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James Gregory 2013-12-30 12:50:32 +11:00
commit 500e7f5654
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49-02.html
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@ -37,10 +37,10 @@
</CENTER>
<P><BR></P>
<H4 ALIGN="LEFT"><A NAME="Heading4"></A><FONT COLOR="#000077">Faster Masked Copying</FONT></H4>
<P>In the previous chapter we saw how the VGA&#146;s latches can be used to copy four pixels at a time from one area of display memory to another in Mode X. We&#146;ve further seen that in Mode X the Map Mask register can be used to select which planes are copied. That&#146;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.
<P>In the previous chapter we saw how the VGA&rsquo;s latches can be used to copy four pixels at a time from one area of display memory to another in Mode X. We&rsquo;ve further seen that in Mode X the Map Mask register can be used to select which planes are copied. That&rsquo;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.
</P>
<P>There&#146;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 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&#146;s appropriate for the destination&#146;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.</P>
<P>Listing 49.2 performs fast masked copying. This code expects to receive a pointer to a <B>MaskedImage</B> structure, which in turn points to four <B>AlignedMaskedImage</B> structures that describe the four possible image 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&#146;s left edge alignment.</P>
<P>There&rsquo;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 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&rsquo;s appropriate for the destination&rsquo;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.</P>
<P>Listing 49.2 performs fast masked copying. This code expects to receive a pointer to a <B>MaskedImage</B> structure, which in turn points to four <B>AlignedMaskedImage</B> structures that describe the four possible image 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&rsquo;s left edge alignment.</P>
<P><B>LISTING 49.2 L49-2.ASM</B></P>
<!-- CODE //-->
<PRE>
@ -101,53 +101,53 @@ AlignedMaskedImage ends
.code
public _CopyScreenToScreenMaskedX
_CopyScreenToScreenMaskedX proc near
push bp ;preserve caller&#146;s stack frame
push bp ;preserve caller&rsquo;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&#146;s register variables
push si ;preserve caller&rsquo;s register variables
push di
cld
mov dx,GC_INDEX ;set the bit mask to select all bits
mov ax,00000h&#43;BIT_MASK ; from the latches and none from
mov ax,00000h+BIT_MASK ; from the latches and none from
out dx,ax ; the CPU, so that we can write the
; latch contents directly to memory
mov ax,SCREEN_SEG ;point ES to display memory
mov es,ax
mov ax,[bp&#43;DestBitmapWidth]
mov ax,[bp+DestBitmapWidth]
shr ax,1 ;convert to width in addresses
shr ax,1
mul [bp&#43;DestStartY] ;top dest rect scan line
mov di,[bp&#43;DestStartX]
mul [bp+DestStartY] ;top dest rect scan line
mov di,[bp+DestStartX]
mov si,di
shr di,1 ;X/4 = offset of first dest rect pixel in
shr di,1 ; scan line
add di,ax ;offset of first dest rect pixel in page
add di,[bp&#43;DestPageBase] ;offset of first dest rect pixel in display
; memory. now look up the image that&#146;s
add di,[bp+DestPageBase] ;offset of first dest rect pixel in display
; memory. now look up the image that&rsquo;s
; aligned to match left-edge alignment
; of destination
and si,3 ;DestStartX modulo 4
mov cx,si ;set aside alignment for later
shl si,1 ;prepare for word look-up
mov bx,[bp&#43;Source] ;point to source MaskedImage structure
mov bx,[bx&#43;Alignments&#43;si] ;point to AlignedMaskedImage
mov bx,[bp+Source] ;point to source MaskedImage structure
mov bx,[bx+Alignments+si] ;point to AlignedMaskedImage
; struc for current left edge alignment
mov ax,[bx&#43;ImageWidth] ;image width in addresses
mov [bp&#43;SourceBitmapWidth],ax ;remember image width in addresses
mul [bp&#43;SourceStartY] ;top source rect scan line
mov si,[bp&#43;SourceStartX]
mov ax,[bx+ImageWidth] ;image width in addresses
mov [bp+SourceBitmapWidth],ax ;remember image width in addresses
mul [bp+SourceStartY] ;top source rect scan line
mov si,[bp+SourceStartX]
shr si,1 ;X/4 = address of first source rect pixel in
shr si,1 ; scan line
add si,ax ;offset of first source rect pixel in image
mov ax,si
add si,[bx&#43;MaskPtr] ;point to mask offset of first mask pixel in DS
mov bx,[bx&#43;ImagePtr] ;offset of first source rect pixel
add si,[bx+MaskPtr] ;point to mask offset of first mask pixel in DS
mov bx,[bx+ImagePtr] ;offset of first source rect pixel
add bx,ax ; in display memory
mov ax,[bp&#43;SourceStartX] ;calculate # of addresses across
mov ax,[bp+SourceStartX] ;calculate # of addresses across
add ax,cx ; rect, shifting if necessary to
add cx,[bp&#43;SourceEndX] ; account for alignment
add cx,[bp+SourceEndX] ; account for alignment
cmp cx,ax
jle CopyDone ;skip if 0 or negative width
add cx,3
@ -155,26 +155,26 @@ _CopyScreenToScreenMaskedX proc near
sub cx,ax
shr cx,1
shr cx,1 ;# of addresses across rectangle to copy
mov ax,[bp&#43;SourceEndY]
sub ax,[bp&#43;SourceStartY] ;AX = height of rectangle
mov ax,[bp+SourceEndY]
sub ax,[bp+SourceStartY] ;AX = height of rectangle
jle CopyDone ;skip if 0 or negative height
mov [bp&#43;RectHeight],ax
mov ax,[bp&#43;DestBitmapWidth]
mov [bp+RectHeight],ax
mov ax,[bp+DestBitmapWidth]
shr ax,1 ;convert to width in addresses
shr ax,1
sub ax,cx ;distance from end of one dest scan line to start of next
mov [bp&#43;DestNextScanOffset],ax
mov ax,[bp&#43;SourceBitmapWidth] ;width in addresses
mov [bp+DestNextScanOffset],ax
mov ax,[bp+SourceBitmapWidth] ;width in addresses
sub ax,cx ;distance from end of source scan line to start of next
mov [bp&#43;SourceNextScanOffset],ax
mov [bp&#43;RectAddrWidth],cx ;remember width in addresses
mov [bp+SourceNextScanOffset],ax
mov [bp+RectAddrWidth],cx ;remember width in addresses
mov dx,SC_INDEX
mov al,MAP_MASK
out dx,al ;point SC Index register to Map Mask
inc dx ;point to SC Data register
CopyRowsLoop:
mov cx,[bp&#43;RectAddrWidth] ;width across
mov cx,[bp+RectAddrWidth] ;width across
CopyScanLineLoop:
lodsb ;get the mask for this four-pixel set
; and advance the mask pointer
@ -186,21 +186,21 @@ CopyScanLineLoop:
dec cx ;count off four-pixel sets
jnz CopyScanLineLoop
mov ax,[bp&#43;SourceNextScanOffset]
mov ax,[bp+SourceNextScanOffset]
add si,ax ;point to the start of
add bx,ax ; the next source, mask,
add di,[bp&#43;DestNextScanOffset] ; and dest lines
dec word ptr [bp&#43;RectHeight] ;count down scan lines
add di,[bp+DestNextScanOffset] ; and dest lines
dec word ptr [bp+RectHeight] ;count down scan lines
jnz CopyRowsLoop
CopyDone:
mov dx,GC_INDEX&#43;1 ;restore the bit mask to its default,
mov dx,GC_INDEX+1 ;restore the bit mask to its default,
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&#146;s register variables
pop di ;restore caller&rsquo;s register variables
pop si
mov sp,bp ;discard storage for local variables
pop bp ;restore caller&#146;s stack frame
pop bp ;restore caller&rsquo;s stack frame
ret
_CopyScreenToScreenMaskedX endp
end