Re: R6502AP

From: Nate Lawson <>
Date: Sat, 29 Nov 2014 12:46:11 -0800
Message-Id: <>
On Nov 27, 2014, at 12:53 AM, Marko Mäkelä <> 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?

I haven’t tested it, but Nintendo just removed the gates that implemented BCD mode. Like you say, it would be interesting to find out how far they went.

It didn’t matter much to them since it was a game console, and BCD mattered more for cash registers and industrial applications that MOS had in mind before computers.


       Message was sent through the cbm-hackers mailing list
Received on 2014-11-29 21:01:13

Archive generated by hypermail 2.2.0.