Date   : Tue, 05 Sep 2000 19:42:53 -0700 (PDT)
From   : Angus Duggan <angus@...>
Subject: Re: FRAK!

Thomas Harte writes:
>So, does the Rockwell not do the unofficial opcodes? The only documentation
>I have seen claims that the only way not to get the unofficial opcodes is to
>switch NMOS to CMOS (or the other way, I forget), which also changes which
>instructions do care about low byte to high byte carry, and therefore is not
>what Acorn have done.
>
>So, what exactly is the deal with the Rockwell & the unofficial ops?

The Rockwell chip had a different set of undocumented opcodes. These
"unofficial/undocumented" opcodes are simply by-products of the way the
instruction decode works, which is why they are different for different
implementations of the architecture (they reflect different design decisions
by the chip designers). I believe that Rockwell produced the CMOS version of
the chip and Mostek (?) the NMOS version.

Using the undocumented opcodes is at best a hack. It only worked because (as
that BBC documentary on disappeared machine a few days pointed out) the BBC
was made by one manufacturer, and sourced chips from the same supplier. A
different implementation of the architecture could very well have used
different chips (the 6502 Tube uses the 65C02, and I believe there was a late
prototype version using one of the 16-bit 6502 developments).

a.