Category : BASIC Source Code
Archive   : PSETMA.ZIP
Filename : CPIXADDR.ASM

NAME CGAPixelAddr

;
; Name: CGAPixelAddr
;
; Function: Determine buffer address of pixel in 640x200 2-color mode
;
; Caller: AX = y-coordinate (0-199)
; BX = x-coordinate (0-639)
;
; Returns: AH = bit mask
; BX = byte offset in buffer
; CL = number of bits to shift left
; ES = video buffer segment
;


OriginOffset EQU 0 ; byte offset of (0,0)
VideoBufferSeg EQU 0B800h

CGAParams SEGMENT byte public 'CODE'
ASSUME cs:CGAParams

PUBLIC CGAPixelAddr
CGAPixelAddr PROC near

mov cl,bl ; CL := low-order byte of x

xchg ah,al ; AX := 100h * y
shr bx,1 ; BX := x/2
shr ax,1 ; AL := 80h*(y&1)
add bh,al ; BX := x/2 + 8000h*(y&1)
xor al,al ; AX := 100h*(y/2)
add bx,ax ; BX := x/2 + 8000h*(y&1) + 100h*(y/2)
shr ax,1
shr ax,1 ; AX := 40h*(y/2)
add bx,ax ; BX := x/2 + 8000h*(y&1) + 140h*(y/2)
shr bx,1
shr bx,1 ; BX := x/8 + 2000h*(y&1) + 50h*(y/2)
add bx,OriginOffset ; BX := byte offset in video buffer

mov ax,VideoBufferSeg

mov es,ax ; ES:BX := byte address of pixel

and cl,7 ; CL := x & 7
xor cl,7 ; CL := number of bits to shift left
mov ah,1 ; AH := unshifted bit mask

ret

CGAPixelAddr ENDP

CGAParams ENDS

END