Hi All...
I have a question about the bzoneRepos (horizontal positionting)
subroutine, as demonstrated in Kirk's Cookbook document:
http://alienbill.com/2600/cookbook/subpixel.html
I've used the routine as presented on that page, but I notice that
when you move across a horizontal position of 15, it sort of stutters.
It seems to be off by about three pixels. You can see this if you move
the ghost (in the demo program on that page) close to the left margin
of the screen. The routine exhibits the same behavior in my program.
Adding a "nop" to it seems to fix it. So the new routine looks like
this:
bzoneRepos
sta WSYNC ; 00 Sync to start of scanline.
sec ; 02 Set the carry flag so no borrow
will be applied during the division.
NOP ;THIS NOP SEEMS TO PREVENT A STUTTER AT 15 PIXELS
.divideby15
sbc #15 ; 04 Waste the necessary amount of
time dividing X-pos by 15!
bcs .divideby15 ; 06/07
11/16/21/26/31/36/41/46/51/56/61/66
tay
lda fineAdjustTable,y ; 13 -> Consume 5 cycles by guaranteeing
we cross a page boundary
sta HMP0,x
sta RESP0,x ; 21/ 26/31/36/41/46/51/56/61/66/71 - Set
the rough position.
rts
It does it in both Stella and z26. Does the problem that I'm seeing
exist on real hardware, too? Or am I "fixing" something to run in the
emulators that will break on hardware? I searched the archive for
"bzoneRepos", but I didn't find any discussions on this routine.
Thanks!
-Rich
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