Re: 6502 illegal opcodes questions

[Eric Smith <eric@brouhaha.com>]

Michael J. Mahon wrote:
> Not so.  The logical elements of the NMOS design are not present in
> the same form in CMOS.  And the instruction decoding was clearly
> redesigned, as was the control section (changed timings and bus cycle
> patterns, new instructions).

Jim Brain <brain@jbrain.com> writes:
> I guess we'll agree to disagree on the point.  I say all the design
> elements are there (the 8 bit ALU, decimal mode, the 16 bit
> instruction pointer, etc.)  As for the instruction decoding, Bill just
> cleaned up the don't cares, as I see it.

You're talking about two completely different design levels.  Michael
is talking about transistors, gates, PLAs, and flip-flops.  You (Jim)
are talking about the programmer-visible processor architecture.

It's possible to produce two processors with vastly different
implementations that have the same architecture, e.g., IBM System 360/30
versus System 360/67, which shared almost no commonality of hardware
design, but could run the same software.

It's also possible to produce two processors with nearly the same
hardware implementation, but dramatically different architecture.

The 65C02 architecture is almost a superset of the 6502.  The implementation
is *much* different.

> Hardly a huge new design.

Tell us that after you design a 6502 replacement in a new technology.

> The physical elements changed, yes, but the logical perspective stayed
> the same. When I interviewed him back in 1995, he said as much.

The "physical elements changed" is basically stating that the entire
chip was redesigned.

We could build a very slow 6502-compatible processor out of a lot of
tinkertoys, and still have the same "logical perspective".  I don't think
you'd try to claim that it wasn't a huge new design.