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---
title: Michael Abrash's Graphics Programming Black Book, Special Edition
author: Michael Abrash
date: '1997-07-01'
identifier:
- scheme: ISBN
text: 1576101746
publisher: The Coriolis Group
category: 'Web and Software Development: Game Development,Web and Software Development:
Graphics and Multimedia Development'
chapter: '31'
pages: 587-605
---
## Chapter 31 -- Higher 256-Color Resolution on the VGA
### When Is 320x200 Really 320x400?
One of the more appealing features of the VGA is its ability to display
256 simultaneous colors. Unfortunately, one of the *less* appealing
features of the VGA is the limited resolution (320x200) of the one
256-color mode the IBM-standard BIOS supports. (There are, of course,
higher resolution 256-color modes in the legion of SuperVGAs, but they
are by no means a standard, and differences between seemingly identical
modes from different manufacturers can be vexing.) More colors can often
compensate for less resolution, but the resolution difference between
the 640x480 16-color mode and the 320x200 256-color mode is so great
that many programmers must regretfully decide that they simply can't
afford to use the 256-color mode.
If there's one thing we've learned about the VGA, however, it's that
there's *never* just one way to do things. With the VGA, alternatives
always exist for the clever programmer, and that's more true than you
might imagine with 256-color mode. Not only is there a high 256-color
resolution, there are *lots* of higher 256-color resolutions, going all
the way up to 360x480—and that's with the vanilla IBM VGA!
In this chapter, I'm going to focus on one of my favorite 256-color
modes, which provides 320x400 resolution and two graphics pages and can
be set up with very little reof the VGA. In the next chapter, I'll
discuss higher-resolution 256-color modes, and starting in Chapter 47,
I'll cover the high-performance "Mode X" 256-color programming that many
games use.
So. Let's get started.
### Why 320x200? Only IBM Knows for Sure
The first question, of course, is, "How can it be possible to get higher
256-color resolutions out of the VGA?" After all, there were no unused
higher resolutions to be found in the CGA, Hercules card, or EGA.
The answer is another question: "Why did IBM *not* use the
higher-resolution 256-color modes of the VGA?" The VGA is easily capable
of twice the 200-scan-line vertical resolution of mode 13H, the
256-color mode, and IBM clearly made a decision not to support a
higher-resolution 256-color mode. In fact, mode 13H *does* display 400
scan lines, but each row of pixels is displayed on two successive scan
lines, resulting in an effective resolution of 320x200. This is the same
scan-doubling approach used by the VGA to convert the CGA's
200-scan-line modes to 400 scan lines; however, the resolution of the
CGA has long been fixed at 200 scan lines, so IBM had no choice with the
CGA modes but to scan-double the lines. Mode 13H has no such historical
limitation—it's the first 256-color mode ever offered by IBM, if you
don't count the late and unlamented Professional Graphics Controller
(PGC). Why, then, would IBM choose to limit the resolution of mode 13H?
There's no way to know, but one good guess is that IBM wanted a standard
256-color mode across all PS/2 computers (for which the VGA was
originally created), and mode 13H is the highest-resolution 256-color
mode that could fill the bill. You see, each 256-color pixel requires
one byte of display memory, so a 320x200 256-color mode requires 64,000
bytes of display memory. That's no problem for the VGA, which has 256K
of display memory, but it's a stretch for the MCGA of the Model 30,
since the MCGA comes with only 64K.
On the other hand, the smaller display memory size of the MCGA also
limits the number of colors supported in 640x480 mode to 2, rather than
the 16 supported by the VGA. In this case, though, IBM simply created
two modes and made both available on the VGA: mode 11H for 640x480
2-color graphics and mode 12H for 640x480 16-color graphics. The same
could have been done for 256-color graphics—but wasn't. Why? I don't
know. Maybe IBM just didn't like the odd aspect ratio of a 320x400
graphics mode. Maybe they didn't want to have to worry about how to map
in more than 64K of display memory. Heck, maybe they made a mistake in
designing the chip. Whatever the reason, mode 13H is really a
400-scan-line mode masquerading as a 200-scan-line mode, and we can
readily end that masquerade.
### 320x400 256-Color Mode
Okay, what's so great about 320x400 256-color mode? Two things: easy,
safe mode sets and page flipping.
As I said above, mode 13H is really a 320x400 mode, albeit with each
line doubled to produce an effective resolution of 320x200. That means
that we don't need to change any display timings, widths, or heights in
order to tweak mode 13H into 320x400 mode—and that makes 320x400 a safe
choice. Basically, 320x400 mode differs from mode 13H only in the
settings of *mode* bits, which are sure to be consistent from one VGA
clone to the next and which work equally well with all monitors. The
other hi-res 256-color modes differ from mode 13H not only in the
settings of the mode bits but also in the settings of timing and
dimension registers, which may not be exactly the same on all VGA clones
and particularly not on all multisync monitors. (Because multisyncs
sometimes shrink the active area of the screen when used with standard
VGA modes, some VGAs use alternate register settings for multisync
monitors that adjust the CRT Controller timings to use as much of the
screen area as possible for displaying pixels.)
The other good thing about 320x400 256-color mode is that two pages are
supported. Each 320x400 256-color mode requires 128,000 bytes of display
memory, so we can just barely manage two pages in 320x400 mode, one
starting at offset 0 in display memory and the other starting at offset
8000H. Those two pages are the largest pair of pages that can fit in the
VGA's 256K, though, and the higher-resolution 256-color modes, which use
still larger bitmaps (areas of display memory that control pixels on the
screen), can't support two pages at all. As we've seen in earlier
chapters and will see again in this book, paging is very useful for
off-screen construction of images and fast, smooth animation.
That's why I like 320x400 256-color mode. The next step is to understand
how display memory is organized in 320x400 mode, and that's not so
simple.
#### Display Memory Organization in 320x400 Mode
First, let's look at why display memory must be organized differently in
320x400 256-color mode than in mode 13H. The designers of the VGA
intentionally limited the maximum size of the bitmap in mode 13H to 64K,
thereby limiting resolution to 320x200. This was accomplished *in
hardware*, so there is no way to extend the bitmap organization of mode
13H to 320x400 mode.
That's a shame, because mode 13H has the simplest bitmap organization of
any mode—one long, linear bitmap, with each byte controlling one pixel.
We can't have that organization, though, so we'll have to find an
acceptable substitute if we want to use a higher 256-color resolution.
We're talking about the VGA, so of course there are actually *several*
bitmap organizations that let us use higher 256-color resolutions than
mode 13H. The one I like best is shown in Figure 31.1. Each byte
controls one 256-color pixel. Pixel 0 is at address 0 in plane 0, pixel
1 is at address 0 in plane 1, pixel 2 is at address 0 in plane 2, pixel
3 is at address 0 in plane 3, pixel 4 is at address 1 in plane 0, and so
on.
Let's look at this another way. Ideally, we'd like one long bitmap, with
each pixel at the address that's just after the address of the pixel to
the left. Well, that's true in this case too, *if* you consider the
number of the plane that the pixel is in to be part of the pixel's
address. View the pixel numbers on the screen as increasing from left to
right and from the end of one scan line to the start of the next. Then
the pixel number, n, of the pixel at display memory address *address* in
plane *plane* is:
![**Figure 31.1**  *Bitmap organization in 320x400 256-color mode in
320x400 256-color mode.*](images/31-01.jpg)
*n* = (*address* \* 4) + *plane*
To turn that around, the display memory address of pixel number n is
given by
address = *n* / 4
and the plane of pixel *n* is given by:
plane = *n* modulo 4
Basically, the full address of the pixel, its pixel number, is broken
into two components: the display memory address and the plane.
By the way, because 320x400 mode has a significantly different memory
organization from mode 13H, the BIOS text routines won't work in 320x400
mode. If you want to draw text in 320x400 mode, you'll have to look up a
font in the BIOS ROM and draw the text yourself. Likewise, the BIOS read
pixel and write pixel routines won't work in 320x400 mode, but that's no
problem because I'll provide equivalent routines in the next section.
Our next task is to convert standard mode 13H into 320x400 mode. That's
accomplished by undoing some of the mode bits that are set up especially
for mode 13H, so that from a programming perspective the VGA reverts to
a straightforward planar model of memory. That means taking the VGA out
of chain 4 mode and doubleword mode, turning off the double display of
each scan line, making sure chain mode, odd/even mode, and word mode are
turned off, and selecting byte mode for video data display. All that's
done in the `Set320x400Mode` subroutine in Listing 31.1, which we'll
discuss next.
#### Reading and Writing Pixels
The basic graphics functions in any mode are functions to read and write
single pixels. Any more complex function can be built on these
primitives, although that's rarely the speediest solution. What's more,
once you understand the operation of the read and write pixel functions,
you've got all the knowledge you need to create functions that perform
more complex graphics functions. Consequently, we'll start our
exploration of 320x400 mode with pixel-at-a-time line drawing.
Listing 31.1 draws 8 multicolored octagons in turn, drawing a new one on
top of the old one each time a key is pressed. The main-loop code of
Listing 31.1 should be easily understood; a series of diagonal,
horizontal, and vertical lines are drawn one pixel at a time based on a
list of line descriptors, with the draw colors incremented for each
successive time through the line list.
**LISTING 31.1 L31-1.ASM**
```nasm
; Program to demonstrate pixel drawing in 320x400 256-color
; mode on the VGA. Draws 8 lines to form an octagon, a pixel
; at a time. Draws 8 octagons in all, one on top of the other,
; each in a different color set. Although it's not used, a
; pixel read function is also provided.
;
VGA_SEGMENT equ 0a000h
SC_INDEX equ 3c4h ;Sequence Controller Index register
GC_INDEX equ 3ceh ;Graphics Controller Index register
CRTC_INDEX equ 3d4h ;CRT Controller Index register
MAP_MASK equ 2 ;Map Mask register index in SC
MEMORY_MODE equ 4 ;Memory Mode register index in SC
MAX_SCAN_LINE equ 9 ;Maximum Scan Line reg index in CRTC
START_ADDRESS_HIGH equ 0ch ;Start Address High reg index in CRTC
UNDERLINE equ 14h ;Underline Location reg index in CRTC
MODE_CONTROL equ 17h ;Mode Control register index in CRTC
READ_MAP equ 4 ;Read Map register index in GC
GRAPHICS_MODE equ 5 ;Graphics Mode register index in GC
MISCELLANEOUS equ 6 ;Miscellaneous register index in GC
SCREEN_WIDTH equ 320 ;# of pixels across screen
SCREEN_HEIGHT equ 400 ;# of scan lines on screen
WORD_OUTS_OK equ 1 ;set to 0 to assemble for
; computers that can't handle
; word outs to indexed VGA registers
;
stack segment para stack 'STACK'
db 512 dup (?)
stack ends
;
Data segment word 'DATA'
;
BaseColor db 0
;
; Structure used to control drawing of a line.
;
LineControl struc
StartX dw ?
StartY dw ?
LineXInc dw ?
LineYInc dw ?
BaseLength dw ?
LineColor db ?
LineControl ends
;
; List of descriptors for lines to draw.
;
LineList label LineControl
LineControl <130,110,1,0,60,0>
LineControl <190,110,1,1,60,1>
LineControl <250,170,0,1,60,2>
LineControl <250,230,-1,1,60,3>
LineControl <190,290,-1,0,60,4>
LineControl <130,290,-1,-1,60,5>
LineControl <70,230,0,-1,60,6>
LineControl <70,170,1,-1,60,7>
LineControl <-1,0,0,0,0,0>
Data ends
;
; Macro to output a word value to a port.
;
OUT_WORD macro
if WORD_OUTS_OK
out dx,ax
else
out dx,al
inc dx
xchg ah,al
out dx,al
dec dx
xchg ah,al
endif
endm
;
; Macro to output a constant value to an indexed VGA register.
;
CONSTANT_TO_INDEXED_REGISTERmacroADDRESS, INDEX, VALUE
mov dx,ADDRESS
mov ax,(VALUE shl 8) + INDEX
OUT_WORD
endm
;
Code segment
assume cs:Code, ds:Data
Start proc near
mov ax,Data
mov ds,ax
;
; Set 320x400 256-color mode.
;
call Set320By400Mode
;
; We're in 320x400 256-color mode. Draw each line in turn.
;
ColorLoop:
mov si,offset LineList ;point to the start of the
; line descriptor list
LineLoop:
mov cx,[si+StartX] ;set the initial X coordinate
cmpcx,-1
jz LinesDone ;a descriptor with a -1 X
; coordinate marks the end
; of the list
mov dx,[si+StartY] ;set the initial Y coordinate,
mov bl,[si+LineColor] ; line color,
mov bp,[si+BaseLength] ; and pixel count
add bl,[BaseColor] ;adjust the line color according
; to BaseColor
PixelLoop:
push cx ;save the coordinates
push dx
call WritePixel ;draw this pixel
pop dx ;retrieve the coordinates
pop cx
add cx,[si+LineXInc] ;set the coordinates of the
add dx,[si+LineYInc] ; next point of the line
dec bp ;any more points?
jnz PixelLoop ;yes, draw the next
add si,size LineControl ;point to the next line descriptor
jmp LineLoop ; and draw the next line
LinesDone:
call GetNextKey ;wait for a key, then
inc [BaseColor] ; bump the color selection and
cmp [BaseColor],8 ; see if we're done
jb ColorLoop ;not done yet
;
; Wait for a key and return to text mode and end when
; one is pressed.
;
call GetNextKey
mov ax,0003h
int 10h text mode
mov ah,4ch
int 21h ;done
;
Start endp
;
; Sets up 320x400 256-color modes.
;
; Input: none
;
; Output: none
;
Set320By400Mode proc near
;
; First, go to normal 320x200 256-color mode, which is really a
; 320x400 256-color mode with each line scanned twice.
;
mov ax,0013h ;AH = 0 means mode set, AL = 13h selects
; 256-color graphics mode
int 10h ;BIOS video interrupt
;
; Change CPU addressing of video memory to linear (not odd/even,
; chain, or chain 4), to allow us to access all 256K of display
; memory. When this is done, VGA memory will look just like memory
; in modes 10h and 12h, except that each byte of display memory will
; control one 256-color pixel, with 4 adjacent pixels at any given
; address, one pixel per plane.
;
mov dx,SC_INDEX
mov al,MEMORY_MODE
out dx,al
inc dx
ina l,dx
and al,not 08h ;turn off chain 4
ora l,04h ;turn off odd/even
out dx,al
mov dx,GC_INDEX
mov al,GRAPHICS_MODE
out dx,al
inc dx
ina l,dx
and al,not 10h ;turn off odd/even
out dx,al
dec dx
mov al,MISCELLANEOUS
out dx,al
inc dx
ina l,dx
and al,not 02h ;turn off chain
out dx,al
;
; Now clear the whole screen, since the mode 13h mode set only
; cleared 64K out of the 256K of display memory. Do this before
; we switch the CRTC out of mode 13h, so we don't see garbage
; on the screen when we make the switch.
;
CONSTANT_TO_INDEXED_REGISTER SC_INDEX,MAP_MASK,0fh
;enable writes to all planes, so
; we can clear 4 pixels at a time
mov ax,VGA_SEGMENT
mov es,ax
sub di,di
mov ax,di
mov cx,8000h ;# of words in 64K
cld
rep stosw ;clear all of display memory
;
; Tweak the mode to 320x400 256-color mode by not scanning each
; line twice.
;
mov dx,CRTC_INDEX
mov al,MAX_SCAN_LINE
out dx,al
inc dx
in al,dx
and al,not 1fh ;set maximum scan line = 0
out dx,al
dec dx
;
; Change CRTC scanning from doubleword mode to byte mode, allowing
; the CRTC to scan more than 64K of video data.
;
mov al,UNDERLINE
out dx,al
inc dx
in al,dx
and al,not 40h ;turn off doubleword
out dx,al
dec dx
mov al,MODE_CONTROL
out dx,al
inc dx
in al,dx
or al,40h ;turn on the byte mode bit, so memory is
; scanned for video data in a purely
; linear way, just as in modes 10h and 12h
out dx,al
ret
Set320By400Mode endp
;
; Draws a pixel in the specified color at the specified
; location in 320x400 256-color mode.
;
; Input:
; CX = X coordinate of pixel
; DX = Y coordinate of pixel
; BL = pixel color
;
; Output: none
;
; Registers altered: AX, CX, DX, DI, ES
;
WritePixel proc near
mov ax,VGA_SEGMENT
mov es,ax ;point to display memory
mov ax,SCREEN_WIDTH/4
;there are 4 pixels at each address, so
; each 320-pixel row is 80 bytes wide
; in each plane
mul dx ;point to start of desired row
push cx ;set aside the X coordinate
shr cx,1 ;there are 4 pixels at each address
shr cx,1 ; so divide the X coordinate by 4
add ax,cx ;point to the pixel's address
mov di,ax
pop cx ;get back the X coordinate
and cl,3 ;get the plane # of the pixel
mov ah,1
shl ah,cl ;set the bit corresponding to the plane
; the pixel is in
mov al,MAP_MASK
mov dx,SC_INDEX
OUT_WORD ;set to write to the proper plane for
; the pixel
mov es:[di],bl ;draw the pixel
ret
WritePixelendp
;
; Reads the color of the pixel at the specified location in 320x400
; 256-color mode.
;
; Input:
; CX = X coordinate of pixel to read
; DX = Y coordinate of pixel to read
;
; Output:
; AL = pixel color
;
; Registers altered: AX, CX, DX, SI, ES
;
ReadPixelprocnear
mov ax,VGA_SEGMENT
mov es,ax ;point to display memory
mov ax,SCREEN_WIDTH/4
;there are 4 pixels at each address, so
; each 320-pixel row is 80 bytes wide
; in each plane
mul dx ;point to start of desired row
push cx ;set aside the X coordinate
shr cx,1 ;there are 4 pixels at each address
shr cx,1 ; so divide the X coordinate by 4
add ax,cx ;point to the pixel's address
mov si,ax
pop ax ;get back the X coordinate
and al,3 ;get the plane # of the pixel
mov ah,al
mov al,READ_MAP
mov dx,GC_INDEX
OUT_WORD ;set to read from the proper plane for
; the pixel
lodsbyte ptr es:[si] ;read the pixel
ret
ReadPixelendp
;
; Waits for the next key and returns it in AX.
;
; Input: none
;
; Output:
; AX = full 16-bit code for key pressed
;
GetNextKey proc near
WaitKey:
mov ah,1
int 16h
jz WaitKey ;wait for a key to become available
sub ah,ah
int 16h ;read the key
ret
GetNextKey endp
;
Code ends
;
end Start
```
The interesting aspects of Listing 31.1 are three. First, the
`Set320x400Mode` subroutine selects 320x400 256-color mode. This is
accomplished by performing a mode 13H mode set followed by then putting
the VGA into standard planar byte mode. `Set320x400Mode` zeros display
memory as well. It's necessary to clear display memory even after a mode
13H mode set because the mode 13H mode set clears only the 64K of
display memory that can be accessed in that mode, leaving 192K of
display memory untouched.
The second interesting aspect of Listing 31.1 is the `WritePixel`
subroutine, which draws a colored pixel at any *x,y* addressable
location on the screen. Although it may not be obvious because I've
optimized the code a little, the process of drawing a pixel is
remarkably simple. First, the pixel's display memory address is
calculated as
*address*=(*y* \* (SCREEN\_WIDTH / 4)) + (*x* / 4)
which might be more recognizable as:
*address*=((*y* \* SCREEN\_WIDTH) + *x*) / 4
(There are 4 pixels at each display memory address in 320x400 mode,
hence the division by 4.) Then the pixel's plane is calculated as
*plane*=*x* and 3
which is equivalent to:
*plane*=*x* modulo 4
The pixel's color is then written to the addressed byte in the addressed
plane. That's all there is to it!
The third item of interest in Listing 31.1 is the `ReadPixel`
subroutine. `ReadPixel` is virtually identical to `WritePixel`, save
that in `ReadPixel` the Read Map register is programmed with a plane
number, while `WritePixel` uses a plane *mask* to set the Map Mask
register. Of course, that difference merely reflects a fundamental
difference in the operation of the two registers. (If that's Greek to
you, refer back to Chapters 23-30 for a refresher on VGA programming.)
`ReadPixel` isn't used in Listing 31.1, but I've included it because,
as I said above, the read and write pixel functions together can support
a whole host of more complex graphics functions.
How does 320x400 256-color mode stack up as regards performance? As it
turns out, the programming model of 320x400 mode is actually pretty good
for pixel drawing, pretty much on a par with the model of mode 13H. When
you run Listing 31.1, you'll no doubt notice that the lines are drawn
quite rapidly. (In fact, the drawing could be considerably faster still
with a dedicated line-drawing subroutine, which would avoid the
multiplication associated with each pixel in Listing 31.1.)
In 320x400 mode, the calculation of the memory address is not
significantly slower than in mode 13H, and the calculation and selection
of the target plane is quickly accomplished. As with mode 13H, 320x400
mode benefits tremendously from the byte-per-pixel organization of
256-color mode, which eliminates the need for the time-consuming
pixel-masking of the 16-color modes. Most important, byte-per-pixel
modes never require read-modify-write operations (which can be extremely
slow due to display memory wait states) in order to clip and draw
pixels. To draw a pixel, you just store its color in display memory—what
could be simpler?
More sophisticated operations than pixel drawing are less easy to
accomplish in 320x400 mode, but with a little ingenuity it is possible
to implement a reasonably efficient version of just about any useful
graphics function. A fast line draw for 320x400 256-color mode would be
simple (although not as fast as would be possible in mode 13H). Fast
image copies could be implemented by copying one-quarter of the image to
one plane, one-quarter to the next plane, and so on for all four planes,
thereby eliminating the `OUT` per pixel that sequential processing
requires. If you're really into performance, you could store your images
with all the bytes for plane 0 grouped together, followed by all the
bytes for plane 1, and so on. That would allow a single `REP MOVS`
instruction to copy all the bytes for a given plane, with just four
`REP MOVS` instructions copying the whole image. In a number of cases,
in fact, 320x400 256-color mode can actually be much faster than mode
13H, because the VGA's hardware can be used to draw four or even eight
pixels with a single access; I'll return to the topic of
high-performance programming in 256-color modes other than mode 13H
("non-chain 4" modes) in Chapter 47.
It's all a bit complicated, but as I say, you should be able to design
an adequately fast—and often *very* fast—version for 320x400 mode of
whatever graphics function you need. If you're not all that concerned
with speed, `WritePixel` and `ReadPixel` should meet your needs.
### Two 256-Color Pages
Listing 31.2 demonstrates the two pages of 320x400 256-color mode by
drawing slanting color bars in page 0, then drawing color bars slanting
the other way in page 1 and flipping to page 1 on the next key press.
(Note that page 1 is accessed starting at offset 8000H in display
memory, and is—unsurprisingly—displayed by setting the start address to
8000H.) Finally, Listing 31.2 draws vertical color bars in page 0 and
flips back to page 0 when another key is pressed.
The color bar routines don't use the `WritePixel` subroutine from
Listing 31.1; they go straight to display memory instead for improved
speed. As I mentioned above, better speed yet could be achieved by a
color-bar algorithm that draws all the pixels in plane 0, then all the
pixels in plane 1, and so on, thereby avoiding the overhead of
constantly reprogramming the Map Mask register.
**LISTING 31.2 L31-2.ASM**
```nasm
; Program to demonstrate the two pages available in 320x400
; 256-color modes on a VGA. Draws diagonal color bars in all
; 256 colors in page 0, then does the same in page 1 (but with
; the bars tilted the other way), and finally draws vertical
; color bars in page 0.
;
VGA_SEGMENT equ 0a000h
SC_INDEX equ 3c4h ;Sequence Controller Index register
GC_INDEX equ 3ceh ;Graphics Controller Index register
CRTC_INDEX equ 3d4h ;CRT Controller Index register
MAP_MASK equ 2 ;Map Mask register index in SC
MEMORY_MODE equ 4 ;Memory Mode register index in SC
MAX_SCAN_LINE equ 9 ;Maximum Scan Line reg index in CRTC
START_ADDRESS_HIGH equ 0ch ;Start Address High reg index in CRTC
UNDERLINE equ 14h ;Underline Location reg index in CRTC
MODE_CONTROL equ 17h ;Mode Control register index in CRTC
GRAPHICS_MODE equ 5 ;Graphics Mode register index in GC
MISCELLANEOUS equ 6 ;Miscellaneous register index in GC
SCREEN_WIDTH equ 320 ;# of pixels across screen
SCREEN_HEIGHT equ 400 ;# of scan lines on screen
WORD_OUTS_OK equ 1 ;set to 0 to assemble for
; computers that can't handle
; word outs to indexed VGA registers
;
stack segment para stack 'STACK'
db 512 dup (?)
stack ends
;
; Macro to output a word value to a port.
;
OUT_WORDmacro
if WORD_OUTS_OK
outdx,ax
else
out dx,al
inc dx
xch gah,al
out dx,al
dec dx
xch gah,al
endif
endm
;
; Macro to output a constant value to an indexed VGA register.
;
CONSTANT_TO_INDEXED_REGISTERmacroADDRESS, INDEX, VALUE
mov dx,ADDRESS
mov ax,(VALUE shl 8) + INDEX
OUT_WORD
endm
;
Code segment
assume cs:Code
Start proc near
;
; Set 320x400 256-color mode.
;
callSet320By400Mode
;
; We're in 320x400 256-color mode, with page 0 displayed.
; Let's fill page 0 with color bars slanting down and to the right.
;
sub di,di ;page 0 starts at address 0
mov bl,1 ;make color bars slant down and
; to the right
call ColorBarsUp ;draw the color bars
;
; Now do the same for page 1, but with the color bars
; tilting the other way.
;
mov di,8000h ;page 1 starts at address 8000h
mov bl,-1 ;make color bars slant down and
; to the left
call ColorBarsUp ;draw the color bars
;
; Wait for a key and flip to page 1 when one is pressed.
;
callGetNextKey
CONSTANT_TO_INDEXED_REGISTER CRTC_INDEX,START_ADDRESS_HIGH,80h
;set the Start Address High register
; to 80h, for a start address of 8000h
;
; Draw vertical bars in page 0 while page 1 is displayed.
;
sub di,di ;page 0 starts at address 0
sub bl,bl ;make color bars vertical
call ColorBarsUp ;draw the color bars
;
; Wait for another key and flip back to page 0 when one is pressed.
;
callGetNextKey
CONSTANT_TO_INDEXED_REGISTER CRTC_INDEX,START_ADDRESS_HIGH,00h
;set the Start Address High register
; to 00h, for a start address of 0000h
;
; Wait for yet another key and return to text mode and end when
; one is pressed.
;
call GetNextKey
mov ax,0003h
int 10h ;text mode
mov ah,4ch
int 21h ;done
;
Start endp
;
; Sets up 320x400 256-color modes.
;
; Input: none
;
; Output: none
;
Set320By400Modeprocnear
;
; First, go to normal 320x200 256-color mode, which is really a
; 320x400 256-color mode with each line scanned twice.
;
mov ax,0013h ;AH = 0 means mode set, AL = 13h selects
; 256-color graphics mode
int 10h ;BIOS video interrupt
;
; Change CPU addressing of video memory to linear (not odd/even,
; chain, or chain 4), to allow us to access all 256K of display
; memory. When this is done, VGA memory will look just like memory
; in modes 10h and 12h, except that each byte of display memory will
; control one 256-color pixel, with 4 adjacent pixels at any given
; address, one pixel per plane.
;
mov dx,SC_INDEX
mov al,MEMORY_MODE
out dx,al
inc dx
ina l,dx
and al,not 08h ;turn off chain 4
ora l,04h ;turn off odd/even
out dx,al
mov dx,GC_INDEX
mov al,GRAPHICS_MODE
out dx,al
inc dx
ina l,dx
and al,not 10h ;turn off odd/even
out dx,al
dec dx
mov al,MISCELLANEOUS
out dx,al
inc dx
ina l,dx
and al,not 02h ;turn off chain
out dx,al
;
; Now clear the whole screen, since the mode 13h mode set only
; cleared 64K out of the 256K of display memory. Do this before
; we switch the CRTC out of mode 13h, so we don't see garbage
; on the screen when we make the switch.
;
CONSTANT_TO_INDEXED_REGISTER SC_INDEX,MAP_MASK,0fh
; enable writes to all planes, so
; we can clear 4 pixels at a time
mov ax,VGA_SEGMENT
mov es,ax
sub di,di
mov ax,di
mov cx,8000h ;# of words in 64K
cld
rep stosw ;clear all of display memory
;
; Tweak the mode to 320x400 256-color mode by not scanning each
; line twice.
;
mov dx,CRTC_INDEX
mov al,MAX_SCAN_LINE
out dx,al
inc dx
in al,dx
and al,not 1fh ;set maximum scan line = 0
out dx,al
dec dx
;
; Change CRTC scanning from doubleword mode to byte mode, allowing
; the CRTC to scan more than 64K of video data.
;
mov al,UNDERLINE
out dx,al
inc dx
ina l,dx
and al,not40h ;turn off doubleword
out dx,al
dec dx
mov al,MODE_CONTROL
out dx,al
inc dx
in al,dx
or al,40h ;turn on the byte mode bit, so memory is
; scanned for video data in a purely
; linear way, just as in modes 10h and 12h
out dx,al
ret
Set320By400Mode endp
;
; Draws a full screen of slanting color bars in the specified page.
;
; Input:
; DI = page start address
; BL = 1 to make the bars slant down and to the right, -1 to
; make them slant down and to the left, 0 to make
; them vertical.
;
ColorBarsUpprocnear
mov ax,VGA_SEGMENT
mov es,ax ;point to display memory
sub bh,bh ;start with color 0
mov si,SCREEN_HEIGHT ;# of rows to do
mov dx,SC_INDEX
mov al,MAP_MASK
out dx,al ;point the SC Index reg to the Map Mask reg
inc dx ;point DX to the SC Data register
RowLoop:
mov cx,SCREEN_WIDTH/4
;4 pixels at each address, so
; each 320-pixel row is 80 bytes wide
; in each plane
pus h bx ;save the row-start color
ColumnLoop:
MAP_SELECT = 1
rept 4 ;do all 4 pixels at this address with
; in-line code
mov al,MAP_SELECT
out dx,al ;select planes 0, 1, 2, and 3 in turn
mov es:[di],bh ;write this plane's pixel
inc bh ;set the color for the next pixel
MAP_SELECT = MAP_SELECT shl 1
endm
inc di ;point to the address containing the next
; 4 pixels
loop ColumnLoop ;do any remaining pixels on this line
pop bx ;get back the row-start color
add bh,bl ;select next row-start color (controls
; slanting of color bars)
dec si ;count down lines on the screen
jnz RowLoop
ret
ColorBarsUpendp
;
; Waits for the next key and returns it in AX.
;
GetNextKeyprocnear
WaitKey:
mov ah,1
int 16h
jz WaitKey ;wait for a key to become available
sub ah,ah
int 16h ;read the key
ret
GetNextKey endp
;
Codeends
;
endStart
```
When you run Listing 31.2, note the extremely smooth edges and fine
gradations of color, especially in the screens with slanting color bars.
The displays produced by Listing 31.2 make it clear that 320x400
256-color mode can produce effects that are simply not possible in any
16-color mode.
### Something to Think About
You can, if you wish, use the display memory organization of 320x400
mode in 320x200 mode by modifying `Set320x400Mode` to leave the
maximum scan line setting at 1 in the mode set. (The version of
`Set320x400Mode` in Listings 31.1 and 31.2 forces the maximum scan
line to 0, doubling the effective resolution of the screen.) Why would
you want to do that? For one thing, you could then choose from not two
but *four* 320x200 256-color display pages, starting at offsets 0,
4000H, 8000H, and 0C000H in display memory. For another, having only
half as many pixels per screen can as much as double drawing speeds;
that's one reason that many games run at 320x200, and even then often
limit the active display drawing area to only a portion of the screen.
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