Re: R6502AP

From: Marko Mäkelä <*msmakela_at_gmail.com*> Date: Thu, 27 Nov 2014 10:53:56 +0200 Message-ID: <20141127085356.GB1821@x60s> · msmakela_at_gmail.com

On Thu, Nov 27, 2014 at 09:11:27AM +0100, Gerrit Heitsch wrote:
>So far my impression was that all NMOS-6502 behave the same, including 
>the ones from UMC (I have a 1581 with a UM6502A somewhere). The 
>differences started with the CMOS variants.

I read somewhere that Nintendo used an unauthorized copy of the 6502, 
where the BCD mode was intentionally disabled in order to avoid 
violating the patent that Chuck Peddle got for it.

I wonder how that was done. Is the D flag still working, but just being 
ignored? Does anyone have a NES? Were undocumented opcodes used in any 
NES games?

AFAIU, the later SNES is based on a CMOS design (65C816), so the 
undocumented opcodes would not work there.

	Marko

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