;************************************************
; vgac.asm
; PC VGA graphics control in assembly language
; uses BIOS for keyboard read and setting graphics
; modes, and procedure for setting a VGA pixel
; version for C calling convention :-
; LARGE model
; no MAIN entry point
; assemble only (no link)
; underscore for C-callable functions
; don't pop arguments off stack (caller does this)
;
; J Leis
; 24 May 1994
;************************************************
TITLE vgac.asm - vga assembler program, callable from C
MODEL LARGE
286
DOSSEG
; stack segment directive
STACK
; data segment directive
DATA
; code segment directive
CODE
_VgaMode PROC
pusha
mov ah, 0 ; function 0 = set video mode
mov al, 12h ; mode 12 = vga graphics
int 10h
popa
ret
_VgaMode ENDP
_TextMode PROC
pusha
mov ah, 0 ; function 0 = set video
mov al, 03h ; mode 3 = text
int 10h
popa
ret
_TextMode ENDP
_ShowMessage PROC
pusha ; save registers if necessary
; call DOS interrupt to display a message
mov bx, 01h
lea dx, mesg ; equivalent to mov dx, OFFSET mesg
mov cx, l_mesg
mov ah, 040h
int 021h
popa
ret
_ShowMessage ENDP
_ReadKey PROC
pusha ; save registers if necessary
mov ah, 00h ; function 0 - wait for key & read it
int 16h ; int 16h = keyboard services
; al now equals ascii code of key
popa
ret
_ReadKey ENDP
; setpixel( xc, yc, color )
; stacking order:
; memory near call far call
; color highest [bp+8] [bp+10]
; y-coord [bp+6] [bp+8]
; x-coord lowest [bp+4] [bp+6]
;
_SetPixel PROC
mov ax, [bp+8] ; get y co-ord
mov bx, 640/8 ; 80 bytes/line
mul bx
mov bx, [bp+6] ; get x-coord
mov cl, 3 ; divide by 8 bits/byte
shr bx, cl
add bx, ax
mov al, es:[bx] ; dummy write to latch data in screen RAM
mov cx, [bp+6] ; get x-coord
and cx, 0007h ; get bit mask
mov al, 07h
sub al, cl
mov ah, 80h
shr ah, cl ; shift to bit position
mov al, 08h ; set mask register
mov dx, 03CEh ; dx destroyed by mul
out dx, ax ; write bit mask
mov cx, [bp+10]; color ; write the color value
mov es:[bx], cl
popa
pop bp
ret ; don't pop args off stack - C does this
