Re: R6502AP

From: groepaz_at_gmx.net
Date: Thu, 27 Nov 2014 10:41:07 +0100
Message-ID: <10988866.JJqhXXFVTG@rakete>
On Thursday 27 November 2014, 10:53:56 Marko Mäkelä <msmakela@gmail.com> 
wrote:
> 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?

iirc the flag is ignored... and there are illegal opcodes, but they dont 
exactly match the known ones -> http://en.wikipedia.org/wiki/2A03

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