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

From: Peter C. Wallace <pcw_at_mesanet.com>
Date: Tue Sep 18 12:52:44 2001

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.

> --
> Pete Peter Turnbull
> Network Manager
> University of York

Peter Wallace
Received on Tue Sep 18 2001 - 12:52:44 BST

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