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

From: jkunz_at_unixag-kl.fh-kl.de <(jkunz_at_unixag-kl.fh-kl.de)>
Date: Tue Sep 18 15:02:18 2001

On 18 Sep, Chuck McManis wrote:

>> Didn't some of the VAXen boot their microcode from a floppy?
> Yes, the VAX 11/780 does.
But only a part of it. I know the person that desined the data path unit
of the VAX11/780 CPU. He told me that most (3/4?) of the microcode is in
ROM, with only one RAM bit per micro code word. That is the "patch bit".
If this bit is set, the sequencer jumes to the RAM microcode. With this
technic you can get patchable / loadable microcode without the need of
a 100% (fast and therefore expensive) RAM microcode memory.

An other option would be a Symbolics. (The old lisp machines.) Last
week a friend and I brought 5 Symbolics 3640 / 3670 back to live. It
seams to me that the CPUs of this odd machines are huge bit slice
processors. I think it would be possible to turn such a machine into
nearly everything with the right microcode. Unfortunately a Symbolics
machine is much more rare then a PDP11.
Homepage: http://www.unixag-kl.fh-kl.de/~jkunz/
Received on Tue Sep 18 2001 - 15:02:18 BST

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