; vi:filetype=nasm ts=8 sts=8 sw=8:
;
; list of functions
; -----------------
; init_gfx
;    initializes the video hardware and graphics routines
; clear
;    clears the framebuffer (not vmem)
;    clobbers: ax, cx, di
; swap_buffers
;    copies the framebuffer to video memory
;    clobbers: ax, cx, di, si
; wait_vsync
;    clobbers: al, dx
; set_palette_entry(idx[al], r[ah], g[bl], b[bh])
;    colors are 0-255

VIDMEM_SEG	equ 0a000h
FRAMEBUF_SEG	equ 09000h

REG_CRTC_STATUS	equ 3dah
CRTC_VBLANK_BIT	equ 08h

REG_DAC_ADDR	equ 3c8h
REG_DAC_DATA	equ 3c9h

init_gfx:
	; video mode 13h (320x200 8bpp)
	mov ax, 13h
	int 10h

	call clear
	ret

clear:
	push es
	mov ax, FRAMEBUF_SEG
	mov es, ax
	xor di, di
	xor ax, ax
	mov cx, 16000
	rep stosd
	pop es
	ret

swap_buffers:
	push ds
	push es
	mov ax, FRAMEBUF_SEG
	mov ds, ax
	xor si, si
	mov ax, VIDMEM_SEG
	mov es, ax
	xor di, di
	mov cx, 16000
	rep movsd
	pop es
	pop ds
	ret

wait_vsync:
	mov dx, REG_CRTC_STATUS
.wait_vblank_end:
	in al, dx
	and al, CRTC_VBLANK_BIT
	jnz .wait_vblank_end
.wait_vblank_start:
	in al, dx
	and al, CRTC_VBLANK_BIT
	jz .wait_vblank_start
	ret

set_palette_entry:
	push dx
	mov dx, REG_DAC_ADDR
	out dx, al
	inc dx	; dx <- REG_DAC_DATA
	mov al, ah
	shr al, 2
	out dx, al
	mov al, bl
	shr al, 2
	out dx, al
	mov al, bh
	shr al, 2
	out dx, al
	pop dx
	ret