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

From: Bob Shannon <bshannon_at_tiac.net>
Date: Sun Sep 23 14:45:27 2001

Ok, I'd thought I'd given up, but I can't resist this one...

Tony Duell wrote:

<snip of mercy>

> > Well, here's _one_ textbook definition of microcode:
> >
> > "...programming that is ordinarily not program-addressable, but, unlike
> > hardwired
> > logic, is capable of being modified..."
>
> By that definition, a PERQ is not microprogrammed. PERQ microcode can
> modify itself (in fact the only way of writing to the control store in a
> PERQ is by 3 special microinstructions). So the control store is
> (micro)program addressible....

Tony, it said '...ordinarily not program-addressable', so there are exceptions.

> Thing is, we all know the conventional usage of the terms 'microcode',
> 'machine code', 'emulator' and so on. And I think we all agree that _one_
> way to describe the Pentium (or ARM) + firmware + memory interface is as
> a Pentium (or ARM) running a machine code program that is a PDP11
> emulator.
>
> That does not mean that's the _only_ way to describe that system.

There is a good reason why technical terminology has conventional meanings and
usages. What advantage is there it screwing this up and sowing confusion?

<snip>

> If we are going to have the claim that it's 'totally incorrect' to call
> the emulator 'microcode' then we need to have absolutely unambiguous
> definitions of terms like 'microcode', 'hardware', 'state machine' and so
> on. And while I know what all those terms mean, and can give definitions
> and examples, I am not sure I can give definitons that don't have some
> ambiguities in them somewhere. In fact I am not sure such definitions
> even exist.
>
> -tony

Great, lets embrace the ambiguities, and amplifiy them until no one understands
classic computer technology!
Received on Sun Sep 23 2001 - 14:45:27 BST