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

From: Bob Shannon <bshannon_at_tiac.net>
Date: Tue Sep 18 21:38:26 2001

Does anyone on this thread even understand microprogramming?

Apparently not.

Pentium CPU's can't be microprogrammed, unless your Intel. Even microcode
updates cannot replace the basic instruction set.

If you write a program in Pentium assembly code to run PDP-11 instructions, you
have just written an 'emulator', even if it does not run under windows.

But an writing such an emulator is not 'microprogramming'.

Jonathan Engdahl wrote:

> The idea is to write PDP-11 microcode for the PC platform, rather running an
> "emulator" under Windows or whatever. The Pentium would be viewed as the
> micro-architecture, the PDP-11 as the real machine. It would be table driven
> and fully expanded, using the PC memory rather extravagantly. You should be
> able to emulate simple instructions at the rate of about 4~8 Pentium opcodes
> for every PDP-11 opcode. If you rely on the Pentium MMU to trap accesses to
> the I/O page, you don't have to check for non-memory accesses from within
> the CPU model. The trap routines would emulate PDP-11 I/O, mapping it onto
> the PC hardware, rather than onto file I/O as in an emulator. The Pentium
> MMU can also be used to emulate the PDP-11 MMU. Map the PDP-11 registers
> onto Pentium registers, and never save them in memory except on a trap. This
> gives you a very, very fast PDP-11, IBM 1130, or whatever. If you can figure
> out a way to cause the machine to boot this "microcode" at powerup instead
> of Microsoft Wincrash, I argue that you could legitimately call this a
> PDP-11.
>
> I think I remember hearing that the IBM 360 VM OS did this.
>
> It seems a clean way of preserving classic architecture without having to
> mess with decayed disk drives, and without the compromises imposed by
> emulation.
>
> Sort of like rebuilding the Parthenon with injection-molded faux-marble
> columns and friezes. ;-)
>
> --
> Jonathan Engdahl Rockwell Automation
> Principal Research Engineer 24800 Tungsten Road
> Advanced Technology Euclid, OH 44117, USA
> Euclid Labs engdahl_at_cle.ab.com 216-266-6409
>
> > -----Original Message-----
> > From: owner-classiccmp_at_classiccmp.org
> > [mailto:owner-classiccmp_at_classiccmp.org]On Behalf Of emanuel stiebler
> > Sent: Tuesday, September 18, 2001 1:01 PM
> > To: classiccmp
> > Subject: Re: microcoding a PC into a PDP-11 (was: RE: Classic Computers
> > vs. Classic Computing)
> >
> >
> > Jonathan Engdahl wrote:
> > >
> > > > -----Original Message-----
> > > > [mailto:owner-classiccmp_at_classiccmp.org]On Behalf Of emanuel stiebler
> > > > Jonathan Engdahl wrote:
> > > >
> > > > sizes. Prices are
> > > > > about $100 per meg. Something called "flash DIMM" shows up on
> > > > >
> > > > Looking at this prices, what happened to the old 3.5" floppy drive ?
> > > > If you can start a complete/compressed linux/firewall from
> > there, should
> > > > be enough to start an emulator.
> > > > And, BTW, not all of the pc motherboard chip sets supports FLASH DIMM
> > > > (any ?),
> > > > so you're stuck then with some motherboards.
> > >
> > > Very good point. I was thinking that it would be nice to have a
> > machine that
> > > was a PDP-11 as soon as you flipped on the power, but probably
> > not worth the
> > > cost and hassle of the flash.
> >
> > And, what I forgot to write is that the flash is slower anyway, so you
> > copy the
> > software from flash to *RAM anyway.
> >
> > > Didn't some of the VAXen boot their microcode
> > > from a floppy?
> >
> > yes
> >
> > > Also, that way you could have one microcode floppy for each classic
> > > architecture.
> >
> > works only if you have the OS on the floppy too.
> >
> > > I think that my emulator idea can be made to work under Win32.
> >
> > Sorry, I missed that. What is so special about your idea ?
> > (No offense, just missed you posting I guess ;-))
> >
> > > It appears there are facilities allowing an application
> > > program to catch access violations.
> >
> > Sure. But it is easier to check this yourself.
> >
> > cheers
> >
Received on Tue Sep 18 2001 - 21:38:26 BST