Category : Assembly Language Source Code
Archive   : ASM-SUBR.ZIP
Filename : SETPT

;-------------------------routine begins--------------------------+
; ROUTINE FOR plotting a point on med-res color screen
;
; FUNCTION: This routine plots a point on the medium resolution
; color screen. The pixel at the specified location is given a
; specified color, overwriting the old color.
; INPUT: Upon entry:
; x-coordinate (0-319) of the point is in SI
; y-coordinate (0-199) of the point is in DI
; color (0-3) is in DX
; OUTPUT: Just to the screen
; REGISTERS USED: No registers modified. SI,DI,DX are used for
; input.
; SEGMENTS REFERENCED: Upon entry ES: must point to the video RAM
; at B8000h and DS: must point to a data segment containing the fol-
; lowing look-up table of rotated color masks:
;
ctable dw 0003Fh,0403Fh,0803Fh,0C03Fh
dw 000CFh,010CFh,020CFh,030CFh
dw 000F3h,004F3h,008F3h,00CF3h
dw 000FCh,001FCh,002FCh,003FCh
;
; ROUTINES CALLED: None
; SPECIAL NOTES: No bounds checking is performed. The user must
; make sure that the coordinates and the color are in the proper
; ranges.
;
setpt proc far
;
push bx ; save registers
push si
push ax
;
; multiply y-coord by bytes per row and adjust for even/odd lines
mov ax,di ; get y-coord into low part
mov ah,al ; and into high part.
and ax,01FEh ; mask off unwanted parts
sal ax,1 ; times 41
sal ax,1 ; times 8
sal ax,1 ; times 16
mov bx,ax ; goes into adddress
and bx,7 ; without adjustment
sal ax,1 ; times 32
sal ax,1 ; times 64
add bx,ax ; adress gets y-coord times 80
;
; compute rotated mask and color
and si,3 ; adjust pixel position into the index
sal si,1 ; index times 2
sal si,1 ; index times 4
add si,dx ; 4*pixel position + color
sal si,1 ; 8*pixel position + 2*color
mov ax,ctable[si] ; look up rotated color mask
;
; insert color into the video byte
and al,es:[bx] ; get the old byte and remove old pixel
or al,ah ; insert new color
mov es:[bx],al ; put the byte back
;
pop ax ; restore registers
pop si
pop bx
ret ; return
;
setpt endp
;-------------------------routine ends---------------------------+