vertical sprite positioning issue at leftmost screen positions

[Jan Hermanns]

Hello,

 

I'm desperate, I attached a stripped-down 2-scanline kernal, that shows a

phenomenon I don't understand.

 

Here is a little run-down, of what the kernal does:

 

- In the first scanline the playfield gets drawn as well as sprite GRP1 (only

to trigger GRP0, see below).

- In the second scanline the playfield gets disabled and Sprite GRP0 gets

drawn.

- In order to position Sprite GRP0 vertically accurate I use the VDELP0

register.

 

By and large this works fairly well, except for the leftmost side of the

screen. The attached kernal shows a sprite at a neuralgic position. The left

side of the sprite is messed up, whereas the right side gets drawn correctly.

 

Can anybody explain to me, why accurate vertical positioning at the leftmost

side of the screen does not work? Or am I doing something wrong?

 

kind regards, Jan

stripdown.asm.txt

[Ed Fries]

Looks to me like you are taking too long to store into GRP0:

 

  sta   WSYNC          
  stx   PF2			;3
  lda   #0				;2
  cpy   heroY			;3
  bcs   .skipDrawHero		;2
  lda   (heroSpritePtr,x)	;6
  beq   .skipDrawHero		;2
  inc   heroSpritePtr		;5
.skipDrawHero		
  sta   GRP0			;3 - 26 total (should be less than 68/3)

 

Try moving a little work above the WSYNC.

[Jan Hermanns]

Thank you very much!

 

I didn't know about that "GRP0 68/3" timing thing.

Edited July 30, 2010 by Jan Hermanns