IBM 5100

From: Antonio Carlini <>
Date: Wed Aug 18 15:24:23 2004

> To change the machine, I understand that my VAX11/730 loads the CPU
> microcode from the TU58 tape at power-on (certainly there's
> what looks to
> be the WCS RAMs on one of the CPU boards, and I can't find
> any microcode
> PROMs). Does anyone know how 'general purpose' the 11/730 is
> -- can it be
> changed into some totally different processor by loading the
> right microcode?


> Now, which class does the 11/730 fall into? Is it possible
> to, say, write
> microcde for the 11/730 to turn it into a PDP8, or a Nova, or
> something?

I've never written any VAX-11/730 microcode so this is speculation,
but there is an article in one of the DEC Technical Journals that
describes the MicroVAX II development. Since they were subsetting
the architecture, they wanted to try out various options. Since
the 730 is completely soft-loaded, they recoded the microcode
to look like the proposed DC78032 (uVAX-II chip) and tried out
a few variations.

So no guarantees, but from the description, I would expect the
VAX-11/730 to be able to run as a PDP-11 given sufficient
microcode investment. Subject to the usual constraints wrt
actual microcode space available. It certainly seems to
be capable of replacing existing instructions with stuff
that does something entirely different.

The VAX-11/780 has WCS (as I'm sure you know) but that only
allows you to extend the instruction set. The VAX-11/750 has
PCS, which basically allows you to patch the existing
microcode to some extent, but this is pretty limited and
only intended to fix bugs, not create a new machine.

During the initial Alpha development, they recoded the microcode
of one of the Polarstar machines (VAX 88x0 class) to behave
(in some respects) as an EV4 Alpha box - took days
to boot VMS IIRC.

I know the NVAX chip has some microcode patch ability but
it is completely undocumented (other than its existence)
even in the chip spec and my understanding is that it is
pretty limited in what it can do.

I've never heard of any microcode capability in any
of the VAX 6000/VAX 7000 range - but who knows what's
hidden in there!

I guess everyone knows that Richie Lary was alleged to
have cobbled together some PDP-8 microcode for the
PDP-11/60, thus producing the world's fastest PDP-8!

Received on Wed Aug 18 2004 - 15:24:23 BST

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