Subj : Magic smoke update
To   : Spectre
From : Bob Worm
Date : Fri Oct 13 2023 08:31:22

  Re: Magic smoke update
  By: Spectre to Bob Worm on Thu Oct 12 2023 14:39:00

Hi, Spec!

 > You need to install your 6502 brain :)

The Acorn (A3020) uses an ARM 250 - apparently the first system on a chip. I think that's why I love it so much. Computing heritage aside, it's a 32 bit RISC running at 12 MHz so it's not bonkers level restrictive and even I can program some basic graphics stuff on it. The BASIC lets you jump in and out of assembler, too, so you can get a bearable speed out of it - I love ARM assembler. Free barrel shifts, each opcode has conditional versions and can either set or not set the flags, multi register load / store with ascending or descending stack and full or empty pointer... Phwoar :)

I'll have to get a 6502 head on when I "properly" start work on the BBC, though. That'll require a level of coding efficiency that I can only aspire to at this point!

 > Do the Acorns have "the tube" for a co-pro or alternative processor?  There
 > are some really weird things people have hitched up to it... 65C816's, Z80
 > although that was pretty standard, someone had a 386 hitched up I think too.

That is a really good question. I kind of assumed there would be, however I never spotted one on my repair adventures so I looked it up and couldn't find any reference to tube on the A3020. Thinking about it, as popular as the tube is now for hobbyists to attach way overpowered 40-years-after-the-fact processors to their Beebs, it was originally there to get around a (misguided?) part of the BBC specification which said their school computer must be able to run CP/M. Well, there you go, Mr. BBC - it can run CP/M now. Simply attach 75% of a whole other computer via this convenient tube port...

It did get used a little at the time, as well - not sure if you've heard about the BBC Domesday Project, where they took photos all over the UK and school kids wrote pieces about their local area, rammed it onto a couple of laserdiscs and then accessed it via a Beeb with a special SCSI add on so it could control the laserdisc player and overlay analogue video frames to show the pictures. Now I say all that I think the SCSI module itself attached via a different port, not the tube, but it did need a co-processor to run.

The later Acorn RISC PCs did alien processors with an expansion card, I *think* that was a dedicated co-processor slot but I'm not 100% certain. I know Martin has one :) From memory it was a 486-SX 25 but clocked at 20MHz to make the frequency division maths easier. He always enjoyed taunting me by running Windows... in a window!

Possibly you had to be there...

BobW
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