[CCTECH] Interesting tidbit on 6502

From: Richard Erlacher <edick_at_idcomm.com>
Date: Sat Jun 8 09:19:55 2002

see below, plz.

Dick

----- Original Message -----
From: "Fred Cisin (XenoSoft)" <cisin_at_xenosoft.com>
To: <cctalk_at_classiccmp.org>
Sent: Saturday, June 08, 2002 4:48 AM
Subject: Re: [CCTECH] Interesting tidbit on 6502


> On Fri, 7 Jun 2002, Richard Erlacher wrote:
> > see below, plz.
> > Dick
> > ----- Original Message -----
> > > > Problem with the 6502 is that different versions
> > > > from different manufacturers had different
> > > > undocumented instructions. So for the general
> > > > public this was not much of a boon ....
>
> > Has anybody got examples, not including the CMOS versions, of course,
which
> > are documented to be different, of differences in the NMOS 6502's? In the
> > years that I used 'em, I only had hands-on contact with MOS Technology,
> > Synertek, and Rockwell parts.
>
> Where the UNDOCUMENTED aspects are DOCUMENTED to be different ?
>
There's been quite a bit of effort put into characterizing the behavior of
serveral popular processors of the '80's with respect to their behavior, both
in terms of their behavior when given undocumented opcodes, and in terms of
undocumented behaviors (bugs) when they were executing documented opcodes. A
few of these are well publicized, while some are not.

One example of the latter sort is the beavior of the Z80 executing an INIR or
OTIR, which is not supposed to impact the parity flag, but seemingly does.
>
Since there has been effort in this direction over the past two decades, quite
a bit is published, albeit not by the chip manufacturers.

Of course, this is not "official" data, but anything that is unofficial can be
documented, and the rest is a question of how much you trust the source. The
manufacturer didn't document these unofficially recognized behaviors for a
number of reasons, among which would be that they intended to use the logic
that provided them for something they considered more important, or perhaps
because they simply didn't want to take responsibility for those behaviors.
Received on Sat Jun 08 2002 - 09:19:55 BST