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Re: The Sega Satan WIP/screenshot thread [Re: Kale] #94877 06/30/14 07:22 PM
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Oh, that's *very* nice! Coming right on the heels of noCash's project to get all the PSX CD drive MCUs dumped, no less.

Re: The Sega Satan WIP/screenshot thread [Re: Kale] #94878 06/30/14 07:42 PM
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Haze Offline
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that is good news, and a proper readout method too which is always preferable in terms of being able to trust the results 100%

I wonder if the different revisions will reveal why some games apparently don't work on the later HiSaturn model etc.

Re: The Sega Satan WIP/screenshot thread [Re: Haze] #94879 06/30/14 08:01 PM
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Originally Posted By Haze
I wonder if the different revisions will reveal why some games apparently don't work on the later HiSaturn model etc.


Because VA10 and later Saturns have the SCSP and 68k integrated into a single Yamaha ASIC, and a few games trip up those 68k commands that this ASIC does not support. Exact same bug as why the Genesis 3 can't run Gargoyles.
One of the Sega tech bulletins mention which two commands are not supported.

As far as I know, there are only two games with such a problem: Space Harrier, and Outrun. Both of them got second revisions which fixed the issue. (and no, Metal Slug is NOT affected, unlike what rumors and Wikipedia state)

But, Model 2 HiSaturn is identical to the late 1996 PAL models - same VA9 board. So maybe you are talking about some other problem?

Re: The Sega Satan WIP/screenshot thread [Re: Kale] #94880 06/30/14 08:04 PM
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That's a little weird; the 68k commands are provided and parsed by the sound driver program that's downloaded. It has no 'commands' per se, it's a completely normal 68000.

I'd believe something like an out of bounds access that gets DTACK'ed on the previous boards but not the all-in-one though.

Last edited by R. Belmont; 06/30/14 08:07 PM.
Re: The Sega Satan WIP/screenshot thread [Re: Kale] #94881 06/30/14 08:19 PM
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Haze Offline
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I suspect he means the 68k doesn't implement all opcodes.. (or maybe has a slightly different mapping?)

Gargoyles on the Megadrive is because of the TAS behavior (writeback fails on an original genesis, happens on a Genesis 3)

I'm not aware of Saturn needing Tas writeback disabled, although I've never checked if anything uses the opcode..

Re: The Sega Satan WIP/screenshot thread [Re: Kale] #94882 06/30/14 08:26 PM
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zyrobs Offline
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Sega Saturn tech bulletin #51 states:
Quote:

The following instructions codes cannot be used in programs for the 68000 Sound CPU installed in the Sega Saturn:
RESET instruction
TAS instruction


I suspect the problem is the same as on Gargoyles since the Genesis 3 also uses a Yamaha-built integrated 68k.

Of course it could be anything else. If anyone wants to build a test cd that checks this, I can run it on such a machine.

Re: The Sega Satan WIP/screenshot thread [Re: Kale] #94883 06/30/14 08:42 PM
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TAS was always forbidden on Saturn, but RESET worked on the original boards to reset the SCSP as far as I remember.

Leave it to Yamaha to figure out how to make it actively bad.

Re: The Sega Satan WIP/screenshot thread [Re: Kale] #94884 06/30/14 08:54 PM
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Like I said, write a test program and I can run it on nearly any Saturn.

Alternatively, you can try it on SSF, there is an extra ini-only option called 68000Type that emulates this specific behaviour, so you don't even need to waste discs to check just this. If it's just the TAS instruction returning different data, that should be easy to check, no?

Re: The Sega Satan WIP/screenshot thread [Re: zyrobs] #94885 06/30/14 08:54 PM
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Haze Offline
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Originally Posted By zyrobs
Sega Saturn tech bulletin #51 states:
Quote:

The following instructions codes cannot be used in programs for the 68000 Sound CPU installed in the Sega Saturn:
RESET instruction
TAS instruction


I suspect the problem is the same as on Gargoyles since the Genesis 3 also uses a Yamaha-built integrated 68k.

Of course it could be anything else. If anyone wants to build a test cd that checks this, I can run it on such a machine.


would be interesting to know for sure what TAS does on both systems.. feel free to make a little demo CD to use it to detect models that way if you can ;-)

of course the writeback might not happen on either, in which case we should disable it in all cases like the Genesis.


Re: The Sega Satan WIP/screenshot thread [Re: zyrobs] #94892 07/01/14 10:31 AM
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Kale Offline OP
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Originally Posted By zyrobs
Like I said, write a test program and I can run it on nearly any Saturn.

Alternatively, you can try it on SSF, there is an extra ini-only option called 68000Type that emulates this specific behaviour, so you don't even need to waste discs to check just this. If it's just the TAS instruction returning different data, that should be easy to check, no?


I've written one a year ago or so, I think Charles still has it including source code?

And yes, Outrun, Space Harrier AND Fantasy Zone iirc trips RESET opcode on 68k side, expecting it to reset the sound CPU otherwise it hangs before Sega/Sega Ages logo. Pretty sure it also resets something else in SCSP plus the CPU reset actually happens thru SMPC (m68k RESET opcode can't toggle reset pin independently).

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