- Joined: 21 Apr 2000
- Posts: 598
- Location: Newcastle upon Tyne, England
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Checksum and US BIOS - some info, and a question for Zoop
 Posted: Thu Jul 13, 2000 12:16 pm
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Following on from my earlier post, I have managed to get the SMS Power demo to start and run (after a fashion!) on a SMS with a US BIOS. Incidentally, all the results below also apply to the Alex Kidd BIOS in the SMS 2 (and, I would imagine, all other US/European BIOS versions).
Note that the display (above) is corrupted - the modified demo also doesn't work correctly in Meka. (It works fine with the Japanese BIOS installed.) Zoop - do you know the reason for this?
All I've done is change the last 16 bytes of the ROM to the following:
54 4D 52 20 53 45 47 41 00 00 50 48 00 00 00 4C
incorporating the TMR SEGA message (first 8 bytes), correct checksum (0x4850), country code (4) and cartridge size code (0x0C).
In the process of testing various things I discovered that:
- The cartridge won't be recognized *at all* if the TMR SEGA message is not present. (BIOS behaves as if no cart is inserted.)
- A "SOFTWARE ERROR" message is produced unless (a) the checksum is correct, and (b) the country code is 4 (US/European).
The Japanese BIOS, on the other hand, will boot the cartridge no matter what is present in the last 16 bytes of page 01. Some Japanese SMS games lack the TMR SEGA message, contain an invalid checksum, or both. (Hang On and Machine Gun Joe are two examples.) Additionally, the country code is 3 in some (but not all) Japanese games, meaning they fail to work on a US BIOS.
Mike
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- Joined: 24 Jun 1999
- Posts: 1732
- Location: Paris, France
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Posted: Sat Jul 15, 2000 3:15 pm
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Quote > all the results below also apply to the Alex Kidd BIOS in the SMS 2 (and, I would imagine, all other US/European BIOS versions).
Any ideas if the Game Gear BIOS has any restrictions? It seems to behave like the Master System Japanese one.
Quote > Note that the display (above) is corrupted -
Now that's very interesting. I really wonders why such thing happen, but my first guess would be that I'm not following the delay rules about accessing the VDP.
(Again it is an old demo so I hope everyone will forgive me ;-)
Quote >the modified demo also doesn't work correctly in Meka. [..] Zoop - do you know the reason for this?
Sure I do. In a recent Meka an emulation fixes made that the demo wasn't working well with the emulator (default VDP registers weren't good) so a patch for it is included in MEKA.PAT. I guess I'd better release a fix to the demo itself, though.
Quote >(It works fine with the Japanese BIOS installed.)
I do not understand. Please doesn't tell me if you upload the Japanese BIOS to your SMS the demo then works fine?
Quote > All I've done is change the last 16 bytes of the ROM to the following:
> 54 4D 52 20 53 45 47 41 00 00 50 48 00 00 00 4C
> incorporating the TMR SEGA message (first 8 bytes), correct checksum (0x4850), country code (4) and cartridge size code (0x0C).
> In the process of testing various things I discovered that:
> - The cartridge won't be recognized *at all* if the TMR SEGA message is not present. (BIOS behaves as if no cart is inserted.)
Great finds. Thanks for the informations. Now there is a point to solves out about the checksum calculation:
Some games like "Golvellius" have bad checksum *considering our knowledge of the checksum*. And obviously they works on a SMS2.
Will study the header..
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- Joined: 21 Apr 2000
- Posts: 598
- Location: Newcastle upon Tyne, England
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Posted: Sat Jul 15, 2000 8:01 pm
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Quote > Any ideas if the Game Gear BIOS has any restrictions? It seems to behave like the Master System Japanese one.
The GG has no restrictions as far as I can see - the same is true of the Mega Drive in SMS mode. Certainly they've run everything I've tried on them (apart from SG1000 stuff).
Quote > >(It works fine with the Japanese BIOS installed.)
> I do not understand. Please doesn't tell me if you upload the Japanese BIOS to your SMS the demo then works fine?
Yes, it's true! Given what you've just said, I don't understand it either. But see the screenshot above... It also works perfectly on the MD and GG.
Quote > Great finds. Thanks for the informations. Now there is a point to solves out about the checksum calculation:
> Some games like "Golvellius" have bad checksum *considering our knowledge of the checksum*. And obviously they works on a SMS2.
> Will study the header..
Strange... This is true of the European/US version as well? I can understand the Japanese Golvellius having a "bad" checksum, but not the US one.
Mike
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- Joined: 24 Jun 1999
- Posts: 1732
- Location: Paris, France
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Posted: Sat Jul 15, 2000 9:37 pm
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Quote >> I do not understand. Please doesn't tell me if you upload the Japanese BIOS to your SMS the demo then works fine?
> Yes, it's true! Given what you've just said, I don't understand it either. But see the screenshot above... It also works perfectly on the MD and GG.
Wow. Weird, but I have an idea.
Quote >> Great finds. Thanks for the informations. Now there is a point to solves out about the checksum calculation:
>> Some games like "Golvellius" have bad checksum *considering our knowledge of the checksum*. And obviously they works on a SMS2.
>> Will study the header..
> Strange... This is true of the European/US version as well?
Yes with the old knowledge. But today I have studied the header and found out how to calculate the header correctly for Golvellius! Means I'll have to rewrite the checksum tools ^_^
[..]
Quote >I can understand the Japanese Golvellius having a "bad" checksum, but not the US one.
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