Kool-Aid Man suffers from the "Atari Jr. Bug"

[sneakernets]

There is a bug in some Atari Jrs which make Kool-Aid Man unplayable, he bounces all the time on the upper left corner, and the score isn't rendered correctly. Unfortunately, Javatari does the same thing.

The correct layout:

[ppeccin]

Thanks for the report. Do you have any mo references about this "Atari Jr." bug? Is it a well known problem? Paulo

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On Sat, Nov 16, 2019 at 6:37 PM Shannon Freeman ***@***.***> wrote: There is a bug in some Atari Jrs which make Kool-Aid Man unplayable, he bounces all the time on the upper left corner, and the score isn't rendered correctly. Unfortunately, Javatari does the same thing. [image: image] <https://user-images.githubusercontent.com/5595293/68999520-bd224100-0886-11ea-9961-ec6a8ca9eee5.png> The correct layout: [image: image] <https://user-images.githubusercontent.com/5595293/68999532-ee027600-0886-11ea-99af-c634a6e698b1.png> — You are receiving this because you are subscribed to this thread. Reply to this email directly, view it on GitHub <#15?email_source=notifications&email_token=AAFOLID6REZED4YY6MO4PVTQUBRZBA5CNFSM4JOG7F62YY3PNVWWK3TUL52HS4DFUVEXG43VMWVGG33NNVSW45C7NFSM4HZ2CFTQ>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AAFOLIEUDB77WMS355YETN3QUBRZBANCNFSM4JOG7F6Q> .

[sneakernets]

From the Stella mailing list and AtariAge forums, I was able to piece this together.

Certain functions can act unpredictably when executed right after an HMOVE call: https://atariage.com/forums/topic/165168-kool-aid-man-rom-problem/?do=findComment&comment=2043721 https://www.biglist.com/lists/stella/archives/200211/msg00102.html

I'm pretty convinced this was really a bug that was just fortunate to work out right. I don't see any reason for it to have been done this way except by mistake. The unpredictable results of changing HMP0/HMP1 right after an HMOVE are documented in the Stella Programming Guide and there's no benefit gained except the compatibiltiy problems so it probably just slipped through. [Or maybe Mattel's documentation didn't include mention of this? But then you'd expect that to lead to plenty of problems elsewhere... Anyone have Steve Tatsumi's number (smile) I'm sure he'd remember it like yesterday...]

Yes, it's a problem with certain revisions of the TIA chip (and I think some of the 7800s as well). Although I don't know if it's documented formally, it is a big enough problem that some collectors mention it when they sell a unit or are testing homebrew games.