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@ -9,27 +9,27 @@ Chapter 49\
### How to Make the VGA Really Get up and Dance {#Heading2}
Okay—no amusing stories or informative anecdotes to kick off this
chapter; lotta ground to cover, gotta hurry—you’re impatient, I can
smell it. I won’t talk about the time a friend made the mistake of
loudly saying “\$100 bill” during an animated discussion while walking
chapter; lotta ground to cover, gotta hurry—you're impatient, I can
smell it. I won't talk about the time a friend made the mistake of
loudly saying "\$100 bill" during an animated discussion while walking
among the bums on Market Street in San Francisco one night, thereby
graphically illustrating that context is everything. I can’t spare a
graphically illustrating that context is everything. I can't spare a
word about how my daughter thinks my 11-year-old floppy-disk-based CP/M
machine is more powerful than my 386 with its 100-MB hard disk because
the CP/M machine’s word processor loads and runs twice as fast as the
386’s Windows-based word processor, demonstrating that progress is not
the neat exponential curve we’d like to think it is, and that features
and performance are often conflicting notions. And, lord knows, I can’t
the CP/M machine's word processor loads and runs twice as fast as the
386's Windows-based word processor, demonstrating that progress is not
the neat exponential curve we'd like to think it is, and that features
and performance are often conflicting notions. And, lord knows, I can't
take the time to discuss the habits of small white dogs, notwithstanding
that such dogs seem to be relevant to just about every aspect of
computing, as Jeff Duntemann’s writings make manifest. No lighthearted
computing, as Jeff Duntemann's writings make manifest. No lighthearted
fluff for us; we have real work to do, for today we animate with 256
colors in Mode X.
### Masked Copying {#Heading3}
Over the past two chapters, we’ve put together most of the tools needed
to implement animation in the VGA’s undocumented 320x240 256-color Mode
Over the past two chapters, we've put together most of the tools needed
to implement animation in the VGA's undocumented 320x240 256-color Mode
X. We now have mode set code, solid and 4x4 pattern fills, system
memory-to-display memory block copies, and display memory-to-display
memory block copies. The final piece of the puzzle is the ability to
@ -50,21 +50,21 @@ fact, masked copying even makes it possible to have transparent areas
within images.
Note that another way to achieve this effect is to implement copying
code that supports a transparent color; that is, a color that doesn’t
code that supports a transparent color; that is, a color that doesn't
get copied but rather leaves the destination unchanged. Transparent
copying makes for more compact images, because no separate mask is
needed, and is generally faster in a software-only implementation.
However, Mode X supports masked copying but not transparent copying in
hardware, so we’ll use masked copying in this chapter.
hardware, so we'll use masked copying in this chapter.
The system memory to display memory masked copy routine in Listing 49.1
implements masked copying in a straightforward fashion. In the main
drawing loop, the corresponding mask byte is consulted as each image
pixel is encountered, and the image pixel is copied only if the mask
byte is nonzero. As with most of the system-to-display code I’ve
presented, Listing 49.1 is not heavily optimized, because it’s
inherently slow; there’s a better way to go when performance matters,
and that’s to use the VGA’s hardware.
byte is nonzero. As with most of the system-to-display code I've
presented, Listing 49.1 is not heavily optimized, because it's
inherently slow; there's a better way to go when performance matters,
and that's to use the VGA's hardware.
**LISTING 49.1 L49-1.ASM**
@ -106,7 +106,7 @@ and that’s to use the VGA’s hardware.
DestBitmapWidth dw ? ;# of pixels across dest bitmap (must be multiple of 4)
MaskPtr dw ? ;pointer in DS to start of bitmap in which mask
; resides (byte-per-pixel format, just like the source
; image; 0-bytes mean don’t copy corresponding source
; image; 0-bytes mean don't copy corresponding source
; pixel, 1-bytes mean do copy)
parms ends
@ -118,10 +118,10 @@ and that’s to use the VGA’s hardware.
.code
public _CopySystemToScreenMaskedX
_CopySystemToScreenMaskedX 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
mov ax,SCREEN_SEG ;point ES to display memory
@ -146,10 +146,10 @@ and that’s to use the VGA’s hardware.
add di,ax ;offset of first dest rect pixel in page
add di,[bp+DestPageBase] ;offset of first dest rect pixel
; in display memory
and cl,011b ;CL = first dest pixel’s plane
and cl,011b ;CL = first dest pixel's plane
mov al,11h ;upper nibble comes into play when plane wraps
; from 3 back to 0
shl al,cl ;set the bit for the first dest pixel’s plane
shl al,cl ;set the bit for the first dest pixel's plane
mov [bp+LeftMask],al ; in each nibble to 1
mov ax,[bp+SourceEndX] ;calculate # of pixels across
@ -173,7 +173,7 @@ and that’s to use the VGA’s hardware.
push di ;remember the start offset in the dest
CopyScanLineLoop:
cmp byte ptr [bx],0 ;is this pixel mask-enabled?
jz MaskOff ;no, so don’t draw it
jz MaskOff ;no, so don't draw it
;yes, draw the pixel
out dx,al ;set the plane for this pixel
mov ah,[si] ;get the pixel from the source
@ -181,7 +181,7 @@ and that’s to use the VGA’s hardware.
MaskOff:
inc bx ;advance the mask pointer
inc si ;advance the source pointer
rol al,1 ;set mask for next pixel’s plane
rol al,1 ;set mask for next pixel's plane
adc di,0 ;advance destination address only when
;wrapping from plane 3 to plane 0
loop CopyScanLineLoop
@ -195,10 +195,10 @@ and that’s to use the VGA’s hardware.
dec word ptr [bp+RectHeight] ;count down scan lines
jnz CopyRowsLoop
CopyDone:
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
_CopySystemToScreenMaskedX endp
end