microcoding a PC into a PDP-11 (was: RE: Classic Computers vs. Classic Computing)

From: Bob Shannon <bshannon_at_tiac.net>
Date: Thu Sep 20 22:38:04 2001

"Michael L. Drew" wrote:

> Bob Shannon wrote:
>
> > Does anyone on this thread even understand microprogramming?
> >
> > Apparently not.
> >
>
> Well, I understand microprogramming. I though we were speaking figuratively.
> The "microprogramming" we were talking about was treating the Pentium as
> a very complicated micro engine.... Which IMHO it really is.
>
> Using your definition every machine that has writeable microcode is "emulating"
> something
> else...

Not at all. If a machine has a writable control store, it simply has an extensible
instruction set. Its only emulation if you use that microcode to implement another
processor, like the worlds fasted PDP-8, coded on an 11/60.

> I recall that MIT built a native LISP machine using 2901 bit slice processors.
> Wasn't
> this the basis of the Symbolics machines???

Almost.

MIT developed the CADR, I have one in my garage, and I serviced them at the MIT AI
lab. The CADR uses a TTL cpu based on the 74S181 ALU chips. The MIT CADR became the
LM2, and the Lisp Machines Inc CADR's.

The LM2 begat the whole line of Symbolics machines like the 3600's, etc.

> Does anyone recall the name of the 2901 based machine that could be transformed
> into 8080,Z80,6800,68000, etc. I believe that it came out in the 1980's.
>

I have a dim memory of that machine, but can't recall its name. Wasn't it a Digital
Research machine?
Received on Thu Sep 20 2001 - 22:38:04 BST