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James Gregory 2013-12-30 18:34:31 +11:00
commit 18011c595a
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47-03.md
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@ -3,18 +3,18 @@
------------------------ --------------------------------- --------------------
After setting up mode 13H, Listing 47.1 alters the vertical counts and
timings to select 480 visible scan lines. (There’s no need to alter any
timings to select 480 visible scan lines. (There's no need to alter any
horizontal values, because mode 13H and Mode X both have 320-pixel
horizontal resolutions.) The Maximum Scan Line register is programmed to
double scan each line (that is, repeat each scan line twice), however,
so we get an effective vertical resolution of 240 scan lines. It is, in
fact, possible to get 400 or 480 independent scan lines in 256-color
mode, as discussed in Chapter 31 and 32; however, 400-scan-line modes
lack square pixels and can’t support simultaneous off-screen memory and
lack square pixels and can't support simultaneous off-screen memory and
page flipping. Furthermore, 480-scan-line modes lack page flipping
altogether, due to memory constraints.
At the same time, Listing 47.1 programs the VGA’s bitmap to a planar
At the same time, Listing 47.1 programs the VGA's bitmap to a planar
organization that is similar to that used by the 16-color modes, and
utterly different from the linear bitmap of mode 13H. The bizarre bitmap
organization of Mode X is shown in Figure 47.1. The first pixel (the
@ -42,11 +42,11 @@ written, and likewise for the read pixel code shown in Listing 47.3.
Calculating and mapping in a plane once for each pixel written is
scarcely a recipe for performance.
That’s all right, though, because most graphics software spends little
time drawing individual pixels. I’ve provided the write and read pixel
routines as basic primitives, and so you’ll understand how the bitmap is
That's all right, though, because most graphics software spends little
time drawing individual pixels. I've provided the write and read pixel
routines as basic primitives, and so you'll understand how the bitmap is
organized, but the building blocks of high-performance graphics software
are fills, copies, and bitblts, and it’s there that Mode X shines.
are fills, copies, and bitblts, and it's there that Mode X shines.
![](images/47-01.jpg)\
**Figure 47.1**  *Mode X display memory organization.*
@ -78,30 +78,30 @@ are fills, copies, and bitblts, and it’s there that Mode X shines.
.code
public _WritePixelX
_WritePixelX proc near
push bp ;preserve caller’s stack frame
push bp ;preserve caller's stack frame
mov bp,sp ;point to local stack frame
mov ax,SCREEN_WIDTH
mul [bp+Y] ;offset of pixel’s scan line in page
mul [bp+Y] ;offset of pixel's scan line in page
mov bx,[bp+X]
shr bx,1
shr bx,1 ;X/4 = offset of pixel in scan line
add bx,ax ;offset of pixel in page
add bx,[bp+PageBase] ;offset of pixel in display memory
mov ax,SCREEN_SEG
mov es,ax ;point ES:BX to the pixel’s address
mov es,ax ;point ES:BX to the pixel's address
mov cl,byte ptr [bp+X]
and cl,011b ;CL = pixel’s plane
and cl,011b ;CL = pixel's plane
mov ax,0100h + MAP_MASK ;AL = index in SC of Map Mask reg
shl ah,cl ;set only the bit for the pixel’s plane to 1
shl ah,cl ;set only the bit for the pixel's plane to 1
mov dx,SC_INDEX ;set the Map Mask to enable only the
out dx,ax ; pixel’s plane
out dx,ax ; pixel's plane
mov al,byte ptr [bp+Color]
mov es:[bx],al ;draw the pixel in the desired color
pop bp ;restore caller’s stack frame
pop bp ;restore caller's stack frame
ret
_WritePixelX endp
end
@ -131,29 +131,29 @@ are fills, copies, and bitblts, and it’s there that Mode X shines.
.code
public _ReadPixelX
_ReadPixelX proc near
push bp ;preserve caller’s stack frame
push bp ;preserve caller's stack frame
mov bp,sp ;point to local stack frame
mov ax,SCREEN_WIDTH
mul [bp+Y] ;offset of pixel’s scan line in page
mul [bp+Y] ;offset of pixel's scan line in page
mov bx,[bp+X]
shr bx,1
shr bx,1 ;X/4 = offset of pixel in scan line
add bx,ax ;offset of pixel in page
add bx,[bp+PageBase] ;offset of pixel in display memory
mov ax,SCREEN_SEG
mov es,ax ;point ES:BX to the pixel’s address
mov es,ax ;point ES:BX to the pixel's address
mov ah,byte ptr [bp+X]
and ah,011b ;AH = pixel’s plane
and ah,011b ;AH = pixel's plane
mov al,READ_MAP ;AL = index in GC of the Read Map reg
mov dx,GC_INDEX ;set the Read Map to read the pixel’s
mov dx,GC_INDEX ;set the Read Map to read the pixel's
out dx,ax ; plane
mov al,es:[bx] ;read the pixel’s color
mov al,es:[bx] ;read the pixel's color
sub ah,ah ;convert it to an unsigned int
pop bp ;restore caller’s stack frame
pop bp ;restore caller's stack frame
ret
_ReadPixelX endp
end