f18avramwrite

;;;;;;;;;;;;;;;;;;;;;;;
; graphicsVramWriteBlock
; PARAM1 : Destination VRAM address in F18A, high 2 bits must be 01
; PARAM2 : Source address in main RAM, must be page-aligned
; PARAM3 : Number of bytes to copy
;
; Write data to F18A's VRAM
;
; Trashes PARAM2
;
graphicsVramWriteBlock:
	SAVE_AXY

	; Set up F18A address register
	lda PARAM1_L
	sta F18AREG1
	lda PARAM1_H
	sta F18AREG1

	ldy #0
	ldx PARAM3_H

graphicsVramWriteBlockLoop:
	lda (PARAM2_L),y
	sta F18AREG0
	iny
	cpy PARAM3_L
	beq graphicsVramWriteBlockLoopLowDone

graphicsVramWriteBlockLoopCont:
	cpy #0
	bne graphicsVramWriteBlockLoop

	; Finished 256 byte page, so wrap to next one
	dex
	inc PARAM2_H	; Advance source pointer in RAM to next page
	jmp graphicsVramWriteBlockLoop

graphicsVramWriteBlockLoopLowDone:
	; Low byte of counter matched
	cpx #0		; If high counter is finished, so are we
	bne graphicsVramWriteBlockLoopCont

graphicsVramWriteBlockDone:
	RESTORE_AXY
	rts