6502 vs. 6800 (was Re: kits, definitions, prices...)

From: D. Peschel <dpeschel_at_u.washington.edu>
Date: Wed Oct 7 23:27:28 1998

> I bet it would work, or at least do something of interest. The "illegal
> opcodes" of the NMOS and HMOS 6502s are quite functional -- Commodore programs
> are littered with suspicious opcodes like LAX (load .AR and .X simultaneously).
> With all that hidden functionality, why bother with the CMOS versions? ;-)

I've always wondered about the internal construction of the 6502. Sure, you
can look at a list of illegal instructions but the list doesn't tell you why
the instructions do what they do, only what they do.

I've also always wondered how the opcodes were allocated. (Perhaps to avoid
being sued for 6800 compatibility?) In some cases, there is almost but not
quite a pattern, and switching the opcodes around would make the pattern
MORE apparent instead of less. (The register-transfer instructions are a
good example.)

So, does anyone have a copy of the 6502 mask designs lying around? :)

And did Chuck Peddle ever show up at the VCF? He would be THE person to ask
(OK, maybe Steve Wozniak would know too).

-- Derek
Received on Wed Oct 07 1998 - 23:27:28 BST