Finally, some progress with the IBM VGA ROM
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Jeff Parsons
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6 changed files with 574 additions and 88 deletions
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blog/2015/06/01/README.md
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blog/2015/06/01/README.md
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Debugging the IBM VGA ROM
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---
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The IBM VGA ("Video Graphics Array") standard was introduced as part of the IBM PS/2 line of computers;
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it was not a feature you could purchase or install in older PC, XT or AT-compatible machines. In fact, full VGA
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support was not even available in all PS/2 models. Of the first four PS/2 models -- the 8086-based Model 30, the
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80286-based Model 50 and Model 60, and the 80386-based Model 80 -- VGA support was available only in the three
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higher-end models. The Model 30 came with MCGA ("Multicolor Graphics Array") video hardware that supported a subset
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of VGA modes (eg, 640x480 2-color and 320x200 256-color graphics).
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It wasn't until October 1987 that IBM finally introduced an 8-bit ISA card that brought VGA capability to older PCs.
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The card was called the **IBM PS/2 Display Adapter**. However, I think the name is a bit confusing, since the card
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could only be used in PC, XT, and AT-compatible systems. I'll refer to it here simply as the IBM VGA.
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The VGA ROM used here is assumed to have come from an original IBM VGA. It's unknown if IBM ever made any
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revisions to the VGA ROM. With the introduction of the PS/2 family and the VGA, IBM decided to no longer publish
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the source code for its ROMs, so I've created some assemblable source code from the IBM VGA ROM
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[here](/devices/pc/video/ibm/vga/ibm-vga.nasm).
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I've finally started debugging a machine configuration that uses the IBM VGA ROM. Since the VGA and the 80386 are
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contemporaries, I'm using an [80386 machine configuration](/devices/pc/machine/compaq/deskpro386/vga/2048kb/machine.xml).
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However, I don't expect the IBM VGA ROM to require any 80386 support or PS/2-specific features.
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The first problem I ran into was here:
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;
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; Initialize the ROM BIOS Video Mode Options byte @40:0087 (default to color and 256Kb of RAM)
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;
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mov byte [0x487],0x60 ; 0000008D
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;
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; The x100 subroutine alternately enables port 0x3B? and 0x3D? decoding, verifying that there is
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; no response on opposing ports 0x3D? and 0x3B?, respectively; otherwise, it assumes that another
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; video card must exist and attempts to select co-existing settings for the VGA. For example, if
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; there is an unexpected response on the color ports, the VGA ROM will default to mono operation.
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;
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call x100 ; 00000092
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The Video component installs I/O port handlers for all possible I/O ranges; when a range isn't being used, the
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associated I/O operations are redirected to a dummy Card, so that the active Card isn't affected. Here,
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however, that was insufficient. If the VGA is the only installed video card, the VGA ROM expects *NO RESPONSE*
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on inactive CRTC ports. So I've changed the CRTC I/O handlers to check the Card's fActive flag. This seems
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like a safe and logical change, but we'll still have to check for backward-compatibility issues with older ROMs.
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The VGA ROM programs the card for the first time here:
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EAX=00000000 EBX=00001642 ECX=00000004 EDX=00004AE8
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ESP=000000EA EBP=FFFF00F0 ESI=00006000 EDI=000003D4
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SS=0030 DS=0000 ES=C000 FS=0000 GS=0304 PS=00000246 V0 D0 I1 T0 S0 Z1 A0 P1 C0
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C000:00CB E85910 CALL 1127 ;cycles=5
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videoVGA.outPort(0x03C4,SEQ.INDX,0x00) @C000:112F
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videoVGA.outPort(0x03C5,SEQ.RESET,0x01) @C000:112F
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videoVGA.outPort(0x03C4,SEQ.INDX,0x01) @C000:1204
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videoVGA.outPort(0x03C5,SEQ.CLK,0x29) @C000:1204
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videoVGA.outPort(0x03C4,SEQ.INDX,0x02) @C000:1204
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videoVGA.outPort(0x03C5,SEQ.MAPMASK,0x0F) @C000:1204
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videoVGA.outPort(0x03C4,SEQ.INDX,0x03) @C000:1204
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videoVGA.outPort(0x03C5,SEQ.CHARMAP,0x00) @C000:1204
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videoVGA.outPort(0x03C4,SEQ.INDX,0x04) @C000:1204
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videoVGA.outPort(0x03C5,SEQ.MODE,0x06) @C000:1204
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videoVGA.outPort(0x03C2,MISC,0x63) @C000:1143
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videoVGA.outPort(0x03C4,SEQ.INDX,0x00) @C000:1149
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videoVGA.outPort(0x03C5,SEQ.RESET,0x03) @C000:1149
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videoVGA.outPort(0x03D4,CRTC.INDX,0x11) @C000:1151
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videoVGA.outPort(0x03D5,CRTC.VERT_RETRACE_END,0x00) @C000:1151
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videoVGA.outPort(0x03D4,CRTC.INDX,0x00) @C000:1204
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videoVGA.outPort(0x03D5,CRTC.HORZ_TOTAL,0x5F) @C000:1204
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videoVGA.outPort(0x03D4,CRTC.INDX,0x01) @C000:1204
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videoVGA.outPort(0x03D5,CRTC.HORZ_DISP_END,0x4F) @C000:1204
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videoVGA.outPort(0x03D4,CRTC.INDX,0x02) @C000:1204
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videoVGA.outPort(0x03D5,CRTC.HORZ_BLANK_START,0x50) @C000:1204
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videoVGA.outPort(0x03D4,CRTC.INDX,0x03) @C000:1204
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videoVGA.outPort(0x03D5,CRTC.HORZ_BLANK_END,0x82) @C000:1204
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videoVGA.outPort(0x03D4,CRTC.INDX,0x04) @C000:1204
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videoVGA.outPort(0x03D5,CRTC.HORZ_RETRACE_START,0x55) @C000:1204
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videoVGA.outPort(0x03D4,CRTC.INDX,0x05) @C000:1204
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videoVGA.outPort(0x03D5,CRTC.HORZ_RETRACE_END,0x81) @C000:1204
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videoVGA.outPort(0x03D4,CRTC.INDX,0x06) @C000:1204
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videoVGA.outPort(0x03D5,CRTC.VERT_TOTAL,0xBF) @C000:1204
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videoVGA.outPort(0x03D4,CRTC.INDX,0x07) @C000:1204
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videoVGA.outPort(0x03D5,CRTC.CRTC_OVERFLOW,0x1F) @C000:1204
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videoVGA.outPort(0x03D4,CRTC.INDX,0x08) @C000:1204
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videoVGA.outPort(0x03D5,CRTC.PRESET_ROW_SCAN,0x00) @C000:1204
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videoVGA.outPort(0x03D4,CRTC.INDX,0x09) @C000:1204
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videoVGA.outPort(0x03D5,CRTC.MAX_SCAN_LINE,0x40) @C000:1204
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removeCursor(): removed from 0,0
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videoVGA.outPort(0x03D4,CRTC.INDX,0x0A) @C000:1204
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videoVGA.outPort(0x03D5,CRTC.CURSOR_START,0x00) @C000:1204
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checkCursor(): cursor moved from -1 to 0
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videoVGA.outPort(0x03D4,CRTC.INDX,0x0B) @C000:1204
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videoVGA.outPort(0x03D5,CRTC.CURSOR_END,0x00) @C000:1204
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videoVGA.outPort(0x03D4,CRTC.INDX,0x0C) @C000:1204
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videoVGA.outPort(0x03D5,CRTC.START_ADDR_HI,0x00) @C000:1204
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videoVGA.outPort(0x03D4,CRTC.INDX,0x0D) @C000:1204
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videoVGA.outPort(0x03D5,CRTC.START_ADDR_LO,0x00) @C000:1204
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videoVGA.outPort(0x03D4,CRTC.INDX,0x0E) @C000:1204
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videoVGA.outPort(0x03D5,CRTC.CURSOR_ADDR_HI,0x00) @C000:1204
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videoVGA.outPort(0x03D4,CRTC.INDX,0x0F) @C000:1204
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videoVGA.outPort(0x03D5,CRTC.CURSOR_ADDR_LO,0x00) @C000:1204
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videoVGA.outPort(0x03D4,CRTC.INDX,0x10) @C000:1204
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videoVGA.outPort(0x03D5,CRTC.VERT_RETRACE_START,0x9C) @C000:1204
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videoVGA.outPort(0x03D4,CRTC.INDX,0x11) @C000:1204
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videoVGA.outPort(0x03D5,CRTC.VERT_RETRACE_END,0x8E) @C000:1204
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videoVGA.outPort(0x03D4,CRTC.INDX,0x12) @C000:1204
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videoVGA.outPort(0x03D5,CRTC.VERT_DISP_END,0x8F) @C000:1204
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videoVGA.outPort(0x03D4,CRTC.INDX,0x13) @C000:1204
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videoVGA.outPort(0x03D5,CRTC.OFFSET,0x28) @C000:1204
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videoVGA.outPort(0x03D4,CRTC.INDX,0x14) @C000:1204
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videoVGA.outPort(0x03D5,CRTC.UNDERLINE,0x1F) @C000:1204
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videoVGA.outPort(0x03D4,CRTC.INDX,0x15) @C000:1204
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videoVGA.outPort(0x03D5,CRTC.VERT_BLANK_START,0x96) @C000:1204
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videoVGA.outPort(0x03D4,CRTC.INDX,0x16) @C000:1204
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videoVGA.outPort(0x03D5,CRTC.VERT_BLANK_END,0xB9) @C000:1204
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videoVGA.outPort(0x03D4,CRTC.INDX,0x17) @C000:1204
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videoVGA.outPort(0x03D5,CRTC.MODE_CTRL,0xE3) @C000:1204
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videoVGA.outPort(0x03D4,CRTC.INDX,0x18) @C000:1204
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videoVGA.outPort(0x03D5,CRTC.LINE_COMPARE,0xFF) @C000:1204
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videoVGA.inPort(0x03DA,STATUS1): 0x00 @C000:1167
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videoVGA.outPort(0x03C0,ATC.INDX,0x10) @C000:116C
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videoVGA.outPort(0x03C0,ATC.MODE,0x01) @C000:1171
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videoVGA.outPort(0x03C0,ATC.INDX,0x12) @C000:1174
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videoVGA.outPort(0x03C0,ATC.PLANES,0x0F) @C000:1179
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videoVGA.outPort(0x03C0,ATC.INDX,0x13) @C000:117C
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videoVGA.outPort(0x03C0,ATC.HORZPAN,0x00) @C000:1181
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videoVGA.outPort(0x03CE,GRC.INDX,0x00) @C000:1204
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videoVGA.outPort(0x03CF,GRC.SRESET,0x00) @C000:1204
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videoVGA.outPort(0x03CE,GRC.INDX,0x01) @C000:1204
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videoVGA.outPort(0x03CF,GRC.ESRESET,0x00) @C000:1204
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videoVGA.outPort(0x03CE,GRC.INDX,0x02) @C000:1204
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videoVGA.outPort(0x03CF,GRC.COLRCMP,0x0F) @C000:1204
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videoVGA.outPort(0x03CE,GRC.INDX,0x03) @C000:1204
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videoVGA.outPort(0x03CF,GRC.DATAROT,0x00) @C000:1204
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videoVGA.outPort(0x03CE,GRC.INDX,0x04) @C000:1204
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videoVGA.outPort(0x03CF,GRC.READMAP,0x00) @C000:1204
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videoVGA.outPort(0x03CE,GRC.INDX,0x05) @C000:1204
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videoVGA.outPort(0x03CF,GRC.MODE,0x08) @C000:1204
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setAccess(0x0210)
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videoVGA.outPort(0x03CE,GRC.INDX,0x06) @C000:1204
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videoVGA.outPort(0x03CF,GRC.MISC,0x05) @C000:1204
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setMode(0x0010)
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setMode(16): removing 0x00008000 bytes from 0x000B8000
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setMode(16): adding 0x00010000 bytes to 0x000A0000
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videoVGA.outPort(0x03CE,GRC.INDX,0x07) @C000:1204
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videoVGA.outPort(0x03CF,GRC.COLRDC,0x0F) @C000:1204
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videoVGA.outPort(0x03CE,GRC.INDX,0x08) @C000:1204
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videoVGA.outPort(0x03CF,GRC.BITMASK,0xFF) @C000:1204
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EAX=0000FF09 EBX=00001642 ECX=00000000 EDX=000003DA
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ESP=000000EA EBP=FFFF00F0 ESI=00006000 EDI=000003D4
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SS=0030 DS=0000 ES=C000 FS=0000 GS=0304 PS=00000246 V0 D0 I1 T0 S0 Z1 A0 P1 C0
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C000:00CE 32C0 XOR AL,AL ;cycles=1031
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Now that the card was programmed for the first time, I used the `d video` Debugger command to dump the video
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card's state:
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BIOSMODE: 0x10
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CRTC[0x00]: HORZ_TOTAL 0x5F
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CRTC[0x01]: HORZ_DISP_END 0x4F
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CRTC[0x02]: HORZ_BLANK_START 0x50
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CRTC[0x03]: HORZ_BLANK_END 0x82
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CRTC[0x04]: HORZ_RETRACE_START 0x55
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CRTC[0x05]: HORZ_RETRACE_END 0x81
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CRTC[0x06]: VERT_TOTAL 0xBF
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CRTC[0x07]: CRTC_OVERFLOW 0x1F
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CRTC[0x08]: PRESET_ROW_SCAN 0x00
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CRTC[0x09]: MAX_SCAN_LINE 0x40
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CRTC[0x0A]: CURSOR_START 0x00
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CRTC[0x0B]: CURSOR_END 0x00
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CRTC[0x0C]: START_ADDR_HI 0x00
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CRTC[0x0D]: START_ADDR_LO 0x00
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CRTC[0x0E]: CURSOR_ADDR_HI 0x00
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CRTC[0x0F]: CURSOR_ADDR_LO 0x00
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CRTC[0x10]: VERT_RETRACE_START 0x9C
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CRTC[0x11]: VERT_RETRACE_END 0x8E
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CRTC[0x12]: VERT_DISP_END 0x8F
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CRTC[0x13]: OFFSET 0x28
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CRTC[0x14]: UNDERLINE 0x1F
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CRTC[0x15]: VERT_BLANK_START 0x96
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CRTC[0x16]: VERT_BLANK_END 0xB9
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CRTC[0x17]: MODE_CTRL 0xE3
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CRTC[0x18]: LINE_COMPARE 0xFF*
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STATUS1: 0x00
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ATCDATA: false
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ATC[0x00]: PAL00 0x??
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ATC[0x01]: PAL01 0x??
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ATC[0x02]: PAL02 0x??
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ATC[0x03]: PAL03 0x??
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ATC[0x04]: PAL04 0x??
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ATC[0x05]: PAL05 0x??
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ATC[0x06]: PAL06 0x??
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ATC[0x07]: PAL07 0x??
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ATC[0x08]: PAL08 0x??
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ATC[0x09]: PAL09 0x??
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ATC[0x0A]: PAL0A 0x??
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ATC[0x0B]: PAL0B 0x??
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ATC[0x0C]: PAL0C 0x??
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ATC[0x0D]: PAL0D 0x??
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ATC[0x0E]: PAL0E 0x??
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ATC[0x0F]: PAL0F 0x??
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ATC[0x10]: MODE 0x01
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ATC[0x11]: OVRSCAN 0x??
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ATC[0x12]: PLANES 0x0F
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ATC[0x13]: HORZPAN 0x00*
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GRC[0x00]: SRESET 0x00
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GRC[0x01]: ESRESET 0x00
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GRC[0x02]: COLRCMP 0x0F
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GRC[0x03]: DATAROT 0x00
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GRC[0x04]: READMAP 0x00
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GRC[0x05]: MODE 0x08
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GRC[0x06]: MISC 0x05
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GRC[0x07]: COLRDC 0x0F
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GRC[0x08]: BITMASK 0xFF*
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SEQ[0x00]: RESET 0x03*
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SEQ[0x01]: CLK 0x29
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SEQ[0x02]: MAPMASK 0x0F
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SEQ[0x03]: CHARMAP 0x00
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SEQ[0x04]: MODE 0x06
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FEAT: 0x00
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MISC: 0x63
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STATUS0: 0x00
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LATCHES: 0x00
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ACCESS: 0x0210
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Use 'dump video buffer' to dump video memory
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If you've noticed that none of the ATC palette registers were programmed, well, that's apparently by design.
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*[@jeffpar](http://twitter.com/jeffpar)*
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*June 1, 2015*
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