Use backtick code blocks for syntax highlighting
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James Gregory
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140
58-02.md
140
58-02.md
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@ -12,52 +12,54 @@ pages: 1082-1085
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**LISTING 58.1 L58-1.ASM**
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; Inner loop to draw a single texture-mapped horizontal scanline in
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; Mode X, the VGA's page-flipped 256-color mode. Because adjacent
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; pixels lie in different planes in Mode X, an OUT must be performed
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; to select the proper plane before drawing each pixel.
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;
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; At this point:
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; AL = initial pixel's plane mask
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; DS:BX = initial source texture pointer
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; DX = pointer to VGA's Sequencer Data register
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; SI = # of pixels to fill
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; ES:DI = pointer to initial destination pixel
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```nasm
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; Inner loop to draw a single texture-mapped horizontal scanline in
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; Mode X, the VGA's page-flipped 256-color mode. Because adjacent
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; pixels lie in different planes in Mode X, an OUT must be performed
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; to select the proper plane before drawing each pixel.
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;
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; At this point:
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; AL = initial pixel's plane mask
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; DS:BX = initial source texture pointer
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; DX = pointer to VGA's Sequencer Data register
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; SI = # of pixels to fill
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; ES:DI = pointer to initial destination pixel
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TexScanLoop:
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TexScanLoop:
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; Set the Map Mask for this pixel's plane, then draw the pixel.
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; Set the Map Mask for this pixel's plane, then draw the pixel.
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out dx,al
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mov ah,[bx] ;get texture pixel
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mov es:[di],ah ;set screen pixel
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out dx,al
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mov ah,[bx] ;get texture pixel
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mov es:[di],ah ;set screen pixel
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; Point to the next source pixel.
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; Point to the next source pixel.
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add bx,[bp].lXBaseAdvance ;advance the minimum # of pixels in X
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mov cx,word ptr [bp].lSourceStepX
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add word ptr [bp].lSourceX,cx ;step the source X fractional part
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jnc NoExtraXAdvance ;didn't turn over; no extra advance
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add bx,[bp].lXAdvanceByOne ;did turn over; advance X one extra
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NoExtraXAdvance:
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add bx,[bp].lXBaseAdvance ;advance the minimum # of pixels in X
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mov cx,word ptr [bp].lSourceStepX
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add word ptr [bp].lSourceX,cx ;step the source X fractional part
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jnc NoExtraXAdvance ;didn't turn over; no extra advance
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add bx,[bp].lXAdvanceByOne ;did turn over; advance X one extra
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NoExtraXAdvance:
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add bx,[bp].lYBaseAdvance ;advance the minimum # of pixels in Y
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mov cx,word ptr [bp].lSourceStepY
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add word ptr [bp].lSourceY,cx ;step the source Y fractional part
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jnc NoExtraYAdvance ;didn't turn over; no extra advance
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add bx,[bp].lYAdvanceByOne ;did turn over; advance Y one extra
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NoExtraYAdvance:
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add bx,[bp].lYBaseAdvance ;advance the minimum # of pixels in Y
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mov cx,word ptr [bp].lSourceStepY
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add word ptr [bp].lSourceY,cx ;step the source Y fractional part
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jnc NoExtraYAdvance ;didn't turn over; no extra advance
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add bx,[bp].lYAdvanceByOne ;did turn over; advance Y one extra
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NoExtraYAdvance:
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; Point to the next destination pixel, by cycling to the next plane, and
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; advancing to the next address if the plane wraps from 3 to 0.
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; Point to the next destination pixel, by cycling to the next plane, and
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; advancing to the next address if the plane wraps from 3 to 0.
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rol al,1
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adc di,0
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rol al,1
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adc di,0
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; Continue if there are any more dest pixels to draw.
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; Continue if there are any more dest pixels to draw.
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dec si
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jnz TexScanLoop
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dec si
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jnz TexScanLoop
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```
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Figure 58.2 shows why this cycling is necessary. In Mode X, the
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page-flipped 256-color mode of the VGA, each successive pixel across a
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@ -124,44 +126,46 @@ of *four* times faster than Listing 58.1.)
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**LISTING 58.2 L58-2.ASM**
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; Inner loop to draw a single texture-mapped vertical column, rather
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; than a horizontal scanline. This allows all pixels handled
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; by this code to reside in the same plane, so the time-consuming
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; plane switching can be moved out of the inner loop.
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;
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; At this point:
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; DS:BX = initial source texture pointer
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; DX = offset to advance to the next pixel in the dest column
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; (either positive or negative scanline width)
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; SI = # of pixels to fill
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; ES:DI = pointer to initial destination pixel
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; VGA set up to draw to the correct plane for this column
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```nasm
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; Inner loop to draw a single texture-mapped vertical column, rather
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; than a horizontal scanline. This allows all pixels handled
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; by this code to reside in the same plane, so the time-consuming
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; plane switching can be moved out of the inner loop.
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;
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; At this point:
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; DS:BX = initial source texture pointer
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; DX = offset to advance to the next pixel in the dest column
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; (either positive or negative scanline width)
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; SI = # of pixels to fill
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; ES:DI = pointer to initial destination pixel
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; VGA set up to draw to the correct plane for this column
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REPTLOOP_UNROLL
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REPTLOOP_UNROLL
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; Set the Map Mask for this pixel's plane, then draw the pixel.
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; Set the Map Mask for this pixel's plane, then draw the pixel.
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mov ah,[bx] ;get texture pixel
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mov es:[di],ah ;set screen pixel
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mov ah,[bx] ;get texture pixel
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mov es:[di],ah ;set screen pixel
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; Point to the next source pixel.
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; Point to the next source pixel.
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add bx,[bp].lXBaseAdvance ;advance the minimum # of pixels in X
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mov cx,word ptr [bp].lSourceStepX
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add word ptr [bp].lSourceX,cx ;step the source X fractional part
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jnc NoExtraXAdvance ;didn't turn over; no extra advance
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add bx,[bp].lXAdvanceByOne ;did turn over; advance X one extra
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NoExtraXAdvance:
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add bx,[bp].lXBaseAdvance ;advance the minimum # of pixels in X
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mov cx,word ptr [bp].lSourceStepX
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add word ptr [bp].lSourceX,cx ;step the source X fractional part
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jnc NoExtraXAdvance ;didn't turn over; no extra advance
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add bx,[bp].lXAdvanceByOne ;did turn over; advance X one extra
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NoExtraXAdvance:
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add bx,[bp].lYBaseAdvance ;advance the minimum # of pixels in Y
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mov cx,word ptr [bp].lSourceStepY
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add word ptr [bp].lSourceY,cx ;step the source Y fractional part
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jnc NoExtraYAdvance ;didn't turn over; no extra advance
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add bx,[bp].lYAdvanceByOne ;did turn over; advance Y one extra
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NoExtraYAdvance:
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add bx,[bp].lYBaseAdvance ;advance the minimum # of pixels in Y
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mov cx,word ptr [bp].lSourceStepY
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add word ptr [bp].lSourceY,cx ;step the source Y fractional part
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jnc NoExtraYAdvance ;didn't turn over; no extra advance
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add bx,[bp].lYAdvanceByOne ;did turn over; advance Y one extra
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NoExtraYAdvance:
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; Point to the next destination pixel, which is on the next scan line.
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; Point to the next destination pixel, which is on the next scan line.
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adc di,dx
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adc di,dx
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ENDM
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ENDM
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```
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