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

From: Mike Drew <mldrew_at_drewtech.com>
Date: Tue Sep 18 19:07:55 2001

"Peter C. Wallace" wrote:

> On Mon, 17 Sep 2001, Pete Turnbull wrote:
>
> > On Sep 17, 9:42, Jonathan Engdahl wrote:
> >
> > > Have you ever soldered down a flash ROM? Not fun. I used 36 gauge
> > self-strip
> > > magnet wire and tweezers under a 40X stereoscope. I'd rather buy one
> > > prebuilt if available. If you build you own, it might be hard getting the
> > > interfacing and timing compatible with DRAMs.
> >
> > The ones I've seen have been regular SMD (0.5" pitch) devices which are not
> > too hard to do with hot air, or are ordinary DIL (0.1" pitch). The last
> > one I soldered was a DIL chip.
> >
> > Timing might be a problem; I have almost no idea what access time DIMMs are
> > supposed to be but I expect it's faster than SIMMs. SIMMs are usually in
> > the 60ns-120ns range, and it's not too hard to get FLASH or EPROM in that
> > range too. You'd just need to demultiplex the addresses (latch the
> > addresses presented on the first part of the memory cycle). Of course,
> > modern motherboards use DIMMs.
> >
> > > Of course it would be feasible to implement a PDP-11 emulator in the BIOS
> > > ROM, it just would not be as fast as the one I'm thinking of. Back in the
> > > days of QEMM/386, I remember allocating 64K for the BIOS ROM. I haven't
> > > really looked that closely at a BIOS ROM since then. I was wondering if
> > > modern mainboards had larger ROMs, or if they still are stuck with 64K.
> >
> > I don't know for sure, but I seem to recall seeing at least 128KB. Not
> > megabytes, though!
>
> Newer motherboards have at least 128K, but if you completely replace the
> BIOS you will be stuck with the ugly,nasty,unportable mess of initializing
> the chipset.
>

Use the expansion ROM provision of the BIOS to take control.
This mechanosim is used by video cards.... The rest of the machine is up and
running before they scan the BIOS expansion ROMS...

Then all you need to do is a simple ISA card. ISA slots are becomming rare on
modern machines but not impossible to find...

Question, is the PDP-11 MMU similar to the x86 MMU?

Wouldn't the MMU functionality have to be in "microcode" to be really compatible?

Mike
Received on Tue Sep 18 2001 - 19:07:55 BST