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

;-------------------------xorpt routine begins--------------------------+
;
; FROM Bluebook of Assembly Routines for the IBM PC & XT
; Page : 133
;
; NAME XORPT
;
; ROUTINE FOR XOR plotting a point on med-res color screen
;
; FUNCTION: This routine plots a point on the medium resolution
; color screen using the "exclusive OR" operation. The pixel at the
; specified location is colored with a color obtained by "exclusive or-ing"
; its original color with a specified color. This function is useful for
; making cursors.
;
; 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.
;
; ROUTINE TO XOR A POINT ONTO MEDIUM RESOLUTION COLOR SCREEN
;
xorpt 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 4
sal ax,1 ; times 8
sal ax,1 ; times 16
mov bx,ax ; goes into adddress
and bh,7 ; without adjustment
sal ax,1 ; times 32
sal ax,1 ; times 64
add bx,ax ; adress gets y-coord times 80
;
; add x-coordinate to address
mov ax,si ; get x coordinate
sar ax,1 ; divide
sar ax,1 ; by 4
add bx,ax ; here is the address
;
; 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
xor es:[bx],ah ; xor the byte withthe color
;
pop ax ; restore registers
pop si
pop bx
ret ; return
;
xorpt endp
;-------------------------xorpt routine ends---------------------------+