Re: 6502 illegal opcodes questions

["Michael J. Mahon" <mjmahon@aol.com>]

Jim Brain wrote:
> Michael J. Mahon wrote:

> > I don't fault them for what they did, either--I was just pointing out
> > that we don't do things that way anymore, and for good reason: we
> > found out by (sad) experience that programmers cannot be trusted
> > to "keep off the undocumented grass".  ;-)
>
> I wasn't directing that at you per se.  I just saw a lot of posts on 
> this thread along the lines of "If I had designed the CPU..."  Bah, I 
> say.  Those guys taped out a CPU when you still had to use tape.  Kudos 
> to them.

Actually, they didn't even use mag tape--they used rubylith tape to make
the optical masters for the masks.

The actual layout was done on the floor of a large room using little
scooting "skateboards" like mechanics use to go under cars, and using
Xacto knives!

Ah, those were the days of "wooden chips and iron men".  ;-)

> > And I also agree that for any platform with a single, unchanging
> > implementation, as most retro platforms are today, the use of
> > stable undocumented behavior is not a practical problem.
>
> It does cause issues, as emulator authors have to jump through hoops to 
> support it, and the DTV designer left illops out of the original version 
> of the unit, due to complexity and time issues.

Yes, that's true.  Emulators *are* the "new implementation" for these
old platforms, and they incur the full penalty for any exploitation
of undocumented behavior--both in determing what it is (since it's
undocumented ;-), and then in replicating it.

-michael

Music synthesis for 8-bit Apple II's!
Home page:  http://members.aol.com/MJMahon/

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