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The blit subroutine reads a two-color (1 bit per pixel) block of graphics data from ROM and displays it on the screen, using several parameters. It works much in the same way as the drawchar routine except for arbitrarily sized data. The code was originally taken from Videocart 26 and adapted to be more flexible and reusable.

The routine is separated into two parts, blitGraphic and blit. blit takes parameters in registers r1-r6, and retrieves graphics data from the location DC0 points to. It can be called independently of blitGraphic. blitGraphic takes hard-coded parameters from ROM at the address pointed at by DC0 and then calls the blit routine. When the parameters do not need to be dynamic, storing the parameters in ROM and calling the blitGraphic routine directly can be more efficient than writing them to the registers and calling blit.

; Blit Code ;

; Blit Graphic ;

; takes graphic parameters from ROM, stores them in r1-r6, 
; changes the DC and calls the blit function with the parameters
; modifies: r1-r6, Q, DC

	; load six bytes from the parameters into r0-r5
	lisu	0
	lisl	1
	lr	I, A						; store byte and increase ISAR
	br7	.blitGraphicGetParms				; not finished with the registers, loop

	; load the graphics address
	lr	Qu, A						; into Q
	lr	Ql, A
	lr	DC, Q						; load it into the DC

	; call the blit function
	jmp	blit

; Blit Function ;

; this function blits a graphic based on parameters set in r1-r6,
; and the graphic data pointed to by DC0, onto the screen
; originally from cart 26, modified and annotated
; modifies: r1-r9, DC

; register reference:
; -------------------
; r1 = color 1 (off)
; r2 = color 2 (on)
; r3 = x position
; r4 = y position
; r5 = width
; r6 = height (and vertical counter)
; r7 = horizontal counter
; r8 = graphics byte
; r9 = bit counter
; DC = pointer to graphics

	; fix the x coordinate
	lis	4
	as	3
	lr	3, A
	; fix the y coordinate
	lis	4
	as	4
	lr	4, A

	lis	1
	lr	9, A						; load #1 into r9 so it'll be reset when we start
	lr	A, 4						; load the y offset
	com							; invert it
	outs	5						; load accumulator into port 5 (row)

	; check vertical counter
	ds	6						; decrease r6 (vertical counter)
	bnc	.blitExit					; if it rolls over exit

	; load the width into the horizontal counter
	lr	A, 5
	lr	7, A

	lr	A, 3						; load the x position
	com							; complement it
	outs	4						; use the accumulator as our initial column
	; check to see if this byte is finished
	ds	9						; decrease r9 (bit counter)
	bnz	.blitDrawBit					; if we aren't done with this byte, branch

	; get the next graphics byte and set related registers
	lis	8
	lr	9, A						; load #8 into r9 (bit counter)
	lr	8, A						; load a graphics byte into r8

	; shift graphics byte
	lr	A, 8						; load r8 (graphics byte)
	as	8						; shift left one (with carry)
	lr	8, A						; save it

	; check color to use
	lr	A, 2						; load color 1
	bc	.blitSavePixel					; if this bit is on, draw the color
	lr	A, 1						; load color 2
	bc	.blitCheckColumn				; branch if the color is "clear"
	outs	1						; output A in p1 (color)

	; transfer the pixel data
	li	$60
	outs	0
	li	$c0
	outs	0
	; and delay a little bit
	ai	$60						; add 96
	bnz	.blitSavePixelDelay				; loop if not 0 (small delay)

	ds	7						; decrease r7 (horizontal counter)
	bz	.blitCheckRow					; if it's 0, branch

	ins	4						; get p4 (column)
	ai	$ff						; add 1 (complemented)
	br	.blitColumn					; branch

	ins	5						; get p5 (row)
	ai	$ff						; add 1 (complemented)
	br	.blitRow					; branch

	; return from the subroutine

Note: the adjustment of x and y in blit are to allow the coodinates 0, 0 to access the upper left pixel visible to the screen, not to VRAM, which expands four pixels above and to the left of that pixel.

To call the above functions, you should use:

	dci	graphic.parameters	; address of parameters
	pi	blitGraphic

for blitGraphic, and:

	; parameters have been loaded into r1-r6 beforehand
	dci		; address of graphic data
	pi	blit

for blit. If you're using parameters stored in ROM, they should be in this order:

	.byte	bkg			; color 1
	.byte	blue			; color 2
	.byte	4			; x position
	.byte	18			; y position
	.byte	$60			; width
	.byte	$13			; height
	.word		; address for the graphics

See Also