DG MVs (was RE: microcoding a PC into a PDP-11 (was: RE: Classic Computers

From: Derek Peschel <dpeschel_at_eskimo.com>
Date: Tue Sep 25 03:34:39 2001

On Mon, Sep 24, 2001 at 09:28:19PM -0700, Chris Kennedy wrote:
> Derek Peschel wrote:
>
> > But where is the microcode for the brain-damaged Nova 800 instruction set
> > stored, and can it be overwritten?
>
> PROMs, and no, I know of no way to overwrite _that_ code. Instead you
> execute a load microstore operation, which moves data from memory to
> microstore and resets the machine such that it begins execution from
> soft microstore rather than the proms.

... And once you've loaded your own microcode, do the PROMs take up any
room in the microaddress space? And does the microcode handle the load
operation, or is there some sort of dedicated circuit?

I assume the load operation isn't supposed to happen while an ordinary
program is running (though that _would_ be an opportunity for spectacularly
efficient and inscrutable code).

It's scary how similar this is to a popular booting technique (switch on
boot ROM, use it to load RAM, switch off boot ROM) except, of course, that
you're one level lower.

-- Derek
Received on Tue Sep 25 2001 - 03:34:39 BST