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James Gregory 2013-12-30 18:34:31 +11:00
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@ -9,16 +9,16 @@ left-to-right copying. Downward scrolls require bottom-to-top copying;
likewise, rightward horizontal scrolls require right-to-left copying. As
it happens, my intended use for Listing 48.3 is to copy images between
off-screen memory and on-screen memory, and to save areas under pop-up
menus and the like, so I don’t really need overlap handling—and I do
menus and the like, so I don't really need overlap handling—and I do
really need to keep the complexity of this discussion down. However, you
will surely want to add overlap handling if you plan to perform
arbitrary scrolling and copying in display memory.
Now that we have a fast way to copy images around in display memory, we
can draw icons and other images as much as four times faster than in
mode 13H, depending on the speed of the VGA’s display memory. (In case
you’re worried about the nibble-alignment limitation on fast copies,
don’t be; I’ll address that fully in due time, but the secret is to
mode 13H, depending on the speed of the VGA's display memory. (In case
you're worried about the nibble-alignment limitation on fast copies,
don't be; I'll address that fully in due time, but the secret is to
store all four possible rotations in off-screen memory, then select the
correct one for each copy.) However, before our fast display
memory-to-display memory copy routine can do us any good, we must have a
@ -42,7 +42,7 @@ in off-screen memory and copied to the screen via the latches much
faster than even the speediest system memory-to-display memory copy
routine could manage.
I’m not going to present a routine to perform Mode X copies from display
I'm not going to present a routine to perform Mode X copies from display
memory to system memory, but such a routine would be a straightforward
inverse of Listing 48.4.
@ -93,10 +93,10 @@ inverse of Listing 48.4.
.code
public _CopySystemToScreenX
_CopySystemToScreenX 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
@ -120,10 +120,10 @@ inverse of Listing 48.4.
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
shl al,cl ;set the bit for the first dest pixel's
mov [bp+LeftMask],al ; plane in each nibble to 1
mov cx,[bp+SourceEndX] ;calculate # of pixels across
@ -145,7 +145,7 @@ inverse of Listing 48.4.
CopyScanLineLoop:
out dx,al ;set the plane for this pixel
movsb ;copy the pixel to the screen
rol al,1 ;set mask for next pixel’s plane
rol al,1 ;set mask for next pixel's plane
cmc ;advance destination address only when
sbb di,0 ; wrapping from plane 3 to plane 0
; (else undo INC DI done by MOVSB)
@ -159,31 +159,31 @@ inverse of Listing 48.4.
dec bx ;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
_CopySystemToScreenX endp
end
### Who Was that Masked Image Copier? {#Heading6}
At this point, it’s getting to be time for us to take all the Mode X
tools we’ve developed, together with one more tool—masked image
At this point, it's getting to be time for us to take all the Mode X
tools we've developed, together with one more tool—masked image
copying—and the remaining unexplored feature of Mode X, page flipping,
and build an animation application. I hope that when we’re done, you’ll
and build an animation application. I hope that when we're done, you'll
agree with me that Mode X is *the* way to animate on the PC.
In truth, though, it matters less whether or not *you* think that Mode X
is the best way to animate than whether or not your users think it’s the
is the best way to animate than whether or not your users think it's the
best way based on results; end users care only about results, not how
you produced them. For my writing, you folks are the end users—and
notice how remarkably little you care about how this book gets written
and produced. You care that it turned up in the bookstore, and you care
about the contents, but you sure as heck don’t care about how it got
that far from a bin of tree pulp. When you’re a creator, the process
matters. When you’re a buyer, results are everything. All important.
about the contents, but you sure as heck don't care about how it got
that far from a bin of tree pulp. When you're a creator, the process
matters. When you're a buyer, results are everything. All important.
*Sine qua non.* The whole enchilada.
If you catch my drift.