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Does anyone know of any (commercial or homebrew) SMS/GG programs that use undocumented (or "unofficial) opcodes?

Also, what about the I register and the IM 0 and IM 2 instructions? Do they need to be implemented for SMS/GG emulation?

Thanks.

Eric Quinn

Quote

> Does anyone know of any (commercial or homebrew) SMS/GG programs that use undocumented (or "unofficial) opcodes?

My understanding is that there are handful that are used with some frequency, but these are all emulated by the common Z80 packages (Marat's Z80, RAZE). I was concerned about this when I started CHASMS, and have had Trap Bad Opcodes on (with Marat's Z80) and have never had this come up with 250+ ROMs that I've tested. If you're coding your own Z80 core, there are a variety of docs listing Z80 bugs and undocumented features at Zophar's (here.

Quote

> Also, what about the I register and the IM 0 and IM 2 instructions? Do they need to be implemented for SMS/GG emulation?

I remember reading somewhere that only IM 1 has any application to the SMS whatsoever. I can't find the reference now, but anyhow, I've never encountered any other IM modes in a SMS or GG ROM.

- vecna

Quote

> Does anyone know of any (commercial or homebrew) SMS/GG programs that use undocumented (or "unofficial) opcodes?

Not that I know of, but it's probably safe to assume that
some game out there makes use of the illegal instructions.
Most of them seem to be pretty useful, as opposed to just
being an oddity.

Quote

> Also, what about the I register and the IM 0 and IM 2 instructions? Do they need to be implemented for SMS/GG emulation?

The only game I know of that applies to this is Xenon 2, which
runs in IM 0 until the actual game starts. It expects to get
0xFF (RST 38h) during that time.

My Homepage

Author	Message
Eric Guest	Undocumented Opcodes in commercial SMS/GG software Posted: Wed Mar 29, 2000 5:53 pm
Eric Guest	Does anyone know of any (commercial or homebrew) SMS/GG programs that use undocumented (or "unofficial) opcodes? Also, what about the I register and the IM 0 and IM 2 instructions? Do they need to be implemented for SMS/GG emulation? Thanks. Eric Quinn

vecna Guest	Posted: Wed Mar 29, 2000 7:34 pm
vecna Guest	Quote > Does anyone know of any (commercial or homebrew) SMS/GG programs that use undocumented (or "unofficial) opcodes? My understanding is that there are handful that are used with some frequency, but these are all emulated by the common Z80 packages (Marat's Z80, RAZE). I was concerned about this when I started CHASMS, and have had Trap Bad Opcodes on (with Marat's Z80) and have never had this come up with 250+ ROMs that I've tested. If you're coding your own Z80 core, there are a variety of docs listing Z80 bugs and undocumented features at Zophar's (here. Quote > Also, what about the I register and the IM 0 and IM 2 instructions? Do they need to be implemented for SMS/GG emulation? I remember reading somewhere that only IM 1 has any application to the SMS whatsoever. I can't find the reference now, but anyhow, I've never encountered any other IM modes in a SMS or GG ROM. - vecna

Charles MacDonald Joined: 28 Sep 1999 Posts: 1197	Posted: Wed Mar 29, 2000 8:49 pm
Charles MacDonald Joined: 28 Sep 1999 Posts: 1197	Quote > Does anyone know of any (commercial or homebrew) SMS/GG programs that use undocumented (or "unofficial) opcodes? Not that I know of, but it's probably safe to assume that some game out there makes use of the illegal instructions. Most of them seem to be pretty useful, as opposed to just being an oddity. Quote > Also, what about the I register and the IM 0 and IM 2 instructions? Do they need to be implemented for SMS/GG emulation? The only game I know of that applies to this is Xenon 2, which runs in IM 0 until the actual game starts. It expects to get 0xFF (RST 38h) during that time. My Homepage

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