6502 Decimal mode

From: Marko Mäkelä (msmakela_at_cc.hut.fi)
Date: 2000-07-25 12:37:11

Someone on the plus4 list asked how the addition and subtraction of
invalid operands works in decimal mode.  I got this interesting follow-up
to my answer.


---------- Forwarded message ----------
Date: Tue, 25 Jul 2000 12:14:23 +0200
From: Tim Böscke <t.boescke@tu-harburg.de>
Reply-To: plus4@c64.rulez.org
To: plus4@c64.rulez.org
Subject: Re: ADC - SBC

> > 1. What exactly happens when I am trying to add a number to a non-
> > decimal coded number (semi-tetrade?) in decimal mode.
> I investigated this in 1993 or so.  The results are documented in a file
> "64doc" that you can find e.g. on ftp.funet.fi.  The document includes a

Did you ever want to know how it really works ? ;) Check out the patent
on it:


The adder was one of the main innovations in the 6502 on hardware

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