Re: [stella] Strange glitches in homebrewed games on the 2600 jr.

Subject: Re: [stella] Strange glitches in homebrewed games on the 2600 jr.
From: "Eckhard Stolberg" <Eckhard_Stolberg@xxxxxx>
Date: Fri, 28 Feb 2003 21:49:22 +0100
Nah, it's really a shot in the dark. Eckhard mentions repeatedly that not
doing uninterupted WSYNCs during vertical sync could cause problems. I hope
that's not the case. Being that these games play fine on other models
including my 7800, it seemed like a great place to start tracking it down :)

Has it been established what type of 2600 Jr. console it is that
has problems with the homebrew games. The problem I mentioned
only seems to affect some of the very late 2600 Jrs. and 7800s.
Unfortunately it seems to affect all PAL-B models of the 7800.
That one came out for Christmas of 1989, so Jrs. from that time
might suffer from the same problems.

These consoles also show different behaviour when object positioning
registers are hit too soon after an HMOVE. In most cases the
positions are the same as on the normal VCS, but sometimes they
are not. The only game that is really affected by this is Kool
Aid Man. In the main game screen the digits of the score display
are positioned on top of each other, so that the game sees a
constant collision detection, which it assumes to be caused by
the KAM character. Therefore it is constantly bouncing in the
top left corner.

I'm not quite sure what it is exactly that makes the difference
for KAM. At first I thought it was because it hits several
position registers too soon after HMOVE, but it might also have
something to do with hitting NUSIZ there. I think John and I
forgot to check this when we did our experiments for z26.

Also these consoles don't do the 18-sprite trick quite right.
the first 18-sprite demo that John posted only showed 17 sprites
on my problematical Jr., while it showed 18 sprites on other
VCSs. Later incarnations of the 18-sprite trick worked normally
on it though.

Looking at the type of problems that we see, I'm assuming that
the change must be internal to the TIA chip. I'm not sure why
Atari would change the design though, since my problematical
JR. and 7800 don't have this one-chip solution that Glenn
mentioned. Coincidentially all my PAL 7800s and the problematical
Jr. Seem to loose the colour in VCS mode when they are running
for a couple of minutes. The colours in 7800 mode stays fine
though. Does anyone have an idea what might cause this?

Ciao, Eckhard Stolberg

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