Card.ACCESS.WRITE.MODE2 | Card.ACCESS.WRITE.OR;
                    break;
                case Card.GRC.DATAROT.XOR:
                    nWriteAccess = Card.ACCESS.WRITE.MODE2 | Card.ACCESS.WRITE.XOR;
                    break;
                }
                break;
            case Card.GRC.MODE.WRITE.MODE3:
                if (this.nCard == Video.CARD.VGA) {
                    nWriteAccess = Card.ACCESS.WRITE.MODE3;
                    card.nDataRotate = regDataRotate & Card.GRC.DATAROT.COUNT;
                }
                break;
            default:
                if (DEBUG && this.messageEnabled()) {
                    this.printMessage("getCardAccess(): invalid GRC mode (" + Str.toHexByte(regGRCMode) + ")");
                }
                break;
            }
            if (regGRCMode & Card.GRC.MODE.READ.MODE1) {
                nReadAccess = Card.ACCESS.READ.MODE1;
            }
            /*
             * I discovered that when the IBM EGA ROM scrolls the screen in graphics modes 0x0D and 0x0E, it
             * reprograms this register for WRITE.MODE1 (which is fine) *and* EVENODD (which is, um, very odd).
             * Moreover, it does NOT make the complementary change to the SEQ.MEMMODE.SEQUENTIAL bit; under
             * normal circumstances, those two bits are always supposed to programmed oppositely.
             *
             * Until I can perform some tests on real hardware, I have to assume that the EGA scroll operation
             * is supposed to actually WORK in modes 0x0D and 0x0E, so I've decided to tie the trigger for my own
             * EVENODD functions to SEQ.MEMMODE.SEQUENTIAL being clear, instead of GRC.MODE.EVENODD being set.
             *
             * It's also possible that my EVENODD read/write functions are not implemented properly; when EVENODD
             * is in effect, which addresses get latched by a read, and to which addresses are latches written?
             * If EVENODD has no effect on the effective address used with the latches, then I should change the
             * EVENODD read/write functions accordingly.
             *
             * However, I've also done some limited testing with an emulated VGA running in text mode, and I've
             * discovered that toggling the GRC.MODE.EVENODD bit *alone* doesn't seem to affect the delivery of
             * text mode attributes from plane 1.  So maybe this is the wiser change after all.
             *
             * TODO: Perform some tests on actual EGA/VGA hardware, to determine the proper course of action.
             *
             *  if (regGRCMode & Card.GRC.MODE.EVENODD) {
             *      nReadAccess |= Card.ACCESS.READ.EVENODD;
             *      nWriteAccess |= Card.ACCESS.WRITE.EVENODD;
             *  }
             */
            var regSEQMode = card.regSEQData[Card.SEQ.MEMMODE.INDX];
            if (regSEQMode != null) {
                if (!(regSEQMode & Card.SEQ.MEMMODE.SEQUENTIAL)) {
                    nReadAccess |= Card.ACCESS.READ.EVENODD;
                    nWriteAccess |= Card.ACCESS.WRITE.EVENODD;
                }
                if (regGRCMode & Card.GRC.MODE.COLOR256) {
                    if (regSEQMode & Card.SEQ.MEMMODE.CHAIN4) {
                        nReadAccess |= Card.ACCESS.READ.CHAIN4;
                        nWriteAccess |= Card.ACCESS.WRITE.CHAIN4;
                    }
                    this.fColor256 = true;
                }
            }
            nAccess = nReadAccess | nWriteAccess;
        }
        return nAccess;
    }

    /**
     * setCardAccess(nAccess)
     *
     * @this {Video}
     * @param {number|undefined} nAccess (one of the Card.ACCESS.* constants)
     * @return {boolean} true if access may have changed, false if not
     */
    setCardAccess(nAccess)
    {
        var card = this.cardActive;
        if (card && nAccess != null && nAccess != card.nAccess) {

            if (MAXDEBUG && this.messageEnabled()) {
                this.printMessage("setCardAccess(" + Str.toHexWord(nAccess) + ")");
            }

            card.setMemoryAccess(nAccess);

            /*
             * Note that setMemoryAccess() can fail, in which case it will an report error, indicating either a
             * misconfiguration or some sort of internal inconsistency; in any case, there's not much we can do about
             * it at this point, other than possibly reverting the current access setting.  There's probably not much
             * point, however, because there's no guarantee that setMemoryAccess() didn't modify one or more blocks
             * before choking.
             */
            this.bus.setMemoryAccess(card.addrBuffer, card.sizeBuffer, card.getMemoryAccess(), true);
            return true;
        }
        return false;
    }

    /**
     * setDimensions()
     *
     * This is the workhorse of setMode()
     *
     * @this {Video}
     */
    setDimensions()
    {
        this.nFont = 0;
        this.nCols = this.nColsDefault;
        this.nRows = this.nRowsDefault;
        this.nColsLogical = this.nCols;
        this.nCellsPerWord = Video.aModeParms[Video.MODE.MDA_80X25][2];

        var cbPadding = 0;
        var modeParms = Video.aModeParms[this.nMode];
        if (modeParms) {

            this.nCols = modeParms[0];
            this.nRows = modeParms[1];
            this.nCellsPerWord = modeParms[2];
            cbPadding = modeParms[3];       // undefined for EGA/VGA graphics modes only
            this.nFont = modeParms[4];      // this will be undefined for all graphics modes

            if (this.nMonitorType == ChipSet.MONITOR.EGACOLOR || this.nMonitorType == ChipSet.MONITOR.VGACOLOR) {
                /*
                 * When an EGA is connected to a CGA monitor, the old aModeParms table is correct: we must
                 * use the hard-coded 8x8 "CGA_80" font.  But when it's connected to an EGA monitor, we want
                 * to use the 9x14 "EGA" color font instead.
                 *
                 * TODO: Can an EGA with a monochrome monitor be programmed for 43-line mode as well?  If so,
                 * then we'll need to load another MDA font variation, because we only load the 9x14 font for MDA.
                 */
                if (this.cardActive === this.cardEGA && this.nFont == Video.FONT.CGA) {
                    if ((this.cardEGA.regCRTData[Card.CRTC.EGA.MAX_SCAN.INDX] & Card.CRTC.EGA.MAX_SCAN.SCAN_LINE) == 7) {
                        /*
                         * Vertical resolution of 350 divided by 8 (ie, scan lines 0-7) yields 43 whole rows.
                         */
                        this.nRows = this.cardEGA.getCRTCReg(Card.CRTC.EGA.VDISP_END) < 350? 43 : 50;
                    }
                    /*
                     * Since we can also be called before any hardware registers have been initialized,
                     * it may be best to not perform the following test (which is why it's commented out).
                     */
                    else /* if (this.cardEGA.regCRTData[Card.CRTC.EGA.MAX_SCAN.INDX] == 13) */ {
                        /*
                         * Vertical resolution of 350 divided by 14 (ie, scan lines 0-13) yields exactly 25 rows.
                         *
                         * Note that a card's default font matches its card ID (eg, Video.CARD.EGA == Video.FONT.EGA,
                         * and Video.CARD.VGA == Video.FONT.VGA)
                         */
                        this.nFont = this.nCard;
                    }
                }
            }
        }

        this.nCells = (this.nCols * this.nRows)|0;
        this.nCellCache = (this.nCells / this.nCellsPerWord)|0;
        this.cbScreen = this.nCellCache;
        this.cbSplit = 0;

        if (cbPadding !== undefined) {
            this.cbScreen = ((this.cbScreen << 1) + cbPadding)|0;
            this.cbSplit = (this.cbScreen + cbPadding) >> 1;
        }

        /*
         * If no fonts were successfully loaded, there's no point in initializing the remaining drawing parameters.
         */
        if (!this.aFonts.length) return;

        this.cxScreenCell = (this.cxScreen / this.nCols)|0;
        this.cyScreenCell = (this.cyScreen / this.nRows)|0;

        /*
         * Now we make the all-important scaling determination: if the font cell dimensions (cxCell, cyCell)
         * don't match the physical screen cell dimensions (cxCell, cyCell), then we look at the caller's
         * fScaleFont setting: if it's false, we draw the characters as-is, with a border if the characters
         * are smaller than the cells; and if fScaleFont is true, we simply tell drawImage to draw the
         * characters to fit.
         *
         * WARNING: The only problem with fScaleFont is that any stretching or shrinking tends to be accompanied
         * by subpixel artifacts along the boundaries of the font images.  Definitely annoying, and apparently
         * there are no standard mechanisms for turning that behavior off. So, for now, I've "neutered" the
         * fScaleFont test slightly, by adding the "nCols == 80" test that prevents scaling from kicking in for
         * 40-column modes.
         *
         * Also, whether scaling or not, if it makes sense to use a "doubled" font, we'll switch the font as
         * well.  Note that the doubled font for any existing font also has an ID that is double the existing ID,
         * making it easy to check for the existence of a font's "double" (shift the ID left by 1).
         *
         * TODO: Since we now use an off-screen buffer for ALL modes, both text and graphics, we should
         * revisit changes that were made to work around subpixel artifacts; those should no longer be an issue.
         */
        if (this.nFont) {
            var font = this.aFonts[this.nFont];
            if (!font) {
                this.assert(false);
                return;
            }
            var fontDoubled = this.aFonts[this.nFont << 1];

            if (this.fScaleFont && this.nCols == 80) {
                if (fontDoubled) {
                    if (this.cxScreenCell >= (fontDoubled.cxCell * 3) >> 2) { // && this.cyScreenCell > (fontDoubled.cyCell * 3) >> 2) {
                        this.nFont <<= 1;
                        font = fontDoubled;
                        if (DEBUG) this.log("setDimensions(): switching to double-size font, scaled");
                    }
                }
            } else {
                if (fontDoubled) {
                    if (this.cxScreenCell >= fontDoubled.cxCell) { // && this.cyScreenCell == fontDoubled.cyCell) {
                        this.nFont <<= 1;
                        font = fontDoubled;
                        if (DEBUG) this.log("setDimensions(): switching to double-size font, unscaled");
                    }
                }
                if (!this.fScaleFont) {
                    this.cxScreenCell = font.cxCell;
                    this.cyScreenCell = font.cyCell;
                }
            }

            /*
             * In text modes, we have the option of setting all the *Buffer variables to null instead of
             * allocating them, because updateChar(), as currently written, is capable of writing characters to
             * either an off-screen or on-screen context.
             *
             *      this.imageBuffer = this.canvasBuffer = this.contextBuffer = null;
             */
            this.cxBuffer = this.cyBuffer = 0;
            if (font) {
                this.cxBuffer = this.nCols * font.cxCell;
                this.cyBuffer = this.nRows * font.cyCell;
            }
        } else {
            /*
             * CGA graphics modes have their "cells" (pixels) split evenly across two halves of the video buffer, with
             * EVEN scan lines in the first half and ODD scan lines in the second half, so unlike text modes, we can't set a
             * limit of what's visible on-screen to "columns * rows", so the screen limit is set to match the buffer limit.
             *
             * In addition, updateScreen() requires an off-screen imageData buffer that matches the size of the entire screen,
             * so that updateScreen() can set all pixels that have changed and then update the screen with a single drawImage().
             *
             * An alternative approach, with a smaller footprint, would be to allocate an off-screen buffer large enough for a
             * single scan line, and redraw one scan line at a time, but given how EVEN and ODD scan lines are spread across the
             * entire buffer, it's not clear there would be enough unchanged scan lines on average to make that approach faster.
             */
            this.cxScreenCell = this.cyScreenCell = 1;  // in graphics mode, a cell is exactly one pixel
            this.cxBuffer = this.nCols;
            this.cyBuffer = this.nRows;
        }

        /*
         * Allocate the off-screen buffers
         */
        this.imageBuffer = this.contextScreen.createImageData(this.cxBuffer, this.cyBuffer);
        this.canvasBuffer = document.createElement("canvas");
        this.canvasBuffer.width = this.cxBuffer;
        this.canvasBuffer.height = this.cyBuffer;
        this.contextBuffer = this.canvasBuffer.getContext("2d");

        /*
         * Since cxCell and cyCell were originally defined in terms of cxScreen/nCols and cyScreen/nRows, you might think
         * these border calculations would always be zero, but that would mean you overlooked the code above which tries to
         * avoid stretching 40-column modes into an unpleasantly wide shape.
         */
        this.xScreenOffset = this.yScreenOffset = 0;
        this.cxScreenOffset = this.cxScreen;
        this.cyScreenOffset = this.cyScreen;

        var cxBorder = this.cxScreen - (this.nCols * this.cxScreenCell);
        var cyBorder = this.cyScreen - (this.nRows * this.cyScreenCell);
        if (cxBorder > 0) {
            this.xScreenOffset = (cxBorder >> 1);
            this.cxScreenOffset -= cxBorder;
        }
        if (cyBorder > 0) {
            this.yScreenOffset = (cyBorder >> 1);
            this.cyScreenOffset -= cyBorder;
        }
        if (cxBorder || cyBorder) {
            this.contextScreen.fillStyle = this.canvasScreen.style.backgroundColor;
            this.contextScreen.fillRect(0, 0, this.cxScreen, this.cyScreen);
        }
    }

    /**
     * checkMode(fForce)
     *
     * Called whenever the MDA/CGA's mode register (eg, Card.MDA.MODE.PORT, Card.CGA.MODE.PORT) is updated,
     * or whenever the EGA/VGA's GRC.MISC register is updated, or when we've just finished a restore().
     *
     * @this {Video}
     * @param {boolean} [fForce] is used to force a mode update, if we recognize the current mode
     * @return {boolean} true if successful, false if not
     */
    checkMode(fForce)
    {
        var nAccess;
        var nMode = this.nMode;
        var card = this.cardActive;

        if (!card) {
            /*
             * We are likely being called after a restore(), which needs us to call setMode() to insure the proper video
             * buffer is mapped in.  So we unset this.nMode to guarantee that setMode() will be called, and if it wasn't set
             * to anything before, then we fall-back to the default mode.
             */
            this.nMode = null;
            if (nMode == null) nMode = this.nModeDefault;
        }
        else {
            if (card.nCard == Video.CARD.MDA) {
                nMode = Video.MODE.MDA_80X25;
            }
            else if (card.nCard >= Video.CARD.EGA) {
                /*
                 * The sizeBuffer we choose reflects the amount of physical address space that all 4 planes
                 * of EGA memory normally span, NOT the total amount of EGA memory.  So for a 64Kb EGA card,
                 * we would set card.sizeBuffer to 16Kb (0x4000).
                 *
                 * TODO: Need to take into account modes that "chain" planes together (eg, mode 0x0F, and
                 * presumably mode 0x10, on an EGA card with only 64Kb).
                 */
                nMode = null;
                var cbBuffer = card.cbMemory >> 2;
                var cbBufferText = (cbBuffer > 0x8000? 0x8000 : cbBuffer);

                var regGRCMisc = card.regGRCData[Card.GRC.MISC.INDX];
                if (regGRCMisc != null) {

                    switch(regGRCMisc & Card.GRC.MISC.MAPMEM) {
                    case Card.GRC.MISC.MAPA0128:
                        card.addrBuffer = 0xA0000;
                        card.sizeBuffer = cbBuffer;     // 0x20000
                        nMode = Video.MODE.UNKNOWN;     // no BIOS mode uses this mapping, but we don't want to leave nMode null if we've come this far
                        break;
                    case Card.GRC.MISC.MAPA064:
                        card.addrBuffer = 0xA0000;
                        card.sizeBuffer = cbBuffer;     // 0x10000
                        nMode = (this.nMonitorType == ChipSet.MONITOR.MONO? Video.MODE.EGA_640X350_MONO : Video.MODE.EGA_640X350);
                        break;
                    case Card.GRC.MISC.MAPB032:
                        card.addrBuffer = 0xB0000;
                        card.sizeBuffer = cbBufferText;
                        nMode = Video.MODE.MDA_80X25;
                        break;
                    case Card.GRC.MISC.MAPB832:
                        card.addrBuffer = 0xB8000;
                        card.sizeBuffer = cbBufferText;
                        nMode = (this.nMonitorType == ChipSet.MONITOR.MONO? Video.MODE.CGA_80X25_BW : Video.MODE.CGA_80X25);
                        break;
                    default:
                        break;
                    }
                    /*
                     * TODO: The following mode discrimination code is all a bit haphazard, a byproduct of its slow evolution
                     * from increasingly greater EGA support to increasingly greater VGA support.  Make it more rational someday,
                     * so that as support is added for even more modes (eg, "Mode X" variations, monochrome modes, etc), it
                     * doesn't get totally out of control.
                     *
                     * One of the problems with the current approach is that it depends on the card's registers being programmed
                     * in at least roughly the same order that the IBM EGA and VGA ROMs program them.
                     */
                    var regGRCMode = card.regGRCData[Card.GRC.MODE.INDX];

                    /*
                     * This text/graphics hybrid test detects the way Windows 95 reprograms the VGA on boot; ie, switching
                     * to graphics mode 0x13 (320x200) without disturbing the text buffer contents, then reprogramming it
                     * to enable graphics mode 0x15 (320x400), then drawing a logo in the 2nd half of the video memory, and
                     * finally reprogramming regGRCMode and regGRCMisc to move the frame buffer back to its original text mode
                     * location.
                     */
                    var fTextGraphicsHybrid = (regGRCMode & (Card.GRC.MODE.COLOR256 | Card.GRC.MODE.EVENODD)) == (Card.GRC.MODE.COLOR256 | Card.GRC.MODE.EVENODD);
                    if (fTextGraphicsHybrid) {
                        /*
                         * When fTextGraphicsHybrid is true, we should be at the end of the above process, so addrBuffer
                         * will have changed.  Since we don't (yet) assign a special mode to that configuration, we must at
                         * least set fForce to true, so that setMode() will notice the buffer address change and remap it.
                         */
                        if (card.addrBuffer != this.addrBuffer || card.sizeBuffer != this.sizeBuffer) {
                            fForce = true;
                        }
                    }

                    var nCRTCVertTotal = card.getCRTCReg(Card.CRTC.EGA.VTOTAL);
                    var nCRTCMaxScan = card.regCRTData[Card.CRTC.EGA.MAX_SCAN.INDX];
                    var nCRTCModeCtrl = card.regCRTData[Card.CRTC.EGA.MODE_CTRL.INDX];

                    var fSEQDotClock = (card.regSEQData[Card.SEQ.CLOCKING.INDX] & Card.SEQ.CLOCKING.DOTCLOCK);

                    if (nMode != Video.MODE.UNKNOWN) {
                        if (!(regGRCMisc & Card.GRC.MISC.GRAPHICS)) {
                            /*
                             * Here's where we handle text modes; since nMode will have been assigned a default
                             * of either 0x02 or 0x03, convert that to either 0x05 or 0x04 if we're in a low-res
                             * graphics mode, 0x06 otherwise.
                             */
                            nMode -= (fSEQDotClock? 2 : 0);
                        }
                        else if (card.addrBuffer != 0xA0000 && !fTextGraphicsHybrid && !(nCRTCModeCtrl & Card.CRTC.EGA.MODE_CTRL.COMPAT_MODE)) {
                            /*
                             * Here's where we handle CGA graphics modes; since nMode will have been assigned a
                             * default of either 0x02 or 0x03, convert that to either 0x05 or 0x04 if we're in a
                             * low-res graphics mode, 0x06 otherwise.
                             *
                             * For Windows 95, I've had to add BOTH the fTextGraphicsHybrid test, to avoid misdetecting
                             * the logo display mode, AND the COMPAT_MODE test, to avoid misinterpreting the VDD's physical
                             * (NOT logical) card reprogramming during windowed VM creation; the latter seems like a VDD bug,
                             * because only the Windows display driver should be *physically* reprogramming the card then.
                             */
                            nMode = fSEQDotClock? (7 - nMode) : Video.MODE.CGA_640X200;
                        } else {
                            /*
                             * Here's where we handle EGA/VGA graphics modes, discriminating among modes 0x0D and up;
                             * we've already defaulted to either 0x0F or 0x10.  If COLOR256 is set, then select mode
                             * 0x13 (or greater), else if 200-to-400 scan-line conversion is in effect, select either
                             * mode 0x0D or 0x0E, else if VGA resolution is set, select either mode 0x11 or 0x12.
                             */
                            if (card.regGRCData[Card.GRC.MODE.INDX] & Card.GRC.MODE.COLOR256) {
                                if (nCRTCMaxScan & Card.CRTC.EGA.MAX_SCAN.SCAN_LINE) {
                                    /*
                                     * NOTE: Technically, VDISP_END is one of those CRTC registers that should be read using
                                     * card.getCRTCReg(), because there are overflow bits (8 and 9).  However, all known modes
                                     * always SET bit 8 and CLEAR bit 9, so examining only bits 0-7 is sorta OK.
                                     */
                                    if (card.regCRTData[Card.CRTC.EGA.VDISP_END] <= 0x8F) {
                                        nMode = Video.MODE.VGA_320X200;
                                    }
                                    else { /* (card.regCRTData[Card.CRTC.EGA.VDISP_END] == 0xDF) */
                                        nMode = Video.MODE.VGA_320X240;
                                    }
                                } else {
                                    nMode = Video.MODE.VGA_320X400;
                                }
                            }
                            else if ((nCRTCMaxScan & Card.CRTC.EGA.MAX_SCAN.CONVERT400) || nCRTCVertTotal < 350) {
                                nMode = (fSEQDotClock? Video.MODE.EGA_320X200 : Video.MODE.EGA_640X200);
                            } else if (nCRTCVertTotal >= 480) {
                                nMode = (this.nMonitorType == ChipSet.MONITOR.MONO? Video.MODE.VGA_640X480_MONO : Video.MODE.VGA_640X480);
                            }
                            if (DEBUG && this.messageEnabled()) {
                                this.printMessage("checkMode(): nCRTCVertTotal=" + nCRTCVertTotal + ", mode=" + Str.toHexByte(nMode));
                            }
                        }
                    }
                    nAccess = this.getCardAccess();
                }
            }
            else if (card.regMode & Card.CGA.MODE.VIDEO_ENABLE) {
                /*
                 * NOTE: For the CGA, we precondition any mode change on CGA.MODE.VIDEO_ENABLE being set, otherwise
                 * we'll get spoofed by the ROM BIOS scroll code, which waits for vertical retrace and then turns CGA.MODE.VIDEO_ENABLE
                 * off, using a hard-coded mode value (0x25) that does NOT necessarily match the the CGA video mode currently in effect.
                 */
                if (!(card.regMode & Card.CGA.MODE.GRAPHIC_SEL)) {
                    nMode = ((card.regMode & Card.CGA.MODE._80X25)? Video.MODE.CGA_80X25 : Video.MODE.CGA_40X25);
                    if (card.regMode & Card.CGA.MODE.BW_SEL) nMode -= 1;
                } else {
                    nMode = ((card.regMode & Card.CGA.MODE.HIRES_BW)? Video.MODE.CGA_640X200 : Video.MODE.CGA_320X200_BW);
                    if (!(card.regMode & Card.CGA.MODE.BW_SEL)) nMode -= 1;
                }
            }
        }

        /*
         * NOTE: If setMode() remaps the video memory, that will trigger calls to setCardAccess() to also update the
         * memory's access functions.  However, if the memory access setting (nAccess) is about to change as well, those
         * changes will be moot until the setCardAccess() call that follows.  Basically, whenever both memory mapping AND
         * access functions are changing, the memory will be in an inconsistent state until both setMode() and setCardAccess()
         * are finished.
         *
         * The setMode() call takes precedence; if we called setCardAccess() first, it might attempt to modify memory access
         * functions based on the card's addrBuffer setting, and if that doesn't match what's currently mapped, assertions
         * will be triggered (probably not fatal, but it would defeat the point of the assertions).
         */
        if (!this.setMode(nMode, fForce)) return false;

        this.setCardAccess(nAccess);

        return true;
    }

    /**
     * setMode(nMode, fForce)
     *
     * Set fForce to true to update the mode regardless of previous mode, or false to perform
     * a normal update that bypasses updateScreen() but still calls initCache().
     *
     * @this {Video}
     * @param {number|null} nMode
     * @param {boolean|undefined} [fForce] is set when checkMode() wants to force a mode update
     * @return {boolean} true if successful, false if failure
     */
    setMode(nMode, fForce)
    {
        if (nMode != null && (nMode != this.nMode || fForce)) {

            if (DEBUG && this.messageEnabled()) {
                this.printMessage("setMode(" + Str.toHexByte(nMode) + (fForce? ",force" : "") + ")", true, true);
            }

            this.cUpdates = 0;      // count updateScreen() calls as a means of driving blink updates
            this.nMode = nMode;
            this.fRGBValid = false;

            /*
             * On an EGA, it's CRITICAL that a reset() invalidate cardActive, to ensure that the code below
             * releases the previous video buffer and installs a new one, even if there was no change in the
             * video buffer address or size, because otherwise the Memory blocks installed at the video buffer
             * address may still be using blocks of the EGA's previous memory buffer.
             *
             * When the EGA is reinitialized, a new memory buffer (adwMemory) is allocated (see initEGA()), and
             * this is where the mapping of that EGA memory buffer to the video buffer occurs.  Other cards
             * (MDA or CGA) don't allocate/manage their own memory buffer, but even then, it's still a good idea
             * to always force this operation (eg, in case a switch setting changed the active video card).
             */
            var card = this.cardActive || (nMode == Video.MODE.MDA_80X25? this.cardMono : this.cardColor);

            if (card != this.cardActive || card.addrBuffer != this.addrBuffer || card.sizeBuffer != this.sizeBuffer) {

                this.removeCursor();

                if (this.addrBuffer) {

                    if (DEBUG && this.messageEnabled()) {
                        this.printMessage("setMode(" + Str.toHexByte(nMode) + "): removing " + Str.toHexLong(this.sizeBuffer) + " bytes from " + Str.toHexLong(this.addrBuffer));
                    }

                    if (!this.bus.removeMemory(this.addrBuffer, this.sizeBuffer)) {
                        /*
                         * TODO: Force this failure case and see how well the Video component deals with it.
                         */
                        return false;
                    }
                    if (this.cardActive) this.cardActive.fActive = false;
                }

                this.cardActive = card;
                card.fActive = true;

                this.addrBuffer = card.addrBuffer;
                this.sizeBuffer = card.sizeBuffer;

                if (DEBUG && this.messageEnabled()) {
                    this.printMessage("setMode(" + Str.toHexByte(nMode) + "): adding " + Str.toHexLong(this.sizeBuffer) + " bytes to " + Str.toHexLong(this.addrBuffer));
                }

                var controller = (card === this.cardEGA? card : null);

                if (!this.bus.addMemory(card.addrBuffer, card.sizeBuffer, Memory.TYPE.VIDEO, controller)) {
                    /*
                     * TODO: Force this failure case and see how well the Video component deals with it.
                     */
                    return false;
                }
            }
            this.setDimensions();
            this.invalidateCache(true);
            this.updateScreen();
        }
        return true;
    }

    /**
     * setPixel(imageData, x, y, rgb)
     *
     * Worker function used by createFontColor() and updateScreen() (graphics modes only).
     *
     * @this {Video}
     * @param {Object} imageData
     * @param {number} x
     * @param {number} y
     * @param {Array.<number>} rgb is a 4-element array containing the red, green, blue and alpha values
     */
    setPixel(imageData, x, y, rgb)
    {
        var index = (x + y * imageData.width) * rgb.length;
        imageData.data[index]   = rgb[0];
        imageData.data[index+1] = rgb[1];
        imageData.data[index+2] = rgb[2];
        imageData.data[index+3] = rgb[3];
    }

    /**
     * initCache()
     *
     * Initializes the contents of our internal cell cache.
     *
     * TODO: Consider changing this to a cache of RGB values, so that when the buffer is merely being color-cycled,
     * we don't have to update the entire screen.  This will also allow invalidateCache() to honor the fModified flag,
     * bypassing initCache() when it is false.
     *
     * @this {Video}
     */
    initCache()
    {
        this.cBlinkVisible = -1;                // invalidate the visible blinking character count, to force updateScreen() to recount
        this.fCellCacheValid = false;
        var nCells = this.nCellCache;
        if (this.aCellCache === undefined || this.aCellCache.length != nCells) {
            this.aCellCache = new Array(nCells);
        }
    }

    /**
     * invalidateCache(fModified)
     *
     * Ensure that the next updateScreen() will update every cell; intended for situations where the entire screen needs
     * to be redrawn, even though the underlying data in the video buffer has not changed (and therefore cleanMemory() will
     * report that the buffer is still clean, and/or all the video data still matches everything in our cell cache).
     *
     * For example, when the palette is being cycled, the screen is being panned, the page is being flipped, etc.
     *
     * @this {Video}
     * @param {boolean} [fModified] (true if the buffer may have been modified, false if only color(s) may have changed)
     */
    invalidateCache(fModified)
    {
        if (!fModified) this.fRGBValid = false;
        this.initCache();
    }

    /**
     * updateChar(col, row, data, context)
     *
     * Updates a particular character cell (row,col) in the associated window.
     *
     * The data parameter is the attribute byte from the display buffer (fgnd attribute in the low nibble,
     * bgnd attribute in the high nibble), but updateScreen() supplements data with a couple internal attribute bits:
     *
     *      ATTRS.DRAW_FGND:    set for every cell whose fgnd element is currently on (ie, non-blinking, or whenever blink is on)
     *      ATTRS.DRAW_CURSOR:  set only for the cell containing the cursor, if any
     *
     * To make a character blink, we alternately draw its cell with ATTRS.DRAW_FGND set, and then again with
     * ATTRS.DRAW_FGND clear (meaning only the cell background is drawn).
     *
     * To make the cursor blink, we must alternately draw its entire cell with ATTRS.DRAW_CURSOR set, and then
     * draw it again with ATTRS.DRAW_CURSOR clear.
     *
     * @this {Video}
     * @param {number} col
     * @param {number} row
     * @param {number} data (if text mode, character code in low byte, attribute code in high byte)
     * @param {Object} [context]
     */
    updateChar(col, row, data, context)
    {
        /*
         * The caller MUST promise this.nFont is defined, and that the font in this.aFonts[this.nFont] has been loaded.
         */
        var bChar = data & 0xff;
        var bAttr = data >> 8;
        var iFgnd = bAttr & 0xf;
        var font = this.aFonts[this.nFont];
        if (font.aColorMap) iFgnd = font.aColorMap[iFgnd];

        /*
         * Just as aColorMap maps the foreground attribute to the appropriate foreground character grid,
         * it also maps the background attribute to the appropriate background color.
         */
        var xDst, yDst;
        var iBgnd = (bAttr >> 4) & 0xf;
        if (font.aColorMap) iBgnd = font.aColorMap[iBgnd];

        if (context) {
            xDst = col * font.cxCell;
            yDst = row * font.cyCell;
            context.fillStyle = font.aCSSColors[iBgnd];
            context.fillRect(xDst, yDst, font.cxCell, font.cyCell);
        } else {
            xDst = col * this.cxScreenCell + this.xScreenOffset;
            yDst = row * this.cyScreenCell + this.yScreenOffset;
            this.contextScreen.fillStyle = font.aCSSColors[iBgnd];
            this.contextScreen.fillRect(xDst, yDst, this.cxScreenCell, this.cyScreenCell);
        }

        if (MAXDEBUG && this.messageEnabled(Messages.VIDEO | Messages.LOG)) {
            this.log("updateCharBgnd(" + col + "," + row + "," + bChar + "): filled " + xDst + "," + yDst);
        }

        if (bAttr & Video.ATTRS.DRAW_FGND) {
            /*
             * (bChar & 0xf) is the equivalent of (bChar % 16), and (bChar >> 4) is the equivalent of Math.floor(bChar / 16)
             */
            var xSrcFgnd = (bChar & 0xf) * font.cxCell;
            var ySrcFgnd = (bChar >> 4) * font.cyCell;

            if (MAXDEBUG && this.messageEnabled(Messages.VIDEO | Messages.LOG)) {
                this.log("updateCharFgnd(" + col + "," + row + "," + bChar + "): draw from " + xSrcFgnd + "," + ySrcFgnd + " (" + font.cxCell + "," + font.cyCell + ") to " + xDst + "," + yDst);
            }

            if (context) {
                context.drawImage(font.aCanvas[iFgnd], xSrcFgnd, ySrcFgnd, font.cxCell, font.cyCell, xDst, yDst, font.cxCell, font.cyCell);
            } else {
                this.contextScreen.drawImage(font.aCanvas[iFgnd], xSrcFgnd, ySrcFgnd, font.cxCell, font.cyCell, xDst, yDst, this.cxScreenCell, this.cyScreenCell);
            }
        }

        if (bAttr & Video.ATTRS.DRAW_CURSOR) {
            /*
             * Drawing the cursor with lineTo() seemed logical, but it was complicated by the fact that the
             * TOP of the line must appear at "yDst + this.yCursor", whereas lineTo() wants to know the CENTER
             * of the line. So it's simpler to draw the cursor with another fillRect().  Here's the old code:
             *
             *      this.contextScreen.strokeStyle = font.aCSSColors[iFgnd];
             *      this.contextScreen.lineWidth = this.cyCursor;
             *      this.contextScreen.beginPath();
             *      this.contextScreen.moveTo(xDst, yDst + this.yCursor);
             *      this.contextScreen.lineTo(xDst + this.cxScreenCell, yDst + this.yCursor);
             *      this.contextScreen.stroke();
             *
             * Also, note that we're scaling the yCursor and cyCursor values here, instead of in checkCursor(), because
             * this is where we have all the required information: in the first case (off-screen buffer), the scaling must
             * be based on the font cell size (cxCell, cyCell), whereas in the second case (on-screen buffer), the scaling
             * must be based on the screen cell size (cxScreenCell,cyScreenCell).
             *
             * yCursor and cyCursor are actual hardware values, both relative to another hardware value: cyCursorCell.
             */
            var yCursor = this.yCursor;
            var cyCursor = this.cyCursor;
            if (context) {
                if (this.cyCursorCell && this.cyCursorCell !== font.cyCell) {
                    yCursor = ((yCursor * font.cyCell) / this.cyCursorCell)|0;
                    cyCursor = ((cyCursor * font.cyCell) / this.cyCursorCell)|0;
                }
                context.fillStyle = font.aCSSColors[iFgnd];
                context.fillRect(xDst, yDst + yCursor, font.cxCell, cyCursor);
            } else {
                if (this.cyCursorCell && this.cyCursorCell !== this.cyScreenCell) {
                    yCursor = ((yCursor * this.cyScreenCell) / this.cyCursorCell)|0;
                    cyCursor = ((cyCursor * this.cyScreenCell) / this.cyCursorCell)|0;
                }
                this.contextScreen.fillStyle = font.aCSSColors[iFgnd];
                this.contextScreen.fillRect(xDst, yDst + yCursor, this.cxScreenCell, cyCursor);
            }
        }
    }

    /**
     * updateScreen(fForce)
     *
     * Propagates the video buffer to the cell cache and updates the screen with any changes.  Forced updates
     * are generally internal updates triggered by an I/O operation or other state change, while non-forced updates
     * are the periodic updates coming from the CPU.
     *
     * For every cell in the video buffer, compare it to the cell stored in the cell cache, render if it differs,
     * and then update the cell cache to match.  Since initCache() sets every cell in the cell cache to an
     * invalid value, we're assured that the next call to updateScreen() will redraw the entire (visible) video buffer.
     *
     * @this {Video}
     * @param {boolean} [fForce] is used by setMode() to reset the cell cache and force a redraw
     */
    updateScreen(fForce)
    {
        /*
         * The Computer component maintains the fPowered setting on our behalf, so we use it.
         */
        if (!this.flags.powered) return;

        /*
         * If the card's video signal is disabled (eg, during a mode change), then skip the update,
         * unless fForce is set.
         */
        var fEnabled = false;
        var card = this.cardActive;

        if (card) {
            if (card !== this.cardEGA) {
                if (card.regMode & Card.CGA.MODE.VIDEO_ENABLE) fEnabled = true;
            }
            else {
                if (card.regATCIndx & Card.ATC.INDX_PAL_ENABLE) fEnabled = true;
            }
        }

        if (!fEnabled && !fForce) return;

        if (fForce) {
            this.initCache();
        }
        else {
            /*
             * This should never happen, but since updateScreen() is also called by CPU.updateVideo(),
             * better safe than sorry.
             */
            if (this.aCellCache === undefined) return;
        }

        /*
         * If cBlinks is "enabled" (ie, >= 0), then advance it once every 16 updateScreen() calls
         * (assuming an updateScreen() frequency of 60 per second; see CPU.VIDEO_UPDATES_PER_SECOND).
         *
         * We assume that the CPU is calling us whenever fForce is undefined.
         */
        var fBlinkUpdate = false;
        if (!fForce && !(++this.cUpdates & 0xf) && this.cBlinks >= 0) {
            this.cBlinks++;
            fBlinkUpdate = true;
        }

        var iCell = 0;
        var nCells = this.nCells;

        /*
         * Calculate the VISIBLE start of screen memory (addrScreen), not merely the PHYSICAL start,
         * as well as the extent of it (cbScreen) and use those values for all addressing operations to follow.
         * FYI, in these calculations, offScreen does not refer to "off-screen" memory, but rather the "offset"
         * of the start of visible screen memory.
         */
        var addrBuffer = this.addrBuffer;
        var addrScreen = addrBuffer;
        var addrScreenLimit = addrScreen + this.sizeBuffer;

        /*
         * HACK: To deal with the fTextGraphicsHybrid 320x400 mode that Windows 95 uses (ie, when the buffer
         * is mapped to B800:0000 instead of A000:0000 and is configured for text mode access, but graphics are
         * still being displayed from the second half of video memory), we must ignore the programmed address.
         *
         * In that case, the hard-coded address range below isn't actually active either, but it doesn't matter;
         * we just have to get through the rest of this function and make it to the updateScreenGraphicsVGA() call,
         * which will draw from our video buffer (adwMemory) directly; these addresses are only used for bounds
         * checking.
         */
        if (this.nMode >= Video.MODE.VGA_320X200) {
            addrBuffer = addrScreen = 0xA0000;
            addrScreenLimit = addrScreen + 0x10000;
        }

        /*
         * HACK: The CRTC's START_ADDR_HI and START_ADDR_LO registers are supposed to be "latched" into
         * offStartAddr ONLY at the start of every VRETRACE interval; this is an attempt to honor that behavior,
         * but unfortunately, updateScreen() is currently called at the CPU's discretion, not necessarily in
         * sync with nCyclesVertPeriod.  As a result, we must rely on other criteria, like the number of vertical
         * periods that have elapsed since the last CRTC write, writes to the ATC (see outATC()), etc.
         *
         * TODO: Consider matching the CPU's nCyclesNextVideoUpdate to the card's nCyclesVertPeriod, ensuring
         * that CPU bursts are in sync with VRETRACE.  Note, however, that that will be complicated by other
         * factors, such as the horizontal retrace interval, and the timing requirements of other cards in a
         * multi-display configuration.
         */
        if ((this.getRetraceBits(card) & Card.CGA.STATUS.VRETRACE) || card.nVertPeriodsStartAddr && card.nVertPeriodsStartAddr < card.nVertPeriods) {
            /*
             * PARANOIA: Don't call invalidateCache() unless the address we're about to "latch" actually changed.
             */
            var offStartAddr = ((card.regCRTData[Card.CRTC.START_ADDR_HI] << 8) + card.regCRTData[Card.CRTC.START_ADDR_LO])|0;
            if (card.offStartAddr !== offStartAddr) {
                card.offStartAddr = offStartAddr;
                this.invalidateCache();
            }
            card.nVertPeriodsStartAddr = 0;
        }

        var offScreen = card.offStartAddr;

        /*
         * Any screen (aka "page") offset must be doubled for text modes, due to the attribute bytes.
         * TODO: Come up with a more robust method of deciding when any screen offset should be doubled.
         */
        if (this.nFont) offScreen <<= 1;

        addrScreen += offScreen;
        var cbScreen = this.cbScreen;

        if (this.nCard >= Video.CARD.EGA && card.regCRTData[Card.CRTC.EGA.OFFSET] && (card.regCRTData[Card.CRTC.EGA.OFFSET] << 1) != card.regCRTData[Card.CRTC.EGA.HDISP_END] + 1) {
            /*
             * Pre-EGA, the extent of visible screen memory (cbScreen) was derived from nCols * nRows, but since
             * then, the logical width of screen memory (nColsLogical) can differ from the visible width (nCols).
             * We now calculate the logical width, and the compute a new cbScreen in much the same way the original
             * cbScreen was computed (but without any CGA-related padding considerations).
             *
             * TODO: I'm taking a lot of shortcuts in this calculation (eg, relying on nFont to detect text modes,
             * ignoring MODE_CTRL.BYTE_MODE, etc); generalize this someday.  In addition, dividing the total number of
             * cells by nCellsPerWord yields total WORDS, not BYTES, so we need to double cbScreen -- EXCEPT that the
             * notion of cell has a slightly different meaning for EGA and VGA-specific modes.  nCellsPerWord should
             * not be overloaded like that.
             */
            this.nColsLogical = card.regCRTData[Card.CRTC.EGA.OFFSET] << (this.nFont? 1 : (card.regCRTData[Card.CRTC.EGA.UNDERLINE.INDX] & Card.CRTC.EGA.UNDERLINE.DWORD)? 3 : 4);
            cbScreen = ((this.nColsLogical * (this.nRows-1) + this.nCols) / this.nCellsPerWord)|0;
            if (this.nMode <= Video.MODE.MDA_80X25) cbScreen <<= 1;
        }

        if (addrScreen + cbScreen > addrScreenLimit) {
            cbScreen = addrScreenLimit - addrScreen;
            if (cbScreen < 0) cbScreen = 0;
        }

        /*
         * addrScreenLimit was initially the limit of the entire video buffer, but we now adjust it
         * to the limit of what's visible, since that's all we want to draw.
         */
        addrScreenLimit = addrScreen + cbScreen;

        /*
         * This next bit of code can be completely disabled if we discover problems with the dirty
         * memory block tracking feature, or if we need to remove or disable that feature in the future.
         *
         * We use cleanMemory() to check the video buffer's dirty state.  If the buffer is clean
         * AND there are no visible blinking characters (as of the last updateScreen) AND there is
         * no visible cursor, then we're done; simply return.  Otherwise, if there's only a blinking
         * cursor, then update JUST that one cell.
         *
         * When dealing with blinking characters, note that we need to run through the entire buffer
         * ONLY if the low bits of the blink count just transitioned to 2 or 0; hence, we could return if
         * the blink count was ODD.  But we'd still have to worry about the cursor, so it's simpler to blow
         * that small optimization off.  Further optimizations are certainly possible, such as a hash table
         * of all blinking character locations, but all those optimizations are saved for a rainy day.
         */
        if (!fForce && this.fCellCacheValid && this.bus.cleanMemory(addrScreen, cbScreen)) {
            if (!fBlinkUpdate) return;
            if (!this.cBlinkVisible) {
                /*
                 * Note that since iCellCursor is a cell-based (not byte-based) index, we must subtract
                 * offStartAddr, which is also cell-based; subtracting offScreen would not be appropriate,
                 * as it has already been converted to a byte-based offset (remember that in text modes,
                 * cell are words, not bytes).
                 */
                iCell = this.iCellCursor - card.offStartAddr;
                /*
                 * Note that iCellCursor may have already been negative (-1 hides the cursor), and
                 * since offStartAddr should never be negative, we only need one iCell underflow check.
                 */
                if (iCell < 0) return;
                nCells = iCell + 1;
            }
            // else if (this.cBlinks & 0x1) return;
        }

        if (this.nFont) {
            /*
             * This is the text-mode update case.  We're required to FIRST verify that the current font
             * has been successfully loaded, because we're not allowed to call updateChar() if there's no font.
             */
            if (this.aFonts[this.nFont]) {
                this.updateScreenText(addrScreen, addrScreenLimit, iCell, nCells);
                this.checkBlink();
            }
        }
        else if (this.cbSplit) {
            /*
             * All CGA graphics modes have the goofy split-buffer layout, hence the simple test above.
             */
            this.updateScreenGraphicsCGA(addrScreen, addrScreenLimit);
        }
        else if (!this.fColor256) {
            /*
             * All EGA graphics modes are taken care of here, including all 16-color VGA graphics modes.
             */
            this.updateScreenGraphicsEGA(addrBuffer, addrScreen, addrScreenLimit);
        }
        else {
            /*
             * Finally, all 256-color VGA modes are processed here.
             */
            this.updateScreenGraphicsVGA(addrBuffer, addrScreen, addrScreenLimit);
        }
    }

    /**
     * updateScreenText(addrScreen, addrScreenLimit, iCell, nCells)
     *
     * @param addrScreen
     * @param addrScreenLimit
     * @param iCell
     * @param nCells
     */
    updateScreenText(addrScreen, addrScreenLimit, iCell, nCells)
    {
        var addr, data, cUpdated = 0;

        /*
         * If MDA.MODE.BLINK_ENABLE is set and a cell's blink bit is set, then if (cBlinks & 0x2) != 0,
         * we want the foreground element of the cell to be drawn; otherwise we don't.  So every 16-bit
         * data word we pull from the video buffer will be supplemented with our own special attribute bit
         * (ATTRS.DRAW_FGND = 0x100) accordingly; and to simplify the drawing code, we will also mask the
         * blink bit from the cell's attribute bits.
         *
         * If MDA.MODE.BLINK_ENABLE is clear, then we always set ATTRS.DRAW_FGND and never mask the blink
         * bit in a cell's attributes bits, since it's actually an intensity bit in that case.
         */
        this.cBlinkVisible = 0;
        var dataBlink = 0;
        var dataDraw = (Video.ATTRS.DRAW_FGND << 8);
        var dataMask = 0xfffff;

        var fBlinkEnable = (this.cardActive.regMode & Card.MDA.MODE.BLINK_ENABLE);
        if (this.nCard >= Video.CARD.EGA) {
            fBlinkEnable = (this.cardActive.regATCData[Card.ATC.MODE.INDX] & Card.ATC.MODE.BLINK_ENABLE);
        }

        /*
         * Since iCell is always relative to addrScreen, we must make iCellCursor similarly relative,
         * otherwise the cursor test below fails when the active video page is something other than page 0.
         */
        var iCellCursor = this.iCellCursor - this.cardActive.offStartAddr;

        if (fBlinkEnable) {
            dataBlink = (Video.ATTRS.BGND_BLINK << 8);
            dataMask &= ~dataBlink;
            if (!(this.cBlinks & 0x2)) dataMask &= ~dataDraw;
        }

        addr = addrScreen + (iCell << 1);
        while (addr < addrScreenLimit && iCell < nCells) {
            data = this.bus.getShortDirect(addr);
            data |= dataDraw;
            if (data & dataBlink) {
                this.cBlinkVisible++;
                data &= dataMask;
            }
            if (iCell == iCellCursor) {
                data |= ((this.cBlinks & 0x1)? (Video.ATTRS.DRAW_CURSOR << 8) : 0);
            }
            this.assert(iCell < this.aCellCache.length);
            if (!this.fCellCacheValid || data !== this.aCellCache[iCell]) {
                var col = iCell % this.nCols;
                var row = (iCell / this.nCols)|0;
                this.updateChar(col, row, data, this.contextBuffer);
                this.aCellCache[iCell] = data;
                cUpdated++;
            }
            addr += 2;
            iCell++;
        }

        this.fCellCacheValid = true;

        if (cUpdated && this.contextBuffer) {
            this.contextScreen.drawImage(this.canvasBuffer, 0, 0, this.cxBuffer, this.cyBuffer, this.xScreenOffset, this.yScreenOffset, this.cxScreenOffset, this.cyScreenOffset);
        }
    }

    /**
     * updateScreenGraphicsCGA(addrScreen, addrScreenLimit)
     *
     * @param addrScreen
     * @param addrScreenLimit
     */
    updateScreenGraphicsCGA(addrScreen, addrScreenLimit)
    {
        var addr, data;

        /*
         * This is the CGA graphics-mode update case, where cells are pixels spread across two halves of the buffer.
         */
        addr = addrScreen;
        this.cBlinkVisible = 0;
        var iCell = 0, nPixelsPerCell = this.nCellsPerWord;
        var wPixelMask = (nPixelsPerCell == 16? 0x10000 : 0x30000);
        var nPixelShift = (nPixelsPerCell == 16? 1 : 2);
        var aPixelColors = this.getCardColors(nPixelShift);

        var x = 0, y = 0;
        var xDirty = this.nCols, xMaxDirty = 0, yDirty = this.nRows, yMaxDirty = 0;
        while (addr < addrScreenLimit) {
            data = this.bus.getShortDirect(addr);
            this.assert(iCell < this.aCellCache.length);
            if (this.fCellCacheValid && data === this.aCellCache[iCell]) {
                x += nPixelsPerCell;
            } else {
                this.aCellCache[iCell] = data;
                var wPixels = (data >> 8) | ((data & 0xff) << 8);
                var wMask = wPixelMask, nShift = 16;
                if (x < xDirty) xDirty = x;
                for (var iPixel = 0; iPixel < nPixelsPerCell; iPixel++) {
                    var bPixel = (wPixels & (wMask >>= nPixelShift)) >> (nShift -= nPixelShift);
                    this.setPixel(this.imageBuffer, x++, y, aPixelColors[bPixel]);
                }
                if (x > xMaxDirty) xMaxDirty = x;
                if (y < yDirty) yDirty = y;
                if (y >= yMaxDirty) yMaxDirty = y + 1;
            }
            addr += 2;
            iCell++;
            if (x >= this.nCols) {
                x = 0;
                y += 2;
                if (y > this.nRows)
                    break;
                if (y == this.nRows) {
                    y = 1;
                    addr = addrScreen + this.cbSplit;
                }
            }
        }

        this.fCellCacheValid = true;

        /*
         * Instead of blasting the ENTIRE imageBuffer into contextBuffer, and then blasting the ENTIRE
         * canvasBuffer onto contextScreen, even for the smallest change, let's try to be a bit smarter about
         * the update (well, to the extent that the canvas APIs permit).
         */
        if (xDirty < this.nCols) {
            var cxDirty = xMaxDirty - xDirty;
            var cyDirty = yMaxDirty - yDirty;
            // this.contextBuffer.putImageData(this.imageBuffer, 0, 0);
            this.contextBuffer.putImageData(this.imageBuffer, 0, 0, xDirty, yDirty, cxDirty, cyDirty);
            /*
             * While ideally I would draw only the dirty portion of canvasBuffer, there usually isn't a 1-1 pixel mapping
             * between canvasBuffer and contextScreen.  In fact, the WHOLE POINT of the canvasBuffer is to leverage
             * drawImage()'s scaling ability; for example, a CGA graphics mode might be 640x200, whereas the canvas representing
             * the screen might be 960x400.  In those situations, if we draw interior rectangles, we often end up with subpixel
             * artifacts along the edges of those rectangles.  So it appears I must continue to redraw the entire canvasBuffer
             * on every change.
             *
            var xScreen = (((xDirty * this.cxScreen) / this.nCols) | 0);
            var yScreen = (((yDirty * this.cyScreen) / this.nRows) | 0);
            var cxScreen = (((cxDirty * this.cxScreen) / this.nCols) | 0);
            var cyScreen = (((cyDirty * this.cyScreen) / this.nRows) | 0);
            this.contextScreen.drawImage(this.canvasBuffer, xDirty, yDirty, cxDirty, cyDirty, xScreen, yScreen, cxScreen, cyScreen);
             */
            this.contextScreen.drawImage(this.canvasBuffer, 0, 0, this.nCols, this.nRows, 0, 0, this.cxScreen, this.cyScreen);
        }
    }

    /**
     * updateScreenGraphicsEGA(addrBuffer, addrScreen, addrScreenLimit)
     *
     * TODO: Add support for blinking graphics (ATC.MODE.BLINK_ENABLE)
     *
     * @param {number} addrBuffer
     * @param {number} addrScreen
     * @param {number} addrScreenLimit
     */
    updateScreenGraphicsEGA(addrBuffer, addrScreen, addrScreenLimit)
    {
        var addr, data;

        addr = addrScreen;
        this.cBlinkVisible = 0;

        var iCell = 0;
        var aPixelColors = this.getCardColors();
        var adwMemory = this.cardActive.adwMemory;

        var x = 0, y = 0;
        var xDirty = this.nCols, xMaxDirty = 0, yDirty = this.nRows, yMaxDirty = 0;

        var iPixelFirst = this.cardActive.regATCData[Card.ATC.HPAN.INDX] & Card.ATC.HPAN.SHIFT_LEFT;
        /*
         * TODO: What should happen if the card is programmed such that nColsLogical is LESS THAN nCols?
         */
        var nRowAdjust = (this.nColsLogical > this.nCols? ((this.nColsLogical - this.nCols - iPixelFirst) >> 3) : 0);

        while (addr < addrScreenLimit) {
            var idw = addr++ - addrBuffer;
            this.assert(idw >= 0 && idw < adwMemory.length);
            data = adwMemory[idw];

            /*
             * Figure out how many visible pixels this data represents; usually 8, unless panning is being used.
             */
            var iPixel, nPixels = 8;

            if (iPixelFirst) {
                /*
                 * Notice that we're not using the cell cache when panning is active, because the cached cell data no
                 * longer aligns with the data we're pulling out of the video buffer, and it's not clear that the effort
                 * to realign the data and make a valid cache comparison would save enough work to make it worthwhile.
                 */
                if (!x) {
                    data <<= iPixelFirst;
                    nPixels -= iPixelFirst;
                    /*
                     * This is as good a place as any to invalidate the cell cache when panning is active; this ensures
                     * we don't rely on stale cache contents once panning stops.
                     */
                    this.fCellCacheValid = false;
                } else {
                    iPixel = this.nCols - x;
                    if (nPixels > iPixel) nPixels = iPixel;
                }
            } else {
                this.assert(iCell < this.aCellCache.length);
                if (this.fCellCacheValid && data === this.aCellCache[iCell]) {
                    x += nPixels;
                    nPixels = 0;
                } else {
                    this.aCellCache[iCell] = data;
                }
                iCell++;
            }

            if (nPixels) {
                if (x < xDirty) xDirty = x;
                for (iPixel = 0; iPixel < nPixels; iPixel++) {
                    /*
                     * 0x80808080 may LOOK like a 32-bit value, but it is not, because JavaScript treats it as a POSITIVE
                     * number, and therefore outside the normal 32-bit integer range; however, the AND operator guarantees
                     * that the result will be a 32-bit value, so it doesn't matter.
                     */
                    var dwPixel = data & 0x80808080;
                    this.assert(Video.aEGADWToByte[dwPixel] !== undefined);
                    /*
                     * Since assertions don't fix problems (only catch them, and only in DEBUG builds), I'm also ensuring
                     * that bPixel will default to 0 if an undefined value ever slips through again.
                     *
                     * How did an undefined value slip through?  We had (incorrectly) initialized entries in aEGADWToByte;
                     * for example, we used to set aEGADWToByte[0x80808080] instead of aEGADWToByte[0x80808080|0].  The
                     * former is a POSITIVE index that is outside the 32-bit integer range, whereas the latter is a NEGATIVE
                     * index, which is what this code requires.
                     */
                    var bPixel = Video.aEGADWToByte[dwPixel] || 0;
                    this.setPixel(this.imageBuffer, x++, y, aPixelColors[bPixel]);
                    data <<= 1;
                }
                if (x > xMaxDirty) xMaxDirty = x;
                if (y < yDirty) yDirty = y;
                if (y >= yMaxDirty) yMaxDirty = y + 1;
            }

            this.assert(x <= this.nCols);

            if (x >= this.nCols) {
                x = 0;
                if (++y > this.nRows) break;
                addr += nRowAdjust;
            }
        }

        if (!iPixelFirst) this.fCellCacheValid = true;

        /*
         * For a fascinating discussion of the best way to update the screen canvas at this point, see updateScreenGraphicsCGA().
         */
        if (xDirty < this.nCols) {
            var cxDirty = xMaxDirty - xDirty;
            var cyDirty = yMaxDirty - yDirty;
            this.contextBuffer.putImageData(this.imageBuffer, 0, 0, xDirty, yDirty, cxDirty, cyDirty);
            this.contextScreen.drawImage(this.canvasBuffer, 0, 0, this.nCols, this.nRows, 0, 0, this.cxScreen, this.cyScreen);
        }
    }

    /**
     * updateScreenGraphicsVGA(addrBuffer, addrScreen, addrScreenLimit)
     *
     * This function name is a slight misnomer: updateScreenGraphicsEGA() takes care of all the 4bpp video modes
     * (first introduced by the EGA and later expanded by the VGA), where each pixel's bits are spread across the 4
     * planes, whereas this function takes care of just the 8bpp video modes introduced by the VGA, such as mode 0x13
     * (320x200x256), where each pixel's bits are contained within a single plane.  This is essentially all 256-color
     * modes (CHAIN4, "Mode X", etc), hence the hard-coded call to getCardColors(8).
     *
     * TODO: Add support for blinking graphics (ATC.MODE.BLINK_ENABLE)
     *
     * @param {number} addrBuffer
     * @param {number} addrScreen
     * @param {number} addrScreenLimit
     */
    updateScreenGraphicsVGA(addrBuffer, addrScreen, addrScreenLimit)
    {
        var addr, data;

        addr = addrScreen;
        this.cBlinkVisible = 0;

        var iCell = 0;
        var aPixelColors = this.getCardColors(8);
        var adwMemory = this.cardActive.adwMemory;

        var x = 0, y = 0;
        var xDirty = this.nCols, xMaxDirty = 0, yDirty = this.nRows, yMaxDirty = 0;

        var cbInc = (this.cardActive.regSEQData[Card.SEQ.MEMMODE.INDX] & Card.SEQ.MEMMODE.CHAIN4)? 4 : 1;
        var iPixelFirst = this.cardActive.regATCData[Card.ATC.HPAN.INDX] & Card.ATC.HPAN.SHIFT_LEFT;
        /*
         * TODO: What should happen if the card is programmed such that nColsLogical is LESS THAN nCols?
         */
        var nRowAdjust = (this.nColsLogical > this.nCols? ((this.nColsLogical - this.nCols - iPixelFirst) >> 3) : 0);

        while (addr < addrScreenLimit) {
            var idw = addr - addrBuffer;
            this.assert(idw >= 0 && idw < adwMemory.length);
            data = adwMemory[idw];

            /*
             * Figure out how many visible pixels this data represents; usually 4, unless panning is being used.
             */
            var iPixel, nPixels = 4;

            if (iPixelFirst) {
                /*
                 * TODO: Implement support for 8bpp panning
                 */
            } else {
                this.assert(iCell < this.aCellCache.length);
                if (this.fCellCacheValid && data === this.aCellCache[iCell]) {
                    x += nPixels;
                    nPixels = 0;
                } else {
                    this.aCellCache[iCell] = data;
                }
                iCell++;
            }

            if (nPixels) {
                if (x < xDirty) xDirty = x;
                for (iPixel = 0; iPixel < nPixels; iPixel++) {
                    this.setPixel(this.imageBuffer, x++, y, aPixelColors[data & 0xff]);
                    data >>= 8;
                }
                if (x > xMaxDirty) xMaxDirty = x;
                if (y < yDirty) yDirty = y;
                if (y >= yMaxDirty) yMaxDirty = y + 1;
            }

            this.assert(x <= this.nCols);

            addr += cbInc;

            if (x >= this.nCols) {
                x = 0;
                if (++y > this.nRows) break;
                addr += nRowAdjust;
            }
        }

        if (!iPixelFirst) this.fCellCacheValid = true;

        /*
         * For a fascinating discussion of the best way to update the screen canvas at this point, see updateScreenGraphicsCGA().
         */
        if (xDirty < this.nCols) {
            var cxDirty = xMaxDirty - xDirty;
            var cyDirty = yMaxDirty - yDirty;
            this.contextBuffer.putImageData(this.imageBuffer, 0, 0, xDirty, yDirty, cxDirty, cyDirty);
            this.contextScreen.drawImage(this.canvasBuffer, 0, 0, this.nCols, this.nRows, 0, 0, this.cxScreen, this.cyScreen);
        }
    }

    /**
     * getRetraceBits(card)
     *
     * This returns a byte value with two bits set or clear as appropriate: RETRACE and VRETRACE.
     *
     * @this {Video}
     * @param {Object} card
     * @return {number}
     */
    getRetraceBits(card)
    {
        var b = 0;

        /*
         * NOTE: The CGA bits CGA.STATUS.RETRACE (0x01) and CGA.STATUS.VRETRACE (0x08) match the EGA definitions,
         * and they also correspond to the MDA bits MDA.STATUS.HDRIVE (0x01) and MDA.STATUS.BWVIDEO (0x08); I'm not sure
         * why the MDA uses different designations, but the bits appear to serve the same purpose.
         *
         * TODO: Decide whether this more faithful emulation of the retrace bits should be extended to the MDA/CGA, too;
         * doing so might slow down the BIOS scroll code a bit, though.
         *
         * TODO: Verify that when a saved machine state is restored, both the CPU's cycle count AND the card's nInitCycles
         * are properly restored.  It's probably not a big deal, but details like that bother me.
         */
        var nCycles = this.cpu.getCycles();
        var nElapsedCycles = nCycles - card.nInitCycles;
        if (nElapsedCycles < 0) {           // perhaps the CPU decided to reset its cycle count?
            card.nInitCycles = nElapsedCycles;
            nElapsedCycles = -nElapsedCycles|0;
        }
        var nCyclesHorzRemain = nElapsedCycles % card.nCyclesHorzPeriod;
        if (nCyclesHorzRemain > card.nCyclesHorzActive) b |= Card.CGA.STATUS.RETRACE;
        var nCyclesVertRemain = nElapsedCycles % card.nCyclesVertPeriod;
        if (nCyclesVertRemain > card.nCyclesVertActive) b |= Card.CGA.STATUS.VRETRACE | Card.CGA.STATUS.RETRACE;
        /*
         * Some callers also want to know how many vertical retrace periods have occurred since the last time they checked,
         * so we compute that now.
         */
        card.nVertPeriods = (nElapsedCycles / card.nCyclesVertPeriod)|0;
        /*
         * The number of CPU cycles that remain in the current vertical period is all we USED to keep track of, since
         * keeping track of the total number of cycles since the card was initialized can result in an extremely large
         * delta after a while.
         *
         *      card.nInitCycles = nCycles - nCyclesVertRemain;
         *
         * HOWEVER, now that we're calling getRetraceBits() more frequently (ie, for internal retrace checks), resetting
         * nInitCycles in this fashion alters the horizontal period too much, causing grief in ROM BIOS code that requires
         * strict horizontal retrace times.  Also, the CPU reserves the right to occasionally reset its own cycle count.
         * A final complication is that nVertPeriods would no longer be accurate if we constantly reduced nInitCycles.
         *
         * None of those are insurmountable problems, but the simple solution is to never reduce nInitCycles (well, except
         * when forced to by a reduction in the CPU's cycle count).
         */
        return b;
    }

    /**
     * inMDAIndx(port, addrFrom)
     *
     * @this {Video}
     * @param {number} port (0x3B4)
     * @param {number} [addrFrom] (not defined whenever the Debugger tries to read the specified port)
     * @return {number|undefined}
     */
    inMDAIndx(port, addrFrom)
    {
        return this.inCRTCIndx(this.cardMono, port, addrFrom);
    }

    /**
     * outMDAIndx(port, bOut, addrFrom)
     *
     * @this {Video}
     * @param {number} port (0x3B4)
     * @param {number} bOut
     * @param {number} [addrFrom] (not defined whenever the Debugger tries to read the specified port)
     */
    outMDAIndx(port, bOut, addrFrom)
    {
        this.outCRTCIndx(this.cardMono, port, bOut, addrFrom);
    }

    /**
     * inMDAData(port, addrFrom)
     *
     * @this {Video}
     * @param {number} port (0x3B5)
     * @param {number} [addrFrom] (not defined whenever the Debugger tries to read the specified port)
     * @return {number|undefined}
     */
    inMDAData(port, addrFrom)
    {
        return this.inCRTCData(this.cardMono, port, addrFrom);
    }

    /**
     * outMDAData(port, bOut, addrFrom)
     *
     * @this {Video}
     * @param {number} port (0x3B5)
     * @param {number} bOut
     * @param {number} [addrFrom] (not defined whenever the Debugger tries to read the specified port)
     */
    outMDAData(port, bOut, addrFrom)
    {
        this.outCRTCData(this.cardMono, port, bOut, addrFrom);
    }

    /**
     * inMDAMode(port, addrFrom)
     *
     * @this {Video}
     * @param {number} port (0x3B8)
     * @param {number} [addrFrom] (not defined whenever the Debugger tries to read the specified port)
     * @return {number}
     */
    inMDAMode(port, addrFrom)
    {
        return this.inCardMode(this.cardMono, addrFrom);
    }

    /**
     * outMDAMode(port, bOut, addrFrom)
     *
     * @this {Video}
     * @param {number} port (0x3B8)
     * @param {number} bOut
     * @param {number} [addrFrom] (not defined whenever the Debugger tries to read the specified port)
     */
    outMDAMode(port, bOut, addrFrom)
    {
        this.outCardMode(this.cardMono, bOut, addrFrom);
    }

    /**
     * inMDAStatus(port, addrFrom)
     *
     * @this {Video}
     * @param {number} port (0x3BA)
     * @param {number} [addrFrom] (not defined whenever the Debugger tries to read the specified port)
     * @return {number}
     */
    inMDAStatus(port, addrFrom)
    {
        return this.inCardStatus(this.cardMono, addrFrom);
    }

    /**
     * outFeat(port, bOut, addrFrom)
     *
     * @this {Video}
     * @param {number} port (0x3BA or 0x3DA)
     * @param {number} bOut
     * @param {number} [addrFrom] (not defined whenever the Debugger tries to read the specified port)
     *
     * NOTE: While this port also existed on the MDA and CGA, it existed only as an INPUT port, not an OUTPUT port.
     */
    outFeat(port, bOut, addrFrom)
    {
        this.cardEGA.regFeat = (this.cardEGA.regFeat & ~Card.FEAT_CTRL.BITS) | (bOut & Card.FEAT_CTRL.BITS);
        this.printMessageIO(port, bOut, addrFrom, "FEAT");
    }

    /**
     * inATCIndx(port, addrFrom)
     *
     * Technically, port 0x3C0 is readable only on a VGA, but we allow reads on an EGA as well,
     * primarily for debugging purposes.  Moreover, ATC port reads do NOT toggle the ATC address/data
     * flip-flop; only writes have that effect.
     *
     * @this {Video}
     * @param {number} port (0x3C0)
     * @param {number} [addrFrom] (not defined whenever the Debugger tries to read the specified port)
     * @return {number}
     */
    inATCIndx(port, addrFrom)
    {
        var b = this.cardEGA.regATCIndx;
        if (!addrFrom || this.messageEnabled()) {
            this.printMessageIO(Card.ATC.PORT, null, addrFrom, "ATC.INDX", b);
        }
        return b;
    }

    /**
     * inATCData(port, addrFrom)
     *
     * Technically, port 0x3C0 is readable only on a VGA, but we allow reads on an EGA as well,
     * primarily for debugging purposes.  Moreover, ATC port reads do NOT toggle the ATC address/data
     * flip-flop; only writes have that effect.
     *
     * @this {Video}
     * @param {number} port (0x3C1)
     * @param {number} [addrFrom] (not defined whenever the Debugger tries to read the specified port)
     * @return {number}
     */
    inATCData(port, addrFrom)
    {
        var b = this.cardEGA.regATCData[this.cardEGA.regATCIndx & Card.ATC.INDX_MASK];
        if (!addrFrom || this.messageEnabled()) {
            this.printMessageIO(Card.ATC.PORT, null, addrFrom, "ATC." + this.cardEGA.asATCRegs[this.cardEGA.regATCIndx & Card.ATC.INDX_MASK], b);
        }
        return b;
    }

    /**
     * outATC(port, bOut, addrFrom)
     *
     * @this {Video}
     * @param {number} port (0x3C0)
     * @param {number} bOut
     * @param {number} [addrFrom] (not defined whenever the Debugger tries to read the specified port)
     */
    outATC(port, bOut, addrFrom)
    {
        var card = this.cardEGA;
        var fPalEnabled = (card.regATCIndx & Card.ATC.INDX_PAL_ENABLE);
        if (!card.fATCData) {
            card.regATCIndx = bOut;
            this.printMessageIO(port, bOut, addrFrom, "ATC.INDX");
            card.fATCData = true;
            if ((bOut & Card.ATC.INDX_PAL_ENABLE) && !fPalEnabled) {
                if (!this.buildFonts(true)) {
                    if (DEBUG && (!addrFrom || this.messageEnabled())) {
                        this.printMessage("outATC(" + Str.toHexByte(bOut) + "): no font changes required");
                    }
                } else {
                    if (DEBUG && (!addrFrom || this.messageEnabled())) {
                        this.printMessage("outATC(" + Str.toHexByte(bOut) + "): redraw screen for font changes");
                    }
                    this.updateScreen(true);
                }
            }
            else {
                /*
                 * TODO: We might want a screen blanking function, suitable for any mode, when INDX_PAL_ENABLE is cleared.
                 * powerDown() might like to use such a function, too.  updateScreen() already disables any further screen
                 * updates while INDX_PAL_ENABLE is clear (except when fForce is true), but that's all we currently do.
                 *
                 *      if (!(bOut & Card.ATC.INDX_PAL_ENABLE) && fPalEnabled) this.blankScreen();
                 *
                 * However, there also needs to be a delay, because when the IBM VGA BIOS changes the mode, it updates
                 * the ATC palette registers in such a way that INDX_PAL_ENABLE is constantly toggled; here's one iteration:
                 *
                 *      C000:2B39 EC              IN       AL,DX
                 *      C000:2B3A B2C0            MOV      DL,C0
                 *      C000:2B3C 8BC3            MOV      AX,BX
                 *      C000:2B3E 86C4            XCHG     AL,AH
                 *      C000:2B40 EE              OUT      DX,AL    <-- this ATC index value does NOT contain 0x20
                 *      C000:2B41 86C4            XCHG     AL,AH
                 *      C000:2B43 EE              OUT      DX,AL
                 *      C000:2B44 B020            MOV      AL,20
                 *      C000:2B46 EE              OUT      DX,AL    <-- this ATC index value obviously DOES contain 0x20
                 *
                 * I'm not sure there are any situations where deliberately flickering the screen is a good thing -- unless
                 * someone REALLY wants to recreate the ugly flickering scroll of a CGA...?
                 */
            }
            /*
             * HACK: offStartAddr is supposed to be "latched" ONLY at the start of every VRETRACE interval, but
             * other "triggers" are helpful; see updateScreen() for details.
             *
             * PARANOIA: Don't call invalidateCache() unless the start address we just "latched" actually changed.
             */
            var offStartAddr = ((card.regCRTData[Card.CRTC.START_ADDR_HI] << 8) + card.regCRTData[Card.CRTC.START_ADDR_LO])|0;
            if (card.offStartAddr != offStartAddr) {
                card.offStartAddr = offStartAddr;
                this.invalidateCache();
            }
            card.nVertPeriodsStartAddr = 0;
        } else {
            card.fATCData = false;
            var iReg = card.regATCIndx & Card.ATC.INDX_MASK;
            if (iReg >= Card.ATC.PALETTE_REGS || !fPalEnabled) {
                if (Video.TRAPALL || card.regATCData[iReg] !== bOut) {
                    if (!addrFrom || this.messageEnabled()) {
                        this.printMessageIO(port, bOut, addrFrom, "ATC." + card.asATCRegs[iReg]);
                    }
                    card.regATCData[iReg] = bOut;
                    this.invalidateCache(false);
                }
            }
        }
    }

    /**
     * inStatus0(port, addrFrom)
     *
     * @this {Video}
     * @param {number} port (0x3C2)
     * @param {number} [addrFrom] (not defined whenever the Debugger tries to read the specified port)
     * @return {number}
     */
    inStatus0(port, addrFrom)
    {
        var bSWBit = 0;
        if (this.nCard == Video.CARD.EGA) {
            var iBit = 3 - ((this.cardEGA.regMisc & Card.MISC.CLOCK_SELECT) >> 2);    // this is the desired SW # (0-3)
            bSWBit = (this.bEGASwitches & (1 << iBit)) << (Card.STATUS0.SWSENSE_SHIFT - iBit);
        } else {
            /*
             * The IBM VGA ROM expects the SWSENSE bit to change according to how the DAC is programmed.
             *
             * At C000:0391, the ROM selects the following array at 0x0454:
             *
             *      db  0x12,0x12,0x12,0x10
             *
             * and writes the first 3 bytes to DAC register #0, and then compares SWSENSE to the 4th byte (0x10).
             *
             * If the 4th byte matches, then the ROM clears the BIOS "monochrome monitor" bit, and does the same
             * thing again with 5 more arrays, expecting the 4th byte in all 5 arrays to match SWSENSE, and being
             * very unhappy if they don't:
             *
             *      db  0x14,0x14,0x14,0x10
             *      db  0x2D,0x14,0x14,0x00
             *      db  0x14,0x2D,0x14,0x00
             *      db  0x14,0x14,0x2D,0x00
             *      db  0x2D,0x2D,0x2D,0x00
             *
             * I ensure much happiness by setting SWSENSE unless any of the three 6-bit DAC values contain 0x2D.
             *
             * This hard-coded behavior assumes a color monitor.  If you really want to simulate a monochrome monitor,
             * then the 1st array (above) must mismatch, and a different set of arrays must all match:
             *
             *      db  0x04,0x12,0x04,0x10
             *      db  0x1E,0x12,0x04,0x00
             *      db  0x04,0x2D,0x04,0x00
             *      db  0x04,0x16,0x15,0x00
             *      db  0x00,0x00,0x00,0x10
             *
             * In other words, for a monochrome monitor, set SWSENSE only when DAC register #0 matches the first and last
             * sets of values.
             */
            var dwDAC = this.cardEGA.regDACData[0];
            if ((dwDAC & 0x3f) != 0x2d && (dwDAC & (0x3f << 6)) != (0x2d << 6) && (dwDAC & (0x3f << 12)) != (0x2d << 12)) {
                bSWBit |= Card.STATUS0.SWSENSE;
            }
        }
        var b = ((this.cardEGA.regStatus0 & ~Card.STATUS0.SWSENSE) | bSWBit);
        /*
         * TODO: Figure out where Card.STATUS0.FEAT bits should come from....
         */
        this.cardEGA.regStatus0 = b;
        this.printMessageIO(Card.STATUS0.PORT, null, addrFrom, "STATUS0", b);
        return b;
    }

    /**
     * @this {Video}
     * @param {number} port (0x3C2)
     * @param {number} bOut
     * @param {number} [addrFrom] (not defined whenever the Debugger tries to read the specified port)
     */
    outMisc(port, bOut, addrFrom)
    {
        this.cardEGA.regMisc = bOut;
        this.enableEGA();
        this.printMessageIO(Card.MISC.PORT_WRITE, bOut, addrFrom, "MISC");
    }

    /**
     * inVGAEnable(port, addrFrom)
     *
     * @this {Video}
     * @param {number} port (0x3C3)
     * @param {number} [addrFrom] (not defined whenever the Debugger tries to read the specified port)
     * @return {number}
     */
    inVGAEnable(port, addrFrom)
    {
        var b = this.cardEGA.regVGAEnable;
        this.printMessageIO(Card.VGA_ENABLE.PORT, null, addrFrom, "VGA_ENABLE", b);
        return b;
    }

    /**
     * outVGAEnable(port, bOut, addrFrom)
     *
     * @this {Video}
     * @param {number} port (0x3C3)
     * @param {number} bOut
     * @param {number} [addrFrom] (not defined whenever the Debugger tries to read the specified port)
     */
    outVGAEnable(port, bOut, addrFrom)
    {
        this.cardEGA.regVGAEnable = bOut;
        this.printMessageIO(Card.VGA_ENABLE.PORT, bOut, addrFrom, "VGA_ENABLE");
    }

    /**
     * inSEQIndx(port, addrFrom)
     *
     * @this {Video}
     * @param {number} port (0x3C4)
     * @param {number} [addrFrom] (not defined whenever the Debugger tries to read the specified port)
     * @return {number}
     */
    inSEQIndx(port, addrFrom)
    {
        var b = this.cardEGA.regSEQIndx;
        this.printMessageIO(Card.SEQ.INDX.PORT, null, addrFrom, "SEQ.INDX", b);
        return b;
    }

    /**
     * outSEQIndx(port, bOut, addrFrom)
     *
     * @this {Video}
     * @param {number} port (0x3C4)
     * @param {number} bOut
     * @param {number} [addrFrom] (not defined whenever the Debugger tries to read the specified port)
     */
    outSEQIndx(port, bOut, addrFrom)
    {
        this.cardEGA.regSEQIndx = bOut;
        this.printMessageIO(Card.SEQ.INDX.PORT, bOut, addrFrom, "SEQ.INDX");
    }

    /**
     * inSEQData(port, addrFrom)
     *
     * @this {Video}
     * @param {number} port (0x3C5)
     * @param {number} [addrFrom] (not defined whenever the Debugger tries to read the specified port)
     * @return {number}
     */
    inSEQData(port, addrFrom)
    {
        var b = this.cardEGA.regSEQData[this.cardEGA.regSEQIndx];
        if (!addrFrom || this.messageEnabled()) {
            this.printMessageIO(Card.SEQ.DATA.PORT, null, addrFrom, "SEQ." + this.cardEGA.asSEQRegs[this.cardEGA.regSEQIndx], b);
        }
        return b;
    }

    /**
     * outSEQData(port, bOut, addrFrom)
     *
     * @this {Video}
     * @param {number} port (0x3C5)
     * @param {number} bOut
     * @param {number} [addrFrom] (not defined whenever the Debugger tries to read the specified port)
     */
    outSEQData(port, bOut, addrFrom)
    {
        if (Video.TRAPALL || this.cardEGA.regSEQData[this.cardEGA.regSEQIndx] !== bOut) {
            if (!addrFrom || this.messageEnabled()) {
                this.printMessageIO(Card.SEQ.DATA.PORT, bOut, addrFrom, "SEQ." + this.cardEGA.asSEQRegs[this.cardEGA.regSEQIndx]);
            }
            this.cardEGA.regSEQData[this.cardEGA.regSEQIndx] = bOut;
        }
        switch(this.cardEGA.regSEQIndx) {
        case Card.SEQ.MAPMASK.INDX:
            this.cardEGA.nSeqMapMask = Video.aEGAByteToDW[bOut & Card.SEQ.MAPMASK.MAPS];
            break;
        case Card.SEQ.MEMMODE.INDX:
            if (this.setCardAccess(this.getCardAccess())) {
                /*
                 * When switching screens (via SysReq) on early revisions of OS/2 (eg, FOOTBALL), the screen would go
                 * blank; this appeared to be because when the card is reprogrammed, we first think the card is going into
                 * graphics mode, then we reverse course when it becomes clear that the card is going back into text mode,
                 * but unfortunately, at that precise moment, the Sequencer hasn't been fully reprogrammed, so when we're
                 * reading screen memory, we're getting back ZEROS for every odd byte (which are the text attribute bytes),
                 * so the screen is redrawn as black-on-black.
                 *
                 * My solution was to change setCardAccess() to indicate whether it actually altered the video buffer
                 * address and/or format, and if so, then force another screen update.
                 *
                 * TODO: This scenario does not seem unique; it suggests that we should generally force a screen update
                 * whenever the video buffer has undergone a significant change.
                 *
                 * UPDATE: This change was NOT sufficient to resolve the OS/2 screen-switching bug described above; in fact,
                 * it's apparently not even necessary, because the REAL problem was caused by PAGED blocks with stale
                 * physical video memory blocks; the solution was for the Bus addMemory() and removeMemory() functions to
                 * call the the CPU flushPageBlocks() function.  With that change in place, the window now stays in sync
                 * with the buffer.
                 *
                 * However, calling updateScreen() here still seems like a good idea, and it shouldn't hurt performance,
                 * since we're doing it only when setCardAccess() indicates a change, so I'm leaving this addition in place.
                 */
                this.updateScreen(true);
            }
            break;
        default:
            break;
        }
    }

    /**
     * inDACMask(port, addrFrom)
     *
     * @this {Video}
     * @param {number} port (0x3C6)
     * @param {number} [addrFrom] (not defined whenever the Debugger tries to read the specified port)
     * @return {number}
     */
    inDACMask(port, addrFrom)
    {
        var b = this.cardEGA.regDACMask;
        if (!addrFrom || this.messageEnabled()) {
            this.printMessageIO(Card.DAC.MASK.PORT, null, addrFrom, "DAC.MASK", b);
        }
        return b;
    }

    /**
     * outDACMask(port, bOut, addrFrom)
     *
     * @this {Video}
     * @param {number} port (0x3C6)
     * @param {number} bOut
     * @param {number} [addrFrom] (not defined whenever the Debugger tries to read the specified port)
     */
    outDACMask(port, bOut, addrFrom)
    {
        if (Video.TRAPALL || this.cardEGA.regDACMask !== bOut) {
            if (!addrFrom || this.messageEnabled()) {
                this.printMessageIO(Card.DAC.MASK.PORT, bOut, addrFrom, "DAC.MASK");
            }
            this.cardEGA.regDACMask = bOut;
        }
    }

    /**
     * inDACState(port, addrFrom)
     *
     * @this {Video}
     * @param {number} port (0x3C7)
     * @param {number} [addrFrom] (not defined whenever the Debugger tries to read the specified port)
     * @return {number}
     */
    inDACState(port, addrFrom)
    {
        var b = this.cardEGA.regDACState;
        if (!addrFrom || this.messageEnabled()) {
            this.printMessageIO(Card.DAC.STATE.PORT, null, addrFrom, "DAC.STATE", b);
        }
        return b;
    }

    /**
     * outDACRead(port, bOut, addrFrom)
     *
     * @this {Video}
     * @param {number} port (0x3C7)
     * @param {number} bOut
     * @param {number} [addrFrom] (not defined whenever the Debugger tries to read the specified port)
     */
    outDACRead(port, bOut, addrFrom)
    {
        if (!addrFrom || this.messageEnabled()) {
            this.printMessageIO(Card.DAC.ADDR.PORT_READ, bOut, addrFrom, "DAC.READ");
        }
        this.cardEGA.regDACAddr = bOut;
        this.cardEGA.regDACState = Card.DAC.STATE.MODE_READ;
        this.cardEGA.regDACShift = 0;
    }

    /**
     * outDACWrite(port, bOut, addrFrom)
     *
     * @this {Video}
     * @param {number} port (0x3C8)
     * @param {number} bOut
     * @param {number} [addrFrom] (not defined whenever the Debugger tries to read the specified port)
     */
    outDACWrite(port, bOut, addrFrom)
    {
        if (!addrFrom || this.messageEnabled()) {
            this.printMessageIO(Card.DAC.ADDR.PORT_WRITE, bOut, addrFrom, "DAC.WRITE");
        }
        this.cardEGA.regDACAddr = bOut;
        this.cardEGA.regDACState = Card.DAC.STATE.MODE_WRITE;
        this.cardEGA.regDACShift = 0;
    }

    /**
     * inDACData(port, addrFrom)
     *
     * @this {Video}
     * @param {number} port (0x3C9)
     * @param {number} [addrFrom] (not defined whenever the Debugger tries to read the specified port)
     * @return {number}
     */
    inDACData(port, addrFrom)
    {
        var b = (this.cardEGA.regDACData[this.cardEGA.regDACAddr] >> this.cardEGA.regDACShift) & 0x3f;
        if (!addrFrom || this.messageEnabled()) {
            this.printMessageIO(Card.DAC.DATA.PORT, null, addrFrom, "DAC.DATA[" + Str.toHexByte(this.cardEGA.regDACAddr) + "][" + Str.toHexByte(this.cardEGA.regDACShift) + "]", b);
        }
        this.cardEGA.regDACShift += 6;
        if (this.cardEGA.regDACShift > 12) {
            this.cardEGA.regDACShift = 0;
            this.cardEGA.regDACAddr = (this.cardEGA.regDACAddr + 1) & (Card.DAC.TOTAL_REGS-1);
        }
        return b;
    }

    /**
     * outDACData(port, bOut, addrFrom)
     *
     * @this {Video}
     * @param {number} port (0x3C9)
     * @param {number} bOut
     * @param {number} [addrFrom] (not defined whenever the Debugger tries to read the specified port)
     */
    outDACData(port, bOut, addrFrom)
    {
        var dw = this.cardEGA.regDACData[this.cardEGA.regDACAddr];
        if (!addrFrom || this.messageEnabled()) {
            this.printMessageIO(Card.DAC.DATA.PORT, bOut, addrFrom, "DAC.DATA[" + Str.toHexByte(this.cardEGA.regDACAddr) + "][" + Str.toHexByte(this.cardEGA.regDACShift) + "]");
        }
        var dwNew = (dw & ~(0x3f << this.cardEGA.regDACShift)) | ((bOut & 0x3f) << this.cardEGA.regDACShift);
        if (dw !== dwNew) {
            this.cardEGA.regDACData[this.cardEGA.regDACAddr] = dwNew;
            this.invalidateCache(false);
        }
        this.cardEGA.regDACShift += 6;
        if (this.cardEGA.regDACShift > 12) {
            this.cardEGA.regDACShift = 0;
            this.cardEGA.regDACAddr = (this.cardEGA.regDACAddr + 1) & (Card.DAC.TOTAL_REGS-1);
        }
    }

    /**
     * inVGAFeat(port, addrFrom)
     *
     * @this {Video}
     * @param {number} port (0x3CA)
     * @param {number} [addrFrom] (not defined whenever the Debugger tries to read the specified port)
     * @return {number}
     */
    inVGAFeat(port, addrFrom)
    {
        var b = this.cardEGA.regFeat;
        this.printMessageIO(Card.FEAT_CTRL.PORT_READ, null, addrFrom, "FEAT", b);
        return b;
    }

    /**
     * outGRCPos2(port, bOut, addrFrom)
     *
     * "The EGA was originally implemented by IBM using two Graphics Controller Chips. This register is used to program
     * the Graphics #2 chip. See the Graphics #1 Position Register for details."
     *
     * "A one should be loaded into this location to map host data bus bits 2 and 3 to display planes 2 and 3, respectively."
     *
     * @this {Video}
     * @param {number} port (0x3CA)
     * @param {number} bOut
     * @param {number} [addrFrom] (not defined whenever the Debugger tries to read the specified port)
     */
    outGRCPos2(port, bOut, addrFrom)
    {
        this.cardEGA.regGRCPos2 = bOut;
        this.printMessageIO(Card.GRC.POS2_PORT, bOut, addrFrom, "GRC2");
    }

    /**
     * inVGAMisc(port, addrFrom)
     *
     * @this {Video}
     * @param {number} port (0x3CC)
     * @param {number} [addrFrom] (not defined whenever the Debugger tries to read the specified port)
     * @return {number}
     */
    inVGAMisc(port, addrFrom)
    {
        var b = this.cardEGA.regMisc;
        this.printMessageIO(Card.MISC.PORT_READ, null, addrFrom, "MISC", b);
        return b;
    }

    /**
     * outGRCPos1(port, bOut, addrFrom)
     *
     * "The EGA was originally implemented by IBM using two Graphics Controller Chips. It was necessary to program
     * each to respond to a different set of two consecutive bits of the 8-bit host data bus. In the IBM EGA implementation,
     * a 0 must be loaded into this register. In the VGA, there is no analogous register."
     *
     * "A zero should be loaded into this location to map host data bus bits 0 and 1 to display planes 0 and 1 respectively."
     *
     * Note that this register was not readable on the EGA, and when the VGA came along, reads of this port read the Misc reg.
     *
     * @this {Video}
     * @param {number} port (0x3CC)
     * @param {number} bOut
     * @param {number} [addrFrom] (not defined whenever the Debugger tries to read the specified port)
     */
    outGRCPos1(port, bOut, addrFrom)
    {
        this.cardEGA.regGRCPos1 = bOut;
        this.printMessageIO(Card.GRC.POS1_PORT, bOut, addrFrom, "GRC1");
    }

    /**
     * inGRCIndx(port, addrFrom)
     *
     * @this {Video}
     * @param {number} port (0x3CE)
     * @param {number} [addrFrom] (not defined whenever the Debugger tries to read the specified port)
     * @return {number}
     */
    inGRCIndx(port, addrFrom)
    {
        var b = this.cardEGA.regGRCIndx;
        this.printMessageIO(Card.GRC.INDX.PORT, null, addrFrom, "GRC.INDX", b);
        return b;
    }

    /**
     * outGRCIndx(port, bOut, addrFrom)
     *
     * @this {Video}
     * @param {number} port (0x3CE)
     * @param {number} bOut
     * @param {number} [addrFrom] (not defined whenever the Debugger tries to read the specified port)
     */
    outGRCIndx(port, bOut, addrFrom)
    {
        this.cardEGA.regGRCIndx = bOut;
        this.printMessageIO(Card.GRC.INDX.PORT, bOut, addrFrom, "GRC.INDX");
    }

    /**
     * inGRCData(port, addrFrom)
     *
     * @this {Video}
     * @param {number} port (0x3CF)
     * @param {number} [addrFrom] (not defined whenever the Debugger tries to read the specified port)
     * @return {number}
     */
    inGRCData(port, addrFrom)
    {
        var b = this.cardEGA.regGRCData[this.cardEGA.regGRCIndx];
        if (!addrFrom || this.messageEnabled()) {
            this.printMessageIO(Card.GRC.DATA.PORT, null, addrFrom, "GRC." + this.cardEGA.asGRCRegs[this.cardEGA.regGRCIndx], b);
        }
        return b;
    }

    /**
     * outGRCData(port, bOut, addrFrom)
     *
     * @this {Video}
     * @param {number} port (0x3CF)
     * @param {number} bOut
     * @param {number} [addrFrom] (not defined whenever the Debugger tries to read the specified port)
     */
    outGRCData(port, bOut, addrFrom)
    {
        if (Video.TRAPALL || this.cardEGA.regGRCData[this.cardEGA.regGRCIndx] !== bOut) {
            if (!addrFrom || this.messageEnabled()) {
                this.printMessageIO(Card.GRC.DATA.PORT, bOut, addrFrom, "GRC." + this.cardEGA.asGRCRegs[this.cardEGA.regGRCIndx]);
            }
            this.cardEGA.regGRCData[this.cardEGA.regGRCIndx] = bOut;
        }
        switch(this.cardEGA.regGRCIndx) {
        case Card.GRC.SRESET.INDX:
            this.cardEGA.nSetMapData = Video.aEGAByteToDW[bOut & 0xf];
            this.cardEGA.nSetMapBits = this.cardEGA.nSetMapData & ~this.cardEGA.nSetMapMask;
            break;
        case Card.GRC.ESRESET.INDX:
            this.cardEGA.nSetMapMask = ~Video.aEGAByteToDW[bOut & 0xf];
            this.cardEGA.nSetMapBits = this.cardEGA.nSetMapData & ~this.cardEGA.nSetMapMask;
            break;
        case Card.GRC.COLORCMP.INDX:
            this.cardEGA.nColorCompare = Video.aEGAByteToDW[bOut & 0xf] & (0x80808080|0);
            break;
        case Card.GRC.DATAROT.INDX:
        case Card.GRC.MODE.INDX:
            this.setCardAccess(this.getCardAccess());
            break;
        case Card.GRC.READMAP.INDX:
            this.cardEGA.nReadMapShift = (bOut & Card.GRC.READMAP.NUM) << 3;
            break;
        case Card.GRC.MISC.INDX:
            this.checkMode(false);
            break;
        case Card.GRC.COLORDC.INDX:
            this.cardEGA.nColorDontCare = Video.aEGAByteToDW[bOut & 0xf] & (0x80808080|0);
            break;
        case Card.GRC.BITMASK.INDX:
            this.cardEGA.nBitMapMask = bOut | (bOut << 8) | (bOut << 16) | (bOut << 24);
            break;
        default:
            break;
        }
    }

    /**
     * inCGAIndx(port, addrFrom)
     *
     * @this {Video}
     * @param {number} port (0x3D4)
     * @param {number} [addrFrom] (not defined whenever the Debugger tries to read the specified port)
     * @return {number|undefined}
     */
    inCGAIndx(port, addrFrom)
    {
        return this.inCRTCIndx(this.cardColor, port, addrFrom);
    }

    /**
     * outCGAIndx(port, bOut, addrFrom)
     *
     * @this {Video}
     * @param {number} port (0x3D4)
     * @param {number} bOut
     * @param {number} [addrFrom] (not defined whenever the Debugger tries to read the specified port)
     */
    outCGAIndx(port, bOut, addrFrom)
    {
        this.outCRTCIndx(this.cardColor, port, bOut, addrFrom);
    }

    /**
     * inCGAData(port, addrFrom)
     *
     * @this {Video}
     * @param {number} port (0x3D5)
     * @param {number} [addrFrom] (not defined whenever the Debugger tries to read the specified port)
     * @return {number|undefined}
     */
    inCGAData(port, addrFrom)
    {
        return this.inCRTCData(this.cardColor, port, addrFrom);
    }

    /**
     * outCGAData(port, bOut, addrFrom)
     *
     * @this {Video}
     * @param {number} port (0x3D5)
     * @param {number} bOut
     * @param {number} [addrFrom] (not defined whenever the Debugger tries to read the specified port)
     */
    outCGAData(port, bOut, addrFrom)
    {
        this.outCRTCData(this.cardColor, port, bOut, addrFrom);
    }

    /**
     * inCGAMode(port, addrFrom)
     *
     * @this {Video}
     * @param {number} port (0x3D8)
     * @param {number} [addrFrom] (not defined whenever the Debugger tries to read the specified port)
     * @return {number}
     */
    inCGAMode(port, addrFrom)
    {
        return this.inCardMode(this.cardColor, addrFrom);
    }

    /**
     * outCGAMode(port, bOut, addrFrom)
     *
     * @this {Video}
     * @param {number} port (0x3D8)
     * @param {number} bOut
     * @param {number} [addrFrom] (not defined whenever the Debugger tries to read the specified port)
     */
    outCGAMode(port, bOut, addrFrom)
    {
        this.outCardMode(this.cardColor, bOut, addrFrom);
    }

    /**
     * inCGAColor(port, addrFrom)
     *
     * @this {Video}
     * @param {number} port (0x3D9)
     * @param {number} [addrFrom] (not defined whenever the Debugger tries to read the specified port)
     * @return {number}
     */
    inCGAColor(port, addrFrom)
    {
        var b = this.cardColor.regColor;
        if (!addrFrom || this.messageEnabled()) {
            this.printMessageIO(port /* this.cardColor.port + 5 */, null, addrFrom, this.cardColor.type + ".COLOR", b);
        }
        return b;
    }

    /**
     * outCGAColor(port, bOut, addrFrom)
     *
     * @this {Video}
     * @param {number} port (0x3D9)
     * @param {number} bOut
     * @param {number} [addrFrom] (not defined whenever the Debugger tries to read the specified port)
     */
    outCGAColor(port, bOut, addrFrom)
    {
        if (!addrFrom || this.messageEnabled()) {
            this.printMessageIO(port /* this.cardColor.port + 5 */, bOut, addrFrom, this.cardColor.type + ".COLOR");
        }
        if (this.cardColor.regColor !== bOut) {
            this.cardColor.regColor = bOut;
            this.invalidateCache(false);
        }
    }

    /**
     * inCGAStatus(port, addrFrom)
     *
     * @this {Video}
     * @param {number} port (0x3DA)
     * @param {number} [addrFrom] (not defined whenever the Debugger tries to read the specified port)
     * @return {number}
     */
    inCGAStatus(port, addrFrom)
    {
        return this.inCardStatus(this.cardColor, addrFrom);
    }

    /**
     * inCRTCIndx(card, port, addrFrom)
     *
     * @this {Video}
     * @param {Object} card
     * @param {number} port
     * @param {number} [addrFrom] (not defined whenever the Debugger tries to read the specified port)
     * @return {number|undefined}
     */
    inCRTCIndx(card, port, addrFrom)
    {
        var b;
        /*
         * The IBM VGA ROM makes some hardware determinations based on how the CRTC controller responds when
         * the IO_SELECT bit in the Miscellaneous Output Register is cleared; normally, that would mean ports
         * 0x3B? are decoded and ports 0x3D? are ignored.  We didn't used to bother ignoring them, but the
         * VGA ROM's logic requires it, so now we also check fActive.  However, we ignore only CTRC reads;
         * we retain any writes in case that information proves useful later.
         *
         * Note that returning an undefined value now signals the Bus component to return whatever default value
         * it prefers (normally 0xff).
         */
        if (card.fActive) b = card.regCRTIndx;
        this.printMessageIO(port, null, addrFrom, "CRTC.INDX", b);
        return b;
    }

    /**
     * outCRTCIndx(card, port, bOut, addrFrom)
     *
     * @this {Video}
     * @param {Object} card
     * @param {number} port
     * @param {number} bOut
     * @param {number} [addrFrom] (not defined whenever the Debugger tries to read the specified port)
     */
    outCRTCIndx(card, port, bOut, addrFrom)
    {
        card.regCRTPrev = card.regCRTIndx;
        card.regCRTIndx = bOut & Card.CGA.CRTC.INDX.MASK;
        this.printMessageIO(port /* card.port */, bOut, addrFrom, "CRTC.INDX");
    }

    /**
     * inCRTCData(card, port, addrFrom)
     *
     * @this {Video}
     * @param {Object} card
     * @param {number} port
     * @param {number} [addrFrom] (not defined whenever the Debugger tries to read the specified port)
     * @return {number|undefined}
     */
    inCRTCData(card, port, addrFrom)
    {
        var b;
        /*
         * The IBM VGA ROM makes some hardware determinations based on how the CRTC controller responds when
         * the IO_SELECT bit in the Miscellaneous Output Register is cleared; normally, that would mean ports
         * 0x3B? are decoded and ports 0x3D? are ignored.  We didn't used to bother ignoring them, but the
         * VGA ROM's logic requires it, so now we also check fActive.  However, we ignore only CTRC reads;
         * we retain any writes in case that information proves useful later.
         *
         * Note that returning an undefined value now signals the Bus component to return whatever default value
         * it prefers (normally 0xff).
         */
        if (card.fActive && card.regCRTIndx < card.nCRTCRegs) b = card.regCRTData[card.regCRTIndx];
        if (!addrFrom || this.messageEnabled()) {
            this.printMessageIO(port /* card.port + 1 */, null, addrFrom, "CRTC." + card.asCRTCRegs[card.regCRTIndx], b);
        }
        return b;
    }

    /**
     * outCRTCData(card, port, bOut, addrFrom)
     *
     * @this {Video}
     * @param {Object} card
     * @param {number} port
     * @param {number} bOut
     * @param {number} [addrFrom] (not defined whenever the Debugger tries to read the specified port)
     */
    outCRTCData(card, port, bOut, addrFrom)
    {
        if (card.regCRTIndx < card.nCRTCRegs) {
            if (Video.TRAPALL || card.regCRTData[card.regCRTIndx] !== bOut) {
                if (!addrFrom || this.messageEnabled()) {
                    this.printMessageIO(port /* card.port + 1 */, bOut, addrFrom, "CRTC." + card.asCRTCRegs[card.regCRTIndx]);
                }
                card.regCRTData[card.regCRTIndx] = bOut;
            }
            if (card.regCRTIndx == Card.CRTC.START_ADDR_HI || card.regCRTIndx == Card.CRTC.START_ADDR_LO) {
                /*
                 * HACK: offStartAddr is supposed to be "latched" ONLY at the start of every VRETRACE interval,
                 * but the best we can currently do is latch it during retrace; beyond that, all we can do is snap
                 * the vertical period count and latch it later, in updateScreen(), once the count has advanced.
                 */
                if (this.getRetraceBits(card) & Card.CGA.STATUS.RETRACE) {
                    /*
                     * PARANOIA: Don't call invalidateCache() unless the address we're about to "latch" actually changed.
                     */
                    var offStartAddr = ((card.regCRTData[Card.CRTC.START_ADDR_HI] << 8) + card.regCRTData[Card.CRTC.START_ADDR_LO])|0;
                    if (card.offStartAddr !== offStartAddr) {
                        card.offStartAddr = offStartAddr;
                        this.invalidateCache();
                    }
                } else if (!card.nVertPeriodsStartAddr) {
                    card.nVertPeriodsStartAddr = card.nVertPeriods;
                }
            }
            /*
             * During mode changes on the EGA, all the CRTC regs are typically programmed in sequence,
             * and if that's all that's happening with Card.CRTC.MAX_SCAN.INDX, then we don't want to treat
             * it special; let the mode change be detected normally (eg, when the GRC regs are written later).
             *
             * On the other hand, if this was an out-of-sequence write to Card.CRTC.MAX_SCAN.INDX, then
             * yes, we want to force setMode() to call setDimensions(), which is key to setting the proper
             * number of screen rows.
             *
             * The second part of the check is required to promptly detect a switch to "Mode X"; if we assume
             * that anyone switching to "Mode X" will first switch to mode 0x13, then it's a given that they
             * must reprogram the VDISP_END register, and that they will probably change it from 0x8F to 0xDF.
             *
             * Originally, I wasn't going to check specifically for 0xDF, to help catch other "Mode X" variations,
             * but if I don't, then some spurious mode changes are triggered (eg, when Windows 1.0 switches from
             * CGA graphics mode 0x06 to an EGA graphics mode).
             */
            if (card.regCRTIndx == Card.CRTC.MAX_SCAN.INDX && card.regCRTPrev != Card.CRTC.MAX_SCAN.INDX-1 || card.regCRTIndx == Card.CRTC.EGA.VDISP_END && bOut == 0xDF) {
                this.checkMode(true);
            }
            this.checkCursor();
        } else {
            if (DEBUG && (!addrFrom || this.messageEnabled())) {
                this.printMessage("outCRTCData(): ignoring unexpected write to CRTC[" + Str.toHexByte(card.regCRTIndx) + "]: " + Str.toHexByte(bOut));
            }
        }
    }

    /**
     * inCardMode(card, addrFrom)
     *
     * @this {Video}
     * @param {Object} card
     * @param {number} [addrFrom] (not defined whenever the Debugger tries to read the specified port)
     * @return {number}
     */
    inCardMode(card, addrFrom)
    {
        var b = card.regMode;
        this.printMessageIO(card.port + 4, null, addrFrom, "MODE", b);
        return b;
    }

    /**
     * outCardMode(card, bOut, addrFrom)
     *
     * @this {Video}
     * @param {Object} card
     * @param {number} bOut
     * @param {number} [addrFrom] (not defined whenever the Debugger tries to read the specified port)
     */
    outCardMode(card, bOut, addrFrom)
    {
        this.printMessageIO(card.port + 4, bOut, addrFrom, "MODE");
        card.regMode = bOut;
        this.checkMode(false);
    }

    /**
     * inCardStatus(card, addrFrom)
     *
     * On an EGA, this register is called "Status Register One" (0x3BA/0x3DA aka STATUS1), to distinguish it from
     * "Status Register Zero" (0x3C2 aka STATUS0).  One of the side-effects of reading STATUS1 is that it resets the
     * ATC address/data flip-flop to "address" mode, which we emulate by setting cardEGA.fATCData to false, indicating
     * that the ATC is not in "data" mode.
     *
     * @this {Video}
     * @param {Object} card
     * @param {number} [addrFrom] (not defined whenever the Debugger tries to read the specified port)
     * @return {number}
     */
    inCardStatus(card, addrFrom)
    {
        var b = this.getRetraceBits(card);

        if (card === this.cardEGA) {
            /*
             * STATUS1 diagnostic bits 5 and 4 are set according to the Card.ATC.PLANES.MUX bits:
             *
             *      MUX     Bit 5   Bit 4
             *      ---     ----    ----
             *      00:     Red     Blue
             *      01:     SecBlue Green
             *      10:     SecRed  SecGreen
             *      11:     unused  unused
             *
             * Depending on where we are in the horizontal and vertical periods (which can be inferred from the
             * same elapsed cycle count that we used to simulate the retrace bits above), we could extract 4 bits
             * from a corresponding region of the video buffer, "and" them with Card.ATC.PLANES.MASK, use
             * that to index into the palette registers (cardEGA.regATCData), and use the resulting palette register
             * bits to set these diagnostics bits.  However, that's all rather tedious, and the process of extracting
             * 4 appropriate bits from the video buffer varies depending on the video mode.
             *
             * Why are we even considering this?  Because the EGA BIOS diagnostic code draws a bright reverse-video
             * line of text blocks across the top of the screen, writes 0x3F to palette register 0x0f, and then
             * monitors the STATUS1 diagnostic bits, waiting for those palette bits to show up.  It turns out, however,
             * that we can easily fool the EGA BIOS by simply toggling the diagnostic bits.  So we take the easy way out.
             *
             * TODO: Faithful emulation of these bits is certainly doable, so consider doing that at some point.
             */
            b |= ((card.regStatus & Card.STATUS1.DIAGNOSTIC) ^ Card.STATUS1.DIAGNOSTIC);

            /*
             * Last but not least, we must reset the EGA's ATC flip-flop whenever this register is read.
             */
            card.fATCData = false;
        }
        else {
            /*
             * On the MDA/CGA, to satisfy ROM BIOS testing ("TEST.10"), it's sufficient to do a simple toggle of
             * bits 0 and 3 on every read.
             *
             * Also, according to http://www.seasip.info/VintagePC/mda.html, on an MDA, bits 7-4 are always ON and
             * bits 2-1 are always OFF, hence the "OR" of 0xf0.
             *
             * TODO: Decide whether to preserve the bits from getRetraceBits() on the MDA/CGA; we're continuing
             * to do a simple toggle, partly on the theory that that may speed up the CGA BIOS scroll code a bit.
             */
            b = (card.regStatus ^= (Card.CGA.STATUS.RETRACE | Card.CGA.STATUS.VRETRACE)) | 0xf0;
        }

        card.regStatus = b;
        this.printMessageIO(card.port + 6, null, addrFrom, (card === this.cardEGA? "STATUS1" : "STATUS"), b);
        return b;
    }

    /**
     * dumpVideo(asArgs)
     *
     * @this {Video}
     * @param {Array.<string>} asArgs
     */
    dumpVideo(asArgs)
    {
        if (DEBUGGER) {
            if (!this.cardActive) {
                this.dbg.println("no active video card");
                return;
            }
            if (asArgs[0]) {
                this.cardActive.dumpVideoBuffer(asArgs);
                return;
            }
            this.dbg.println("BIOSMODE: " + Str.toHexByte(this.nMode));
            this.cardActive.dumpVideoCard();
        }
    }

    /*
     releaseTouch()
     {
     }
     */

    /**
     * doBlink()
     *
     * This function is obsolete, now that the checkBlink() function is called on every updateScreen()
     * and checkCursor() call.  updateScreen() is driven by CPU bursts, so piggy-backing on that to drive
     * blink updates seems preferable to having another active timer in the system.
     *
     * @this {Video}
     * @param {boolean} [fStart]
     *
     doBlink(fStart)
     {
        if (this.cBlinks >= 0) {
            this.cBlinks++;
            if (this.cBlinkVisible || this.iCellCursor >= 0) {
                if (!fStart && !this.cpu.isRunning()) {
                    this.updateScreen();
                }
                setTimeout(function(video) { return function onBlinkTimeout() {video.doBlink();}; }(this), 266);
                return;
            }
            this.cBlinks = -1;
        }
    },
     */

    /**
     * Video.init()
     *
     * This function operates on every HTML element of class "video", extracting the
     * JSON-encoded parameters for the Video constructor from the element's "data-value"
     * attribute, invoking the constructor to create a Video component, and then binding
     * any associated HTML controls to the new component.
     */
    static init()
    {
        var aeVideo = Component.getElementsByClass(document, PCX86.APPCLASS, "video");
        for (var iVideo = 0; iVideo < aeVideo.length; iVideo++) {
            var eVideo = aeVideo[iVideo];
            var parmsVideo = Component.getComponentParms(eVideo);

            var eCanvas = document.createElement("canvas");
            if (eCanvas === undefined || !eCanvas.getContext) {
                eVideo.innerHTML = "<br/>Missing &lt;canvas&gt; support. Please try a newer web browser.";
                return;
            }

            eCanvas.setAttribute("class", "pcjs-canvas");
            eCanvas.setAttribute("width", parmsVideo['screenWidth']);
            eCanvas.setAttribute("height", parmsVideo['screenHeight']);
            eCanvas.style.backgroundColor = parmsVideo['screenColor'];

            /*
             * The "contenteditable" attribute on a canvas element NOTICEABLY slows down canvas drawing on
             * Safari as soon as you give the canvas focus (ie, click away from the canvas, and drawing speeds
             * up; click on the canvas, and drawing slows down).  So the "transparent textarea hack" that we
             * once employed as only a work-around for Android devices is now our default.
             *
             *      eCanvas.setAttribute("contenteditable", "true");
             *
             * HACK: A canvas style of "auto" provides for excellent responsive canvas scaling in EVERY browser
             * except IE9/IE10, so I recalculate the appropriate CSS height every time the parent DIV is resized;
             * IE11 works without this hack, so we take advantage of the fact that IE11 doesn't identify as "MSIE".
             *
             * The other reason it's good to keep this particular hack limited to IE9/IE10 is that most other
             * browsers don't actually support an 'onresize' handler on anything but the window object.
             */
            eCanvas.style.height = "auto";
            if (Web.getUserAgent().indexOf("MSIE") >= 0) {
                eVideo.onresize = function(eParent, eChild, cx, cy) {
                    return function onResizeVideo() {
                        eChild.style.height = (((eParent.clientWidth * cy) / cx) | 0) + "px";
                    };
                }(eVideo, eCanvas, parmsVideo['screenWidth'], parmsVideo['screenHeight']);
                eVideo.onresize();
            }
            /*
             * The following is a related hack that allows the user to force the screen to use a particular aspect
             * ratio if an 'aspect' attribute or URL parameter is set.  Initially, it's just for testing purposes
             * until we figure out a better UI.  And note that we use our Web.onPageEvent() helper function to make
             * sure we don't trample any other 'onresize' handler(s) attached to the window object.
             */
            var aspect = +(parmsVideo['aspect'] || Web.getURLParm('aspect'));
            /*
             * No 'aspect' parameter yields NaN, which is falsey, and anything else must satisfy my arbitrary
             * constraints of 0.3 <= aspect <= 3.33, to prevent any useless (or worse, browser-blowing) results.
             */
            if (aspect && aspect >= 0.3 && aspect <= 3.33) {
                Web.onPageEvent('onresize', function(eParent, eChild, aspectRatio) {
                    return function onResizeWindow() {
                        /*
                         * Since aspectRatio is the target width/height, we have:
                         *
                         *      eParent.clientWidth / eChild.style.height = aspectRatio
                         *
                         * which means that:
                         *
                         *      eChild.style.height = eParent.clientWidth / aspectRatio
                         *
                         * so for example, if aspectRatio is 16:9, or 1.78, and clientWidth = 640,
                         * then the calculated height should approximately 360.
                         */
                        eChild.style.height = ((eParent.clientWidth / aspectRatio)|0) + "px";
                    };
                }(eVideo, eCanvas, aspect));
                window['onresize']();
            }
            eVideo.appendChild(eCanvas);

            /*
             * HACK: Android-based browsers, like the Silk (Amazon) browser and Chrome for Android, don't honor the
             * "contenteditable" attribute; that is, when the canvas receives focus, they don't activate the on-screen
             * keyboard.  So my fallback is to create a transparent textarea on top of the canvas.
             *
             * The parent DIV must have a style of "position:relative" (alternatively, a class of "pcjs-container"),
             * so that we can position the textarea using absolute coordinates.  Also, we don't want the textarea to be
             * visible, but we must use "opacity:0" instead of "visibility:hidden", because the latter seems to prevent
             * the element from receiving events.  These styling requirements are taken care of in components.css
             * (see references to the "pcjs-video-object" class).
             *
             * UPDATE: Unfortunately, Android keyboards like to compose whole words before transmitting any of the
             * intervening characters; our textarea's keyDown/keyUp event handlers DO receive intervening key events,
             * but their keyCode property is ZERO.  Virtually the only usable key event we receive is the Enter key.
             * Android users will have to use machines that include their own on-screen "soft keyboard", or use an
             * external keyboard.
             *
             * The following attempt to use a password-enabled input field didn't work any better on Android.  You could
             * clearly see the overlaid semi-transparent input field, but none of the input characters were passed along,
             * with the exception of the "Go" (Enter) key.
             *
             *      var eInput = document.createElement("input");
             *      eInput.setAttribute("type", "password");
             *      eInput.setAttribute("style", "position:absolute; left:0; top:0; width:100%; height:100%; opacity:0.5");
             *      eVideo.appendChild(eInput);
             *
             * See this Chromium issue for more information: https://code.google.com/p/chromium/issues/detail?id=118639
             */
            var eTextArea = document.createElement("textarea");

            /*
             * As noted in keyboard.js, the keyboard on an iOS device tends to pop up with the SHIFT key depressed,
             * which is not the initial keyboard state that the Keyboard component expects, so hopefully turning off
             * these "auto" attributes will help.
             */
            if (Web.isUserAgent("iOS")) {
                eTextArea.setAttribute("autocapitalize", "off");
                eTextArea.setAttribute("autocorrect", "off");
                /*
                 * One of the problems on iOS devices is that after a soft-key control is clicked, we need to give
                 * focus back to the above textarea, usually by calling cmp.updateFocus(), but in doing so, iOS may
                 * also "zoom" the page rather jarringly.  While it's a simple matter to completely disable zooming,
                 * by fiddling with the page's viewport, that prevents the user from intentionally zooming.  A bit of
                 * Googling reveals that another way to prevent those jarring unintentional zooms is to simply set the
                 * font-size of the text control to 16px.  So that's what we do.
                 */
                eTextArea.style.fontSize = "16px";
            }
            eVideo.appendChild(eTextArea);

            /*
             * Now we can create the Video object, record it, and wire it up to the associated document elements.
             */
            var eContext = eCanvas.getContext("2d");
            var video = new Video(parmsVideo, eCanvas, eContext, eTextArea /* || eInput */, eVideo);

            /*
             * Bind any video-specific controls (eg, the Refresh button). There are no essential controls, however;
             * even the "Refresh" button is just a diagnostic tool, to ensure that the screen contents are up-to-date.
             */
            Component.bindComponentControls(video, eVideo, PCX86.APPCLASS);
        }
    }
}

Video.TRAPALL = true;           // monitor all I/O by default (not just deltas)

/*
 * Supported Modes
 *
 * Although this component is designed to be a video hardware emulation, not a BIOS simulation, we DO
 * look for changes to the hardware state that correspond to standard BIOS mode settings, so our internal
 * mode setting will normally match the current BIOS mode setting; however, this a debugging convenience,
 * not an attempt to monitor or emulate the BIOS.
 *
 * We do have some BIOS awareness (eg, when loading ROM-based fonts, and some special code to ensure all
 * the BIOS diagnostics pass), but for the most part, we treat the BIOS like any other application code.
 *
 * As we expand support to include more programmable cards like the EGA, it becomes quite easy for the card
 * to enter a "mode" that has no BIOS counterpart (eg, non-standard combinations of video buffer address,
 * memory access modes, fonts, display regions, etc).  Our hardware emulation routines will cope with those
 * situations as best they can (and when they don't, it should be considered a bug if some application is
 * broken as a result), but realistically, our hardware emulation is never likely to be 100% accurate.
 */
Video.MODE = {
    CGA_40X25_BW:       0,
    CGA_40X25:          1,
    CGA_80X25_BW:       2,
    CGA_80X25:          3,
    CGA_320X200:        4,
    CGA_320X200_BW:     5,
    CGA_640X200:        6,
    MDA_80X25:          7,
    EGA_320X200:        0x0D,   // mapped at A000:0000, color, 4bpp, planar
    EGA_640X200:        0x0E,   // mapped at A000:0000, color, 4bpp, planar
    EGA_640X350_MONO:   0x0F,   // mapped at A000:0000, mono,  2bpp, planar
    EGA_640X350:        0x10,   // mapped at A000:0000, color, 4bpp, planar
    VGA_640X480_MONO:   0x11,   // mapped at A000:0000, mono,  2bpp, planar
    VGA_640X480:        0x12,   // mapped at A000:0000, color, 4bpp, planar
    VGA_320X200:        0x13,   // mapped at A000:0000, color, 8bpp, linear
    /*
     * The remaining mode identifiers are for internal use only; there is no correlation with any
     * publicly defined BIOS modes, and overlap with any third-party mode numbers is purely coincidental.
     */
    VGA_320X240:        0x14,   // mapped at A000:0000, color, 8bpp, planar ("Mode X")
    VGA_320X400:        0x15,   // mapped at A000:0000, color, 8bpp, planar
    /*
     * Here's where we might assign additional identifiers to certain unique combinations, like the
     * fTextGraphicsHybrid 320x400 mode that Windows 95 uses (ie, when the buffer is mapped to B800:0000
     * instead of A000:0000 and is configured for text mode access, but graphics are still being displayed
     * from the second half of video memory).
     */
    UNKNOWN:            0xFF
};

/*
 * Supported Fonts
 *
 * Once we've finished loading the standard 8K font file, aFonts[] should contain one or more of the
 * entries listed below.  For the standard MDA/CGA font ROM, the first (MDA) font resides in the first 4Kb,
 * and the second and third (CGA) fonts reside in the two 2K halves of the second 4Kb.
 *
 * It may seem odd that the cell size for FONT_CGAD is *larger* than the cell size for FONT_CGA,
 * since 40-column mode is actually lower resolution, but since we don't shrink the screen canvas when we
 * shrink the mode, the characters must be drawn larger, and they look better if we don't have to scale them.
 *
 * From the IBM EGA Manual (p.5):
 *
 *     "In alphanumeric modes, characters are formed from one of two ROM (Read Only Memory) character
 *      generators on the adapter. One character generator defines 7x9 characters in a 9x14 character box.
 *      For Enhanced Color Display support, the 9x14 character set is modified to provide an 8x14 character set.
 *      The second character generator defines 7x7 characters in an 8x8 character box. These generators contain
 *      dot patterns for 256 different characters. The character sets are identical to those provided by the
 *      IBM Monochrome Display Adapter and the IBM Color/Graphics Monitor Adapter."
 */
Video.FONT = {
    MDA:    1,          // 9x14 monochrome font
    MDAD:   2,          // 18x28 monochrome font (this is the 9x14 font doubled)
    CGA:    3,          // 8x8 color font
    CGAD:   6,          // 16x16 color font (this is the 8x8 CGA font doubled)
    EGA:    5,          // 8x14 color font
    EGAD:   10,         // 16x28 color font (this is the 8x14 EGA font doubled)
    VGA:    7,          // 8x16 color font
    VGAD:   14          // 16x32 color font (this is the 8x16 VGA font doubled)
};

/*
 * Supported Cards
 *
 * Note that we choose card IDs that match the default font ID for each card as well, for convenience.
 */
Video.CARD = {
    MDA: Video.FONT.MDA,
    CGA: Video.FONT.CGA,
    EGA: Video.FONT.EGA,
    VGA: Video.FONT.VGA
};

/*
 * Supported Models
 *
 * Each model refers to an array where [0] is the card ID, and [1] is the default mode.
 */
Video.MODEL = {
    "mda": [Video.CARD.MDA, Video.MODE.MDA_80X25],
    "cga": [Video.CARD.CGA, Video.MODE.CGA_80X25],
    "ega": [Video.CARD.EGA, Video.MODE.CGA_80X25],
    "vga": [Video.CARD.VGA, Video.MODE.CGA_80X25]
};

/*
 * Supported Monitors
 *
 * The MDA monitor displays 350 lines of vertical resolution, 720 lines of horizontal resolution, and refreshes
 * at ~50Hz.  The CGA monitor displays 200 lines vertically, 640 horizontally, and refreshes at ~60Hz.
 *
 * Based on actual MDA timings (see http://diylab.atwebpages.com/pressureDev.htm), the total horizontal
 * period (drawing a line and retracing) is ~54.25uSec (1000000uSec / 18432) and the horizontal retrace interval
 * is about 15% of that, or ~8.14uSec.  Vertical sync occurs once every 370 horizontal periods.  Of those 370,
 * only 354 represent actively drawn lines (and of those, only 350 are visible); the remaining 16 horizontal
 * periods, or 4% of the 370 total, represent the vertical retrace interval.
 *
 * I don't have similar numbers for the CGA or EGA, so for now, I assume similar percentages; ie, 15% of
 * the horizontal period will represent horizontal retrace, and 4% of the vertical pixel maximum (262) will
 * represent vertical retrace.  However, 24% of the CGA's 262 vertical maximum represents non-visible lines,
 * whereas only 5% of the MDA's 370 maximum represents non-visible lines; is there really that much "overscan"
 * on the CGA?
 *
 * For each monitor type, there's a Video.monitorSpecs object that describes the horizontal and vertical
 * timings, along with my assumptions about the percentage of time that drawing is "active" within those periods,
 * and then based on the selected monitor type, I compute the number of CPU cycles that each period lasts,
 * as well as the number of CPU cycles that drawing lasts within each period, so that the horizontal and vertical
 * retrace status flags can be quickly calculated.
 *
 * For reference, here are some important numbers to know (from https://github.com/reenigne/reenigne/blob/master/8088/cga/register_values.txt):
 *
 *              CGA          MDA
 *  Pixel clock 14.318 MHz   16.257 MHz (aka "maximum video bandwidth", as IBM Tech Refs sometimes call it)
 *  Horizontal  15.700 KHz   18.432 KHz (aka "horizontal drive", as IBM Tech Refs sometimes call it)
 *  Vertical    59.923 Hz    49.816 Hz
 *  Usage       53.69%       77.22%
 *  H pix       912 = 114*8  882 = 98*9
 *  V pix       262          370
 *  Dots        238944       326340
 */

/**
 * @class MonitorSpecs
 * @property {number} nHorzPeriodsPerSec
 * @property {number} nHorzPeriodsPerFrame
 * @property {number} percentHorzActive
 * @property {number} percentVertActive
 *
 * From these monitor specs, we calculate the following values for a given Card:
 *
 *      nCyclesPerSecond = cpu.getCyclesPerSecond();      // eg, 4772727
 *      nCyclesHorzPeriod = (nCyclesPerSecond / monitorSpecs.nHorzPeriodsPerSec) | 0;
 *      nCyclesHorzActive = (nCyclesHorzPeriod * monitorSpecs.percentHorzActive / 100) | 0;
 *      nCyclesVertPeriod = nCyclesHorzPeriod * monitorSpecs.nHorzPeriodsPerFrame;
 *      nCyclesVertActive = (nCyclesVertPeriod * monitorSpecs.percentVertActive / 100) | 0;
 */

/**
 * @type {Object}
 */
Video.monitorSpecs = {};

/**
 * NOTE: Based on trial-and-error, 208 is the magic number of horizontal syncs per vertical sync that
 * yielded the necessary number of "horizontal enables" (200 or 0xC8) in the EGA ROM BIOS at C000:03D0.
 *
 * @type {{MonitorSpecs}}
 */
Video.monitorSpecs[ChipSet.MONITOR.COLOR] = {
    nHorzPeriodsPerSec: 15700,
    nHorzPeriodsPerFrame: 208,
    percentHorzActive: 85,
    percentVertActive: 96
};

/**
 * NOTE: Based on trial-and-error, 364 is the magic number of horizontal syncs per vertical sync that
 * yielded the necessary number of "horizontal enables" (350 or 0x15E) in the EGA ROM BIOS at C000:03D0.
 *
 * @type {{MonitorSpecs}}
 */
Video.monitorSpecs[ChipSet.MONITOR.MONO] = {
    nHorzPeriodsPerSec: 18432,
    nHorzPeriodsPerFrame: 364,
    percentHorzActive: 85,
    percentVertActive: 96
};

/**
 * @type {{MonitorSpecs}}
 */
Video.monitorSpecs[ChipSet.MONITOR.EGACOLOR] = {
    nHorzPeriodsPerSec: 21850,
    nHorzPeriodsPerFrame: 364,
    percentHorzActive: 85,
    percentVertActive: 96
};

/**
 * NOTE: As above, the following values are based purely on trial-and-error, to yield results that fall
 * squarely within the bounds of the IBM VGA ROM timing requirements; see the IBM VGA ROM code at C000:024A.
 *
 * @type {{MonitorSpecs}}
 */
Video.monitorSpecs[ChipSet.MONITOR.VGACOLOR] = {
    nHorzPeriodsPerSec: 16700,
    nHorzPeriodsPerFrame: 480,
    percentHorzActive: 85,
    percentVertActive: 83
};

/*
 * EGA Miscellaneous ports and SW1-Sw4
 *
 * The Card.MISC.CLOCK_SELECT bits determine which of the EGA board's 4 configuration switches are
 * returned via Card.STATUS0.SWSENSE (when SWSENSE is zero, the switch is closed):
 *
 *      0xC: return SW1
 *      0x8: return SW2
 *      0x4: return SW3
 *      0x0: return SW4
 *
 * These 4 bits are also copied to the byte at 40:88h by the EGA BIOS, where bit 0 is SW1, bit 1 is SW2,
 * bit 2 is SW3 and bit 3 is SW4.  Our switch settings come from bEGASwitches, which in turn comes from
 * sSwitches, which in turn comes from the "switches" property passed to the Video component, if any.
 *
 * As usual, the switch settings are reversed in both direction and sense from the switch settings; the
 * good news, however, is that we can use the parseSwitches() method in the ChipSet component to parse them.
 *
 * The set of valid EGA switch values, after conversion, is stored in the table below.  For each value,
 * there is an array that defines the corresponding monitor type(s) for the EGA adapter and any secondary
 * adapter.  The third value is a boolean indicating whether the EGA is the primary adapter.
 */
Video.aEGAMonitorSwitches = {
    0x06: [ChipSet.MONITOR.TV,           ChipSet.MONITOR.MONO,  true],  // "1001"
    0x07: [ChipSet.MONITOR.COLOR,        ChipSet.MONITOR.MONO,  true],  // "0001"
    0x08: [ChipSet.MONITOR.EGAEMULATION, ChipSet.MONITOR.MONO,  true],  // "1110"
    0x09: [ChipSet.MONITOR.EGACOLOR,     ChipSet.MONITOR.MONO,  true],  // "0110" [our default; see bEGASwitches below]
    0x0a: [ChipSet.MONITOR.MONO,         ChipSet.MONITOR.TV,    true],  // "1010"
    0x0b: [ChipSet.MONITOR.MONO,         ChipSet.MONITOR.COLOR, true],  // "0010"
    0x00: [ChipSet.MONITOR.TV,           ChipSet.MONITOR.MONO,  false], // "1111"
    0x01: [ChipSet.MONITOR.COLOR,        ChipSet.MONITOR.MONO,  false], // "0111"
    0x02: [ChipSet.MONITOR.EGAEMULATION, ChipSet.MONITOR.MONO,  false], // "1011"
    0x03: [ChipSet.MONITOR.EGACOLOR,     ChipSet.MONITOR.MONO,  false], // "0011"
    0x04: [ChipSet.MONITOR.MONO,         ChipSet.MONITOR.TV,    false], // "1101"
    0x05: [ChipSet.MONITOR.MONO,         ChipSet.MONITOR.COLOR, false]  // "0101"
};

/**
 * @class Font
 * @property {number} cxCell
 * @property {number} cyCell
 * @property {Array} aCSSColors
 * @property {Array} aRGBColors
 * @property {Array} aColorMap
 * @property {Array} aCanvas
 */

/*
 * For each video mode, we need to know the following pieces of information:
 *
 *      0: # of columns (nCols)
 *      1: # of rows (nRows)
 *      2: # cells per word (nCellsPerWord: # of characters or pixels per word)
 *      3: # bytes of visible screen padding, if any (used for CGA graphics modes only)
 *      4: font ID (nFont: undefined if graphics mode)
 *
 * For MDA and CGA modes, a "word" of memory is 16 bits of CPU-addressable data, so by calculating
 * ([0] * [1]) / [2], we obtain the number of words that mode actively displays; for example, the
 * amount of visible memory used by mode 0x04 is (320 * 200) / 4, or 16000.
 *
 * However, for EGA and VGA graphics modes, a "word" of memory is a single element in the video buffer
 * containing 32 bits of pixel data.
 *
 * The MODES.CGA_40X25 modes specify FONT_CGA instead of FONT_CGAD because we don't automatically
 * load the FONT_CGAD unless the screen is large enough to accommodate it (see the fDoubleFont calculation).
 *
 * To compensate, we have code in setDimensions() that automatically switches to FONT_CGAD if it's loaded AND
 * the cell size warrants the larger font.  We could hard-code FONT_CGAD here, but then we'd always load it,
 * and it might not always be the best fit.
 */
Video.aModeParms = [];                                                                              // Mode
Video.aModeParms[Video.MODE.CGA_40X25]          = [ 40,  25,  1,   0, Video.FONT.CGA];              // 0x01
Video.aModeParms[Video.MODE.CGA_80X25]          = [ 80,  25,  1,   0, Video.FONT.CGA];              // 0x03
Video.aModeParms[Video.MODE.CGA_320X200]        = [320, 200,  8, 192];                              // 0x04
Video.aModeParms[Video.MODE.CGA_640X200]        = [640, 200, 16, 192];                              // 0x06
Video.aModeParms[Video.MODE.MDA_80X25]          = [ 80,  25,  1,   0, Video.FONT.MDA];              // 0x07
Video.aModeParms[Video.MODE.EGA_320X200]        = [320, 200,  8];                                   // 0x0D
Video.aModeParms[Video.MODE.EGA_640X200]        = [640, 200,  8];                                   // 0x0E
Video.aModeParms[Video.MODE.EGA_640X350_MONO]   = [640, 350,  8];                                   // 0x0F
Video.aModeParms[Video.MODE.EGA_640X350]        = [640, 350,  8];                                   // 0x10
Video.aModeParms[Video.MODE.VGA_640X480_MONO]   = [640, 480,  8];                                   // 0x11
Video.aModeParms[Video.MODE.VGA_640X480]        = [640, 480,  8];                                   // 0x12
Video.aModeParms[Video.MODE.VGA_320X200]        = [320, 200,  1];                                   // 0x13
Video.aModeParms[Video.MODE.VGA_320X240]        = [320, 240,  4];                                   // 0x14
Video.aModeParms[Video.MODE.VGA_320X400]        = [320, 400,  4];                                   // 0x15

Video.aModeParms[Video.MODE.CGA_40X25_BW]       = Video.aModeParms[Video.MODE.CGA_40X25];           // 0x00
Video.aModeParms[Video.MODE.CGA_80X25_BW]       = Video.aModeParms[Video.MODE.CGA_80X25];           // 0x02
Video.aModeParms[Video.MODE.CGA_320X200_BW]     = Video.aModeParms[Video.MODE.CGA_320X200];         // 0x05

/*
 * MDA attribute byte definitions
 *
 * For MDA, only the following group of ATTR definitions are supported; any FGND/BGND value combinations
 * outside this group will be treated as "normal" (ATTR_FGND_WHITE | ATTR_BGND_BLACK).
 *
 * NOTE: Assuming MDA.MODE.BLINK_ENABLE is set (which the ROM BIOS sets by default), ATTR_BGND_BLINK will
 * cause the *foreground* element of the cell to blink, even though it is part of the *background* attribute bits.
 *
 * Regarding blink rate, characters are supposed to blink every 16 vertical frames, which amounts to .26667 blinks
 * per second, assuming a 60Hz vertical refresh rate.  So roughly every 267ms, we need to take care of any blinking
 * characters.  updateScreen() maintains a global count (cBlinkVisible) of blinking characters, to simplify the
 * decision of when to redraw the screen.
 */
Video.ATTRS = {};
Video.ATTRS.FGND_BLACK  = 0x00;
Video.ATTRS.FGND_ULINE  = 0x01;
Video.ATTRS.FGND_WHITE  = 0x07;
Video.ATTRS.FGND_BRIGHT = 0x08;
Video.ATTRS.BGND_BLACK  = 0x00;
Video.ATTRS.BGND_WHITE  = 0x70;
Video.ATTRS.BGND_BLINK  = 0x80;
Video.ATTRS.BGND_BRIGHT = 0x80;
Video.ATTRS.DRAW_FGND   = 0x100;        // this is an internal attribute bit, indicating the foreground should be drawn
Video.ATTRS.DRAW_CURSOR = 0x200;        // this is an internal attribute bit, indicating when the cursor should be drawn

/*
 * Here's a "cheat sheet" for attribute byte combinations that the IBM MDA could have supported.  The original (Aug 1981)
 * IBM Tech Ref is very terse and implies that only those marked with * are actually supported.
 *
 *     *0x00: non-display                       ATTR_FGND_BLACK |                    ATTR_BGND_BLACK
 *     *0x01: underline                         ATTR_FGND_ULINE |                    ATTR_BGND_BLACK
 *     *0x07: normal (white on black)           ATTR_FGND_WHITE |                    ATTR_BGND_BLACK
 *    **0x09: bright underline                  ATTR_FGND_ULINE | ATTR_FGND_BRIGHT | ATTR_BGND_BLACK
 *    **0x0F: bold (bright white on black)      ATTR_FGND_WHITE | ATTR_FGND_BRIGHT | ATTR_BGND_BLACK
 *     *0x70: reverse (black on white)          ATTR_FGND_BLACK |                  | ATTR_BGND_WHITE
 *      0x81: blinking underline                ATTR_FGND_ULINE |                  | ATTR_BGND_BLINK (or dim background if blink disabled)
 *    **0x87: blinking normal                   ATTR_FGND_WHITE |                  | ATTR_BGND_BLINK (or dim background if blink disabled)
 *      0x89: blinking bright underline         ATTR_FGND_ULINE | ATTR_FGND_BRIGHT | ATTR_BGND_BLINK (or dim background if blink disabled)
 *    **0x8F: blinking bold                     ATTR_FGND_WHITE | ATTR_FGND_BRIGHT | ATTR_BGND_BLINK (or dim background if blink disabled)
 *    **0xF0: blinking reverse                  ATTR_FGND_WHITE | ATTR_FGND_BRIGHT | ATTR_BGND_BLINK (or bright background if blink disabled)
 *
 * Unsupported attributes reportedly display as "normal" (ATTR_FGND_WHITE | ATTR_BGND_BLACK).  However, precisely which
 * attributes are unsupported on the MDA varies depending on the source.  Some sources (eg, the IBM Tech Ref) imply that
 * only those marked by * are supported, while others (eg, some--but not all--Peter Norton guides) include those marked
 * by **, and still others include ALL the combinations listed above.
 *
 * Furthermore, according to http://www.seasip.info/VintagePC/mda.html:
 *
 *      Attributes 0x00, 0x08, 0x80 and 0x88 display as black space;
 *      Attribute 0x78 displays as dark green on green; depending on the monitor, there may be a green "halo" where the dark and bright bits meet;
 *      Attribute 0xF0 displays as a blinking version of 0x70 if blink enabled, and black on bright green otherwise;
 *      Attribute 0xF8 displays as a blinking version of 0x78 if blink enabled, and as dark green on bright green otherwise.
 *
 * However, I'm rather skeptical about supporting 0x78 and 0xF8, until I see some evidence that "bright black" actually
 * produced dark green on IBM equipment; it also doesn't sound like a combination many people would have used.  I'll probably
 * treat all of 0x08, 0x80 and 0x88 the same as 0x00, only because it seems logical (they're all "black on black" combinations
 * with only BRIGHT and/or BLINK bits set). Beyond that, I'll likely treat any other combination not listed in the above cheat
 * sheet as "normal".
 *
 * All the discrepancies/disagreements I've found are probably due in part to the proliferation of IBM and non-IBM MDA
 * cards, combined with IBM and non-IBM monochrome monitors, and people assuming that their non-IBM card and/or monitor
 * behaved exactly like the original IBM equipment, which probably wasn't true in all cases.
 *
 * I would like to limit my MDA display support to EXACTLY everything that the IBM MDA supported and nothing more, but
 * since there will be combinations that will logically "fall out" unless I specifically exclude them, it's very likely
 * this implementation will end up being a superset.
 */

/*
 * CGA attribute byte definitions; these simply extend the set of MDA attributes, with the exception of ATTR_FNGD_ULINE,
 * which the CGA can treat only as ATTR_FGND_BLUE.
 */
Video.ATTRS.FGND_BLUE       = 0x01;
Video.ATTRS.FGND_GREEN      = 0x02;
Video.ATTRS.FGND_CYAN       = 0x03;
Video.ATTRS.FGND_RED        = 0x04;
Video.ATTRS.FGND_MAGENTA    = 0x05;
Video.ATTRS.FGND_BROWN      = 0x06;

Video.ATTRS.BGND_BLUE       = 0x10;
Video.ATTRS.BGND_GREEN      = 0x20;
Video.ATTRS.BGND_CYAN       = 0x30;
Video.ATTRS.BGND_RED        = 0x40;
Video.ATTRS.BGND_MAGENTA    = 0x50;
Video.ATTRS.BGND_BROWN      = 0x60;

/*
 * For the MDA, the number of unique "colors" is 5, based on the following supported FGND attribute values:
 *
 *      0x0: black font (attribute value 0x8 is mapped to 0x0)
 *      0x1: green font with underline
 *      0x7: green font without underline (attribute values 0x2-0x6 are mapped to 0x7)
 *      0x9: bright green font with underline
 *      0xf: bright green font without underline (attribute values 0xa-0xe are mapped to 0xf)
 *
 * I'm still not sure about 0x8 (dark green?); for now, I'm mapping it to 0x0, but it may become a 6th supported color.
 *
 * MDA attributes form an index into aMDAColorMap, which in turn provides an index (0-4) into aMDAColors.
 */
Video.aMDAColors = [
    [0x00, 0x00, 0x00, 0xff],
    [0x7f, 0xc0, 0x7f, 0xff],
    [0x7f, 0xc0, 0x7f, 0xff],
    [0x7f, 0xff, 0x7f, 0xff],
    [0x7f, 0xff, 0x7f, 0xff]
];
Video.aMDAColorMap = [0x0, 0x1, 0x2, 0x2, 0x2, 0x2, 0x2, 0x2, 0x0, 0x3, 0x4, 0x4, 0x4, 0x4, 0x4, 0x4];

Video.aCGAColors = [
    [0x00, 0x00, 0x00, 0xff],   // 0x00: ATTR_FGND_BLACK
    [0x00, 0x00, 0xaa, 0xff],   // 0x01: ATTR_FGND_BLUE
    [0x00, 0xaa, 0x00, 0xff],   // 0x02: ATTR_FGND_GREEN
    [0x00, 0xaa, 0xaa, 0xff],   // 0x03: ATTR_FGND_CYAN
    [0xaa, 0x00, 0x00, 0xff],   // 0x04: ATTR_FGND_RED
    [0xaa, 0x00, 0xaa, 0xff],   // 0x05: ATTR_FGND_MAGENTA
    [0xaa, 0x55, 0x00, 0xff],   // 0x06: ATTR_FGND_BROWN
    [0xaa, 0xaa, 0xaa, 0xff],   // 0x07: ATTR_FGND_WHITE                      (aka light gray)
    [0x55, 0x55, 0x55, 0xff],   // 0x08: ATTR_FGND_BLACK   | ATTR_FGND_BRIGHT (aka gray)
    [0x55, 0x55, 0xff, 0xff],   // 0x09: ATTR_FGND_BLUE    | ATTR_FGND_BRIGHT
    [0x55, 0xff, 0x55, 0xff],   // 0x0A: ATTR_FGND_GREEN   | ATTR_FGND_BRIGHT
    [0x55, 0xff, 0xff, 0xff],   // 0x0B: ATTR_FGND_CYAN    | ATTR_FGND_BRIGHT
    [0xff, 0x55, 0x55, 0xff],   // 0x0C: ATTR_FGND_RED     | ATTR_FGND_BRIGHT
    [0xff, 0x55, 0xff, 0xff],   // 0x0D: ATTR_FGND_MAGENTA | ATTR_FGND_BRIGHT
    [0xff, 0xff, 0x55, 0xff],   // 0x0E: ATTR_FGND_BROWN   | ATTR_FGND_BRIGHT (aka yellow)
    [0xff, 0xff, 0xff, 0xff]    // 0x0F: ATTR_FGND_WHITE   | ATTR_FGND_BRIGHT (aka white)
];

Video.aCGAColorSet1 = [Video.ATTRS.FGND_GREEN, Video.ATTRS.FGND_RED,     Video.ATTRS.FGND_BROWN];
Video.aCGAColorSet2 = [Video.ATTRS.FGND_CYAN,  Video.ATTRS.FGND_MAGENTA, Video.ATTRS.FGND_WHITE];

/*
 * Here is the EGA BIOS default ATC palette register set for color text modes, from which getCardColors()
 * builds a default RGB array, similar to aCGAColors above.
 */
Video.aEGAPalDef = [0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x14, 0x07, 0x38, 0x39, 0x3A, 0x3B, 0x3C, 0x3D, 0x3E, 0x3F];

Video.aEGAByteToDW = [
      0x00000000,   0x000000ff,   0x0000ff00,   0x0000ffff,
      0x00ff0000,   0x00ff00ff,   0x00ffff00,   0x00ffffff,
      0xff000000|0, 0xff0000ff|0, 0xff00ff00|0, 0xff00ffff|0,
      0xffff0000|0, 0xffff00ff|0, 0xffffff00|0, 0xffffffff|0
];

Video.aEGADWToByte = [];
Video.aEGADWToByte[0x00000000]   = 0x0;
Video.aEGADWToByte[0x00000080]   = 0x1;
Video.aEGADWToByte[0x00008000]   = 0x2;
Video.aEGADWToByte[0x00008080]   = 0x3;
Video.aEGADWToByte[0x00800000]   = 0x4;
Video.aEGADWToByte[0x00800080]   = 0x5;
Video.aEGADWToByte[0x00808000]   = 0x6;
Video.aEGADWToByte[0x00808080]   = 0x7;
Video.aEGADWToByte[0x80000000|0] = 0x8;
Video.aEGADWToByte[0x80000080|0] = 0x9;
Video.aEGADWToByte[0x80008000|0] = 0xa;
Video.aEGADWToByte[0x80008080|0] = 0xb;
Video.aEGADWToByte[0x80800000|0] = 0xc;
Video.aEGADWToByte[0x80800080|0] = 0xd;
Video.aEGADWToByte[0x80808000|0] = 0xe;
Video.aEGADWToByte[0x80808080|0] = 0xf;

/*
 * Card Specifications
 *
 * We support dynamically switching between MDA and CGA cards by simply flipping switches on
 * the virtual SW1 switch block and resetting the machine.  However, I'm not sure I'll support
 * dynamically switching the EGA card the same way; there's certainly no UI for it at this point.
 *
 * For each supported card, there is a cardSpec array that the Card class uses to initialize the
 * card's defaults:
 *
 *      [0]: card descriptor
 *      [1]: default CRTC port address
 *      [2]: default video buffer address
 *      [3]: default video buffer size
 *      [4]: total on-board memory (if no "memory" parm was specified)
 *      [5]: default monitor type
 *
 * If total on-board memory is zero, then addMemory() will simply add the specified video buffer
 * to the address space; otherwise, we will allocate an internal buffer (adwMemory) and tell addMemory()
 * to map it to the video buffer address.  The latter approach gives us total control over the buffer;
 * refer to getMemoryAccess().
 *
 * TODO: Consider allocating our own buffer for all video cards, not just EGA/VGA.  For MDA/CGA, I'm not
 * sure it would offer any benefits, other than allowing our internal update functions, like updateScreen(),
 * to access the buffer directly, instead of going through the Bus memory interface.
 */
Video.cardSpecs = [];
Video.cardSpecs[Video.CARD.MDA] = ["MDA", Card.MDA.CRTC.INDX.PORT, 0xB0000, 0x01000, 0, ChipSet.MONITOR.MONO];
Video.cardSpecs[Video.CARD.CGA] = ["CGA", Card.CGA.CRTC.INDX.PORT, 0xB8000, 0x04000, 0, ChipSet.MONITOR.COLOR];
Video.cardSpecs[Video.CARD.EGA] = ["EGA", Card.CGA.CRTC.INDX.PORT, 0xB8000, 0x04000, 0x10000, ChipSet.MONITOR.EGACOLOR];
Video.cardSpecs[Video.CARD.VGA] = ["VGA", Card.CGA.CRTC.INDX.PORT, 0xB8000, 0x04000, 0x40000, ChipSet.MONITOR.VGACOLOR];

/*
 * Values for nTouchConfig; a value will be selected based on the sTouchScreen configuration parameter.
 */
Video.TOUCH = {
    NONE:       0,
    KEYGRID:    1,
    MOUSE:      2
};

/*
 * Port input/output notification tables
 *
 * TODO: At one point, I'd added some "duplicate" entries for the MDA because, according to docs I'd read,
 * MDA ports are decoded at multiple addresses.  However, if this is important, then it should be verified
 * and implemented consistently (eg, for CGA as well).  For now, I'm decoding only the standard port addresses.
 *
 * For example, 0x3B5 is apparently also decoded at 0x3B1, 0x3B3, and 0x3B7, while 0x3B4 is also decoded at
 * 0x3B0, 0x3B2, and 0x3B6.
 */
Video.aMDAPortInput = {
    0x3B4: Video.prototype.inMDAIndx,           // technically, not actually readable, but I want the Debugger to be able to read this
    0x3B5: Video.prototype.inMDAData,           // technically, the only CRTC Data registers that are readable are R14-R17
    0x3B8: Video.prototype.inMDAMode,           // technically, not actually readable, but I want the Debugger to be able to read this
    0x3BA: Video.prototype.inMDAStatus
};

Video.aMDAPortOutput = {
    0x3B4: Video.prototype.outMDAIndx,
    0x3B5: Video.prototype.outMDAData,
    0x3B8: Video.prototype.outMDAMode
};

Video.aCGAPortInput = {
    0x3D4: Video.prototype.inCGAIndx,           // technically, not actually readable, but I want the Debugger to be able to read this
    0x3D5: Video.prototype.inCGAData,           // technically, the only CRTC Data registers that are readable are R14-R17
    0x3D8: Video.prototype.inCGAMode,           // technically, not actually readable, but I want the Debugger to be able to read this
    0x3D9: Video.prototype.inCGAColor,          // technically, not actually readable, but I want the Debugger to be able to read this
    0x3DA: Video.prototype.inCGAStatus
};

Video.aCGAPortOutput = {
    0x3D4: Video.prototype.outCGAIndx,
    0x3D5: Video.prototype.outCGAData,
    0x3D8: Video.prototype.outCGAMode,
    0x3D9: Video.prototype.outCGAColor
};

Video.aEGAPortInput = {
    0x3C0: Video.prototype.inATCIndx,           // technically, only readable on a VGA, but I want the Debugger to be able to read this, too
    0x3C1: Video.prototype.inATCData,           // technically, only readable on a VGA, but I want the Debugger to be able to read this, too
    0x3C2: Video.prototype.inStatus0,
    0x3C4: Video.prototype.inSEQIndx,           // technically, only readable on a VGA, but I want the Debugger to be able to read this, too
    0x3C5: Video.prototype.inSEQData,           // technically, only readable on a VGA, but I want the Debugger to be able to read this, too
    0x3CE: Video.prototype.inGRCIndx,           // technically, only readable on a VGA, but I want the Debugger to be able to read this, too
    0x3CF: Video.prototype.inGRCData            // technically, only readable on a VGA, but I want the Debugger to be able to read this, too
};

/*
 * WARNING: Unlike the EGA, a standard VGA does not support writes to 0x3C1, but it's easier for me to leave that
 * ability in place, treating the VGA as a superset of the EGA as much as possible; will any code break because word
 * OUTs to port 0x3C0 (and/or byte OUTs to port 0x3C1) actually work?  Possibly, but highly unlikely.
 */
Video.aEGAPortOutput = {
    0x3BA: Video.prototype.outFeat,
    0x3C0: Video.prototype.outATC,
    0x3C1: Video.prototype.outATC,              // the EGA BIOS writes to this port (see C000:0416), implying that 0x3C0 and 0x3C1 both decode the same register
    0x3C2: Video.prototype.outMisc,             // FYI, since this overlaps with STATUS0.PORT, there's currently no way for the Debugger to read the Misc register
    0x3C4: Video.prototype.outSEQIndx,
    0x3C5: Video.prototype.outSEQData,
    0x3CA: Video.prototype.outGRCPos2,
    0x3CC: Video.prototype.outGRCPos1,
    0x3CE: Video.prototype.outGRCIndx,
    0x3CF: Video.prototype.outGRCData,
    0x3DA: Video.prototype.outFeat
};

Video.aVGAPortInput = {
    0x3C3: Video.prototype.inVGAEnable,
    0x3C6: Video.prototype.inDACMask,
    0x3C7: Video.prototype.inDACState,
    0x3C9: Video.prototype.inDACData,
    0x3CA: Video.prototype.inVGAFeat,
    0x3CC: Video.prototype.inVGAMisc
};

Video.aVGAPortOutput = {
    0x3C3: Video.prototype.outVGAEnable,
    0x3C6: Video.prototype.outDACMask,
    0x3C7: Video.prototype.outDACRead,
    0x3C8: Video.prototype.outDACWrite,
    0x3C9: Video.prototype.outDACData
};

/*
 * Initialize every Video module on the page.
 */
Web.onInit(Video.init);

if (NODE) module.exports = Video;