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Basic sprite question


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I am experimenting with sprites, and I have a simple two-line kernel.  Every even-numbered scanline should show a solid red line (#255), while every odd-numbered scanline should show a dotted (#170) yellow line.  I want to use GRP0 for both odd and even lines.  Nothing I have tried works, and for the life of me, I can't figure out why.  Here is my kernel:



		lda #$70
		sta COLUBK

		lda #0
		sta VDELP0

		lda #0
		sta GRP0
		sta GRP1


		ldx #192

		and #$01
		beq .oddscan

		lda #$48	;red	
		sta COLUP0
		lda #255
		sta GRP0
		sta RESP0

		jmp .join

		lda #$1e	;yellow
		sta COLUP0
		lda #170
		sta GRP0
		sta RESP0

		sta WSYNC

        bne LVScan 
		; 30 lines of overscan
		TIMER_SETUP 32    
		lda #170
		sta GRP0

        jmp NextFrame

Any idea what the problem is?





Edited by wopr67
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18 hours ago, Karl G said:

I'm pretty sure the problem is the RESP0 in your kernel. 

+1 . Writing to RESP0 has ... all kinds of tricky side effects. To keep things simple strobe RESP0 before you go into your display loop. This could be as simple as inserting WSYNC / SLEEP ... / sta RESP0  - before you go into LVScan. By changing the sleep value you'll be able to observe how strobing at different points sets the sprite to different horizontal positions.

It is also super helpful to use stella in debug mode and step through frame by frame / line by line to observe what's happening on each scan line.

Edited by Dave C
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