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BIOS weirdness
Posted: Wed May 31, 2000 9:22 am
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I desoldered the BIOS from my SMS-2 and fitted a socket so I could try different BIOS versions.
The Japanese BIOS (posted by Zoop earlier) works fine, plays all games as expected. Funnily enough I thought fitting a Japanese BIOS would put the SMS into Japanese text mode, but evidently not! I still have no clue how a language switch might be implemented. The US BIOS (bios13.sms) is an odd one though - upon power on it displays the logo then goes straight into the snail maze game, regardless of what you press or what game is inserted. I can't get it to do anything else. Is this a bad dump, or has the bios been hacked in some way? Mike |
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Posted: Wed May 31, 2000 10:46 am |
I'm not sure. I didn't dump it myself but I have heard some models of the SMS behave this way. (if no cartridge is inserted it jump to the snail game directly) |
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Posted: Wed May 31, 2000 8:04 pm |
But this one jumps to the snail game regardless of whether a cartridge is inserted or not! It may be to do with the fact that I'm using it on a Master System II - funny that the Japanese one should work fine though! I haven't an original SMS to hand to test it out on unfortunately. Mike |
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Posted: Wed May 31, 2000 9:27 pm |
Maybe it was hacked then, I don't know :( It's not specific to your SMS, it's clear, in the code. |
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vecna
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Posted: Wed May 31, 2000 9:37 pm |
I tend to think it's a bad dump. I spent a great deal of time pouring over the disassembly of the BIOS while trying to get them to work in CHASMS, and well -- the bios13.sms posted simply isn't right. It loads the cartridge-detect routine into memory, but NEVER EXECUTES IT, among other things. And the code for the instructions screen is in there, but it's never accessed. So I'm quite certain that something just isnt right with the dump -- I can't imagine any situation where it would make sense. I finally just romhacked the BIOS to work the way I want it to -- But until I build a devcart of my own I can't figure out the proper way port $3E works (cartridge detect), so I can't create a truly fixed BIOS ROM yet. - vecna |
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Posted: Thu Jun 01, 2000 5:24 pm |
Based on that, I'm -sure- it was a hack. Most likely it was hacked and released so people with an appopriate backup system (the game gear one, or a MGD^2 if you were lucky enough to find/afford one) could play snail race on a system which didn't come with it. |
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vecna
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Posted: Thu Jun 01, 2000 7:08 pm |
Hnmm. Well then, I've created a 'fixed' USA ROM BIOS. This BIOS will not actually properly run in any existing emulator right now, because none of us seem to know how port $3E works yet -- but this should run properly on a Real SMS. I've patched it to call the cartridge-detect routine and if the cart-detect routine returns, it will return to the instruction screen rather than straigh to the snail game -- and if a cart is there, it should load that up, too. My best guess is that this ROM is identical to what you should get with a proper dump. Mike, I'd be real interested if you could try this out and let me know if it works or not. If it does, then I can confirm I have a pretty good grasp on how the BIOS works, if I can just decypher the damn cart-detect/page-in routine and how port $3E works. You can get it here: http://chasms.midgetrebellion.com/bios13fixed.sms - vecna |
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Posted: Thu Jun 01, 2000 9:58 pm |
Your patch has done the trick, Vecna - the BIOS now behaves exactly as it should! With no cart present, it does go to the instruction screen rather than the snail game - and any cartridge inserted loads just fine. I also tried it out in my hacked cart to test it on the GG and MD, and it works exactly as it does on a SMS with no cart inserted, i.e. instruction screen/snail game. Trying this cart on the SMS (with Japanese BIOS) simply caused the logo sequence to cycle over and over. Mike |
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Posted: Fri Jun 02, 2000 4:41 pm |
So you're putting this bios rom in an alex kidd sms2, right? here's a notion: Try this bios with tetracycline (aka my tetris) and let me know what you get. I suspect you may get a 'Software Error' (rarely happens but I have seen it on my SMS 1) since there's no checksum or 'TMR SEGA' code (YET!). P.S. You are my hero. As much as I like to tinker with low level z80 code and stuff, I have -no- talent for hardware projects like what you've put together. |
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Posted: Sat Jun 03, 2000 1:33 pm |
Well... with the Japanese BIOS installed, the game loads and plays OK. With both the Alex Kidd and original US BIOS, it's as if no cart is inserted - the instruction screen, or Alex Kidd, appears instead. The only time I've seen "Software Error", incidentally, was when I tried to play the Japanese "Alex Kidd in Miracle World" with the Alex Kidd BIOS in place. (Don't ask why!)
Who me? *blush* :) Anyway, I'm hopeless at coding, and anyone who can program stuff for the SMS gets my utmost admiration! Cheers, Mike |
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