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LISTING 47.6 L47-6.ASM
; Mode X (320x240, 256 colors) rectangle fill routine. Works on all
; VGAs. Uses fast approach that fans data out to up to four planes at
; once to draw up to four pixels at once. Fills up to but not
; including the column at EndX and the row at EndY. No clipping is
; performed.
; C near-callable as:
; void FillRectangleX(int StartX, int StartY, int EndX, int EndY,
; unsigned int PageBase, int Color);
SC_INDEX equ 03c4h ;Sequence Controller Index
MAP_MASK equ 02h ;index in SC of Map Mask register
SCREEN_SEG equ 0a000h ;segment of display memory in mode X
SCREEN_WIDTH equ 80 ;width of screen in bytes from one scan line
; to the next
parms struc
dw 2 dup (?) ;pushed BP and return address
StartX dw ? ;X coordinate of upper left corner of rect
StartY dw ? ;Y coordinate of upper left corner of rect
EndX dw ? ;X coordinate of lower right corner of rect
; (the row at EndX is not filled)
EndY dw ? ;Y coordinate of lower right corner of rect
; (the column at EndY is not filled)
PageBase dw ? ;base offset in display memory of page in
; which to fill rectangle
Color dw ? ;color in which to draw pixel
parms ends
.model small
.data
; Plane masks for clipping left and right edges of rectangle.
LeftClipPlaneMask db 00fh,00eh,00ch,008h
RightClipPlaneMask db 00fh,001h,003h,007h
.code
public _FillRectangleX
_FillRectangleX proc near
push bp ;preserve caller’s stack frame
mov bp,sp ;point to local stack frame
push si ;preserve caller’s register variables
push di
cld
mov ax,SCREEN_WIDTH
mul [bp+StartY] ;offset in page of top rectangle scan line
mov di,[bp+StartX]
shr di,1 ;X/4 = offset of first rectangle pixel in scan
shr di,1 ; line
add di,ax ;offset of first rectangle pixel in page
add di,[bp+PageBase] ;offset of first rectangle pixel in
; display memory
mov ax,SCREEN_SEG ;point ES:DI to the first rectangle
mov es,ax ; pixel’s address
mov dx,SC_INDEX ;set the Sequence Controller Index to
mov al,MAP_MASK ; point to the Map Mask register
out dx,al
inc dx ;point DX to the SC Data register
mov si,[bp+StartX]
and si,0003h ;look up left edge plane mask
mov bh,LeftClipPlaneMask[si]; to clip & put in BH
mov si,[bp+EndX]
and si,0003h ;look up right edge plane
mov bl,RightClipPlaneMask[si]; mask to clip & put in BL
mov cx,[bp+EndX] ;calculate # of addresses across rect
mov si,[bp+StartX]
cmp cx,si
jle FillDone ;skip if 0 or negative width
dec cx
and si,not 011b
sub cx,si
shr cx,1
shr cx,1 ;# of addresses across rectangle to fill - 1
jnz MasksSet ;there’s more than one byte to draw
and bh,bl ;there’s only one byte, so combine the left-
; and right-edge clip masks
MasksSet:
mov si,[bp+EndY]
sub si,[bp+StartY] ;BX = height of rectangle
jle FillDone ;skip if 0 or negative height
mov ah,byte ptr [bp+Color] ;color with which to fill
mov bp,SCREEN_WIDTH ;stack frame isn’t needed any more
sub bp,cx ;distance from end of one scan line to start
dec bp ; of next
FillRowsLoop:
push cx ;remember width in addresses - 1
mov al,bh ;put left-edge clip mask in AL
out dx,al ;set the left-edge plane (clip) mask
mov al,ah ;put color in AL
stosb ;draw the left edge
dec cx ;count off left edge byte
js FillLoopBottom ;that’s the only byte
jz DoRightEdge ;there are only two bytes
mov al,00fh ;middle addresses are drawn 4 pixels at a pop
out dx,al ;set the middle pixel mask to no clip
mov al,ah ;put color in AL
rep stosb ;draw the middle addresses four pixels apiece
DoRightEdge:
mov al,bl ;put right-edge clip mask in AL
out dx,al ;set the right-edge plane (clip) mask
mov al,ah ;put color in AL
stosb ;draw the right edge
FillLoopBottom:
add di,bp ;point to the start of the next scan line of
; the rectangle
pop cx ;retrieve width in addresses - 1
dec si ;count down scan lines
jnz FillRowsLoop
FillDone:
pop di ;restore caller’s register variables
pop si
pop bp ;restore caller’s stack frame
ret
_FillRectangleX endp
end
Just so you can see Mode X in action, Listing 47.7 is a sample program that selects Mode X and draws a number of rectangles. Listing 47.7 links to any of the rectangle fill routines I’ve presented.
And now, I hope, you’re beginning to see why I’m so fond of Mode X. In the next chapter, we’ll continue with Mode X by exploring the wonders that the latches and parallel plane hardware can work on scrolls, copies, blits, and pattern fills.
LISTING 47.7 L47-7.C
/* Program to demonstrate mode X (320x240, 256-colors) rectangle
fill by drawing adjacent 20x20 rectangles in successive colors from
0 on up across and down the screen */
#include <conio.h>
#include <dos.h>
void Set320x240Mode(void);
void FillRectangleX(int, int, int, int, unsigned int, int);
void main() {
int i,j;
union REGS regset;
Set320x240Mode();
FillRectangleX(0,0,320,240,0,0); /* clear the screen to black */
for (j = 1; j < 220; j += 21) {
for (i = 1; i < 300; i += 21) {
FillRectangleX(i, j, i+20, j+20, 0, ((j/21*15)+i/21) & 0xFF);
}
}
getch();
regset.x.ax = 0x0003; /* switch back to text mode and done */
int86(0x10, ®set, ®set);
}
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