Re: [stella] Thrust incompatibility? (Possibly illegal opcodes)

Subject: Re: [stella] Thrust incompatibility? (Possibly illegal opcodes)
From: Erik Mooney <emooney@xxxxxxxxxxxxxxxx>
Date: Fri, 02 Feb 2001 01:32:12 -0500
On Thu, 1 Feb 2001 23:51:58 -0600 (CST), you wrote:

>I recently got my working copy of thrust from hozer and was troubled that
>it didn't work.  After a little thought, I realised that it may be a
>compatability issue.  So I dug into my closet, and pulled out all my VCS
>Here's what I found:
>Maker   Model                   Functionality
>-----   ---------------------   ----------------------------
>Atari   2600 (6 switch)         OK
>Atari   2600A (4 switch)        OK
>Atari   2600jr (big rainbow)    OK
>Sears   2600 (6 switch)         OK
>Sears   video arcade 2 (2800)   Ship is constantly thrusting
>Coleco  Gemini                  Breaks in playfield
>Coleco  Coleco with expansion   Breaks in playfield
>I am most puzzled by the sears video arcade 2 not working, because I
>thought the sears models were produced by atari and just rebranded.
>Thomas, do you know what could happen to cause the above problems?  Can it
>be linked to an illegal opcode?
>Everyone, has anyone else seen similar problems?  Has anyone done
>experiments or written other programs useing such opcodes on various
>compatables?  Does anyone have a 1-chip jr that they could try?

Yes.  The problems with the Colecos are due to a playfield register update
timing issue.  I had the same problem in one of my builds of INV - look in
the list archives for April 1997, you may find some info there.  The
Coleco TIAs respond differently than the Atari ones to one of the
playfield registers being updated right on the edge of when it's allowed.

Dunno what's wrong with the 2800 though...

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