Category : Files from Magazines
Archive   : PJ64.ZIP
Filename : L2.ASM

; *** Listing 2 ***
;
; Program to illustrate use of read mode 1 (color compare mode)
; to detect collisions in display memory. Draws a yellow line on a
; blue background, then draws a perpendicular green line until the
; yellow line is reached.
;
; By Michael Abrash 4/2/88
;
stack segment word stack 'STACK'
db 512 dup (?)
stack ends
;
VGA_SEGMENT EQU 0a000h
SCREEN_WIDTH EQU 80 ;in bytes
GC_INDEX EQU 3ceh ;Graphics Controller Index register
SET_RESET EQU 0 ;Set/Reset register index in GC
ENABLE_SET_RESET EQU 1 ;Enable Set/Reset register index in GC
COLOR_COMPARE EQU 2 ;Color Compare register index in GC
GRAPHICS_MODE EQU 5 ;Graphics Mode register index in GC
BIT_MASK EQU 8 ;Bit Mask register index in GC
;
code segment word 'CODE'
assume cs:code
Start proc near
cld
;
; Select graphics mode 10h.
;
mov ax,10h
int 10h
;
; Fill the screen with blue.
;
mov al,1 ;blue is color 1
call SelectSetResetColor ;set to draw in blue
mov ax,VGA_SEGMENT
mov es,ax
sub di,di
mov cx,7000h
rep stosb ;the value written actually doesn't
; matter, since set/reset is providing
; the data written to display memory
;
; Draw a vertical yellow line.
;
mov al,14 ;yellow is color 14
call SelectSetResetColor ;set to draw in yellow
mov dx,GC_INDEX
mov al,BIT_MASK
out dx,al ;point GC Index to Bit Mask
inc dx ;point to GC Data
mov al,10h
out dx,al ;set Bit Mask to 10h
mov di,40 ;start in the middle of the top line
mov cx,350 ;do full height of screen
VLineLoop:
mov al,es:[di] ;load the latches
stosb ;write next pixel of yellow line (set/reset
; provides the data written to display
; memory, and AL is actually ignored)
add di,SCREEN_WIDTH-1 ;point to the next scan line
loop VLineLoop
;
; Select write mode 0 and read mode 1.
;
mov dx,GC_INDEX
mov al,GRAPHICS_MODE
out dx,al ;point GC Index to Graphics Mode register
inc dx ;point to GC Data
mov al,00001000b ;bit 3=1 is read mode 1, bits 1 & 0=00
; is write mode 0
out dx,al ;set Graphics Mode to read mode 1,
; write mode 0
;
; Draw a horizontal green line, one pixel at a time, from left
; to right until color compare reports a yellow pixel is encountered.
;
; Draw in green.
;
mov al,2 ;green is color 2
call SelectSetResetColor ;set to draw in green
;
; Set color compare to look for yellow.
;
mov dx,GC_INDEX
mov al,COLOR_COMPARE
out dx,al ;point GC Index to Color Compare register
inc dx ;point to GC Data
mov al,14 ;we're looking for yellow, color 14
out dx,al ;set color compare to look for yellow
dec dx ;point to GC Index
;
; Set up for quick access to Bit Mask register.
;
mov al,BIT_MASK
out dx,al ;point GC Index to Bit Mask register
inc dx ;point to GC Data
;
; Set initial pixel mask and display memory offset.
;
mov al,80h ;initial pixel mask
mov di,100*SCREEN_WIDTH
;start at left edge of scan line 100
HLineLoop:
mov ah,es:[di] ;do a read mode 1 (color compare) read.
; This also loads the latches.
and ah,al ;is the pixel of current interest yellow?
jnz WaitKeyAndDone ;yes-we've reached the yellow line, so we're
; done
out dx,al ;set the Bit Mask register so that we
; modify only the pixel of interest
mov es:[di],al ;draw the pixel. The value written is
; irrelevant, since set/reset is providing
; the data written to display memory
ror al,1 ;shift pixel mask to the next pixel
adc di,0 ;advance the display memory offset if
; the pixel mask wrapped
jmp HLineLoop
;
; Wait for a key to be pressed to end, then return to text mode and
; return to DOS.
;
WaitKeyAndDone:
WaitKeyLoop:
mov ah,1
int 16h
jz WaitKeyLoop
sub ah,ah
int 16h ;clear the key
mov ax,3
int 10h ;return to text mode
mov ah,4ch
int 21h ;done
Start endp
;
; Enables set/reset for all planes, and sets the set/reset color
; to AL.
;
SelectSetResetColor proc near
mov dx,GC_INDEX
push ax ;preserve color
mov al,SET_RESET
out dx,al ;point GC Index to Set/Reset register
inc dx ;point to GC Data
pop ax ;get back color
out dx,al ;set Set/Reset register to selected color
dec dx ;point to GC Index
mov al,ENABLE_SET_RESET
out dx,al ;point GC Index to Enable Set/Reset register
inc dx ;point to GC Data
mov al,0fh
out dx,al ;enable set/reset for all planes
ret
SelectSetResetColor endp
code ends
end Start