Date   : Fri, 19 Sep 2008 10:26:28 +1000
From   : msmcdoug@... (Mark McDougall)
Subject: BBC Micro on an FPGA?

Michael Firth wrote:

> I did have a look there, but it seems that a lot of the projects are 
> designed to work on either the 'PACE-P2 ' or 'PACE-P3' boards, which 
> seem to be proprietary boards you've developed, for which there doesn't 
> seem to be much details of the hardware spec (beyond the core FPGA).

It's true that much of the development is centered around the P2 proprietary
hardware - it's the most powerful platform I have and built specifically for
PACE. However, the TerASIC DE2 is arguably superior, and everything that
runs on P2 should also run on the DE2. My colleague has a DE2 so when he
brings it into work I can test the ports on it.

> For info, there was a version of the BBC designed for the US market with 
> a 60Hz output.
> I don't know how much standard 50Hz BBC software would run on it though.

I saw scant mention of the NTSC version but figured it's not overly popular
so didn't think there was much utility in emulating it.

> If you have a desire to gain more familiarity with the BBC platform in 
> general, it might be worth spending some time with the 'BeebEm' PC 
> emulator. It emulates most of the BBC hardware, including Econet, Hard 
> Drives and Tube second processors, and is open source.
> The Windows version is available from:
> http://www.mikebuk.dsl.pipex.com/beebem/

I used the BBC emulator in MESS quite extensively during development of the
BBC - most notably when tracking down the bug in the T65 (6502) core - and
also as a reference design. Bob Devries was also kind enough to send me a
real BBC which is being well cared for. However I'm yet to run any of the
other BBC emulators out there...

> Specifically for the Tube, Robert Sprowson has developed a new second 
> processor for the BBC in the 21st century 
> (http://www.sprow.co.uk/bbc/armcopro.htm), and uses an Altera device to 
> perform the Tube processing. As such, it may be possible that the logic 
> required to glue two CPUs together could be made available to add to the 
> PACE BBC project.

It would be nice to get the Tube going, and I'm sure it's not a lot of work
in VHDL either. The ARM copro would be a problem as I'm not aware of any
open-source implementations - but a 6502/Z80/6809 would be doable right now.
Of course you could hook up a real ARM to the FPGA board as well...

> One hope that I have is that an FPGA implemented BBC would allow the 
> possibility of a similar flexibility of system configuration to that 
> available in BeebEm, but, because it is a deterministic hardware 
> solution, it should be possible for it to match the performance of a 
> real BBC system a lot better (e.g. the sound on BeebEm isn't perfect, 
> and if you run more than 2 instances on even a very modern PC they get 
> rather sluggish)

In theory, yes! But emulation is emulation - whether hardware or software -
the performance might be deterministic in an FPGA but the accuracy depends
on the implementation! It is nice that you can run "real hardware" but use
wireless wavebird controllers to play BBC games though! ;)

Regards,

-- 
|              Mark McDougall                | "Electrical Engineers do it
|  <http://members.iinet.net.au/~msmcdoug>   |   with less resistance!"