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

From: Pete Turnbull <pete_at_dunnington.u-net.com>
Date: Mon Sep 17 12:46:05 2001

On Sep 17, 9:42, Jonathan Engdahl wrote:

> Have you ever soldered down a flash ROM? Not fun. I used 36 gauge
> 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!

Pete						Peter Turnbull
						Network Manager
						University of York
Received on Mon Sep 17 2001 - 12:46:05 BST

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