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James Gregory 2013-12-30 18:34:31 +11:00
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49-02.md
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@ -4,15 +4,15 @@
#### Faster Masked Copying {#Heading4}
In the previous chapter we saw how the VGA’s latches can be used to copy
In the previous chapter we saw how the VGA's latches can be used to copy
four pixels at a time from one area of display memory to another in Mode
X. We’ve further seen that in Mode X the Map Mask register can be used
to select which planes are copied. That’s all we need to know to be able
X. We've further seen that in Mode X the Map Mask register can be used
to select which planes are copied. That's all we need to know to be able
to perform fast masked copies; we can store an image in off-screen
display memory, and set the Map Mask to the appropriate mask value as up
to four pixels at a time are copied.
There’s a slight hitch, though. The latches can only be used when the
There's a slight hitch, though. The latches can only be used when the
source and destination left edge coordinates, modulo four, are the same,
as explained in the previous chapter. The solution is to copy all four
possible alignments of each image to display memory, each properly
@ -20,8 +20,8 @@ positioned for one of the four possible
destination-left-edge-modulo-four cases. These aligned images must be
accompanied by the four possible alignments of the image mask, stored in
system memory. Given all four image and mask alignments, masked copying
is a simple matter of selecting the alignment that’s appropriate for the
destination’s left edge, then setting the Map Mask with the 4-bit mask
is a simple matter of selecting the alignment that's appropriate for the
destination's left edge, then setting the Map Mask with the 4-bit mask
corresponding to each four-pixel set as we copy four pixels at a time
via the latches.
@ -32,7 +32,7 @@ and mask alignments. The aligned images are already stored in display
memory, and the aligned masks are already stored in system memory;
further, the masks are predigested into Map Mask register-compatible
form. Given all that ready-to-use data, Listing 49.2 selects and works
with the appropriate image-mask pair for the destination’s left edge
with the appropriate image-mask pair for the destination's left edge
alignment.
**LISTING 49.2 L49-2.ASM**
@ -94,10 +94,10 @@ alignment.
.code
public _CopyScreenToScreenMaskedX
_CopyScreenToScreenMaskedX proc near
push bp ;preserve caller’s stack frame
push bp ;preserve caller's stack frame
mov bp,sp ;point to local stack frame
sub sp,STACK_FRAME_SIZE ;allocate space for local vars
push si ;preserve caller’s register variables
push si ;preserve caller's register variables
push di
cld
@ -117,7 +117,7 @@ alignment.
shr di,1 ; scan line
add di,ax ;offset of first dest rect pixel in page
add di,[bp+DestPageBase] ;offset of first dest rect pixel in display
; memory. now look up the image that’s
; memory. now look up the image that's
; aligned to match left-edge alignment
; of destination
and si,3 ;DestStartX modulo 4
@ -190,10 +190,10 @@ alignment.
mov al,0ffh ; which selects all bits from the CPU
out dx,al ; and none from the latches (the GC
; Index still points to Bit Mask)
pop di ;restore caller’s register variables
pop di ;restore caller's register variables
pop si
mov sp,bp ;discard storage for local variables
pop bp ;restore caller’s stack frame
pop bp ;restore caller's stack frame
ret
_CopyScreenToScreenMaskedX endp
end