Date   : Thu, 22 Apr 1993 10:24:01 +1200
From   : David Andrew Sainty <David.Sainty@...>
Subject: SuperBEEB

Hardware hackers out there... I have a plan!

I want to beef up my BBC a bit. This depends on finding a FAST 65C02. I believe
they run up to about the 10MHz range, so this is what I'm really looking for.

Assuming I find a source, here's some ideas:

Either:

pull the old 65c12 off the motherboard, plug in the new on a board with fast
ram, and clock the new processor down to 2MHz to access the original hardware.

This gives a carbon copy of the BBC, but it does things.... faster!

Of course, it'll cost a BBC if anything goes wrong.

Second idea:

External processor, fast 65c02 hooked to a memory controller (the MEMC so loved
by archimedes users may do this job, it'd probably be the cheapest) and about a
meg of memory (cheap in SIMM cards these days).

This one is great! Can emulate multiple complete BBC's! Could even run it as a
multi-user workstation!

I think a third processor will be required to deal with the MEMC page faults,
the problem being that the 6502 when interrupted completes the current
instruction, but what we really want to do is halt the instruction, deal with
the page fault (a page fault here may mean an access to &FE00, so we need to
emulate the hardware either by reading/writing to hardware on the BBC
motherboard, or making note that that BBC image has altered that hardware
register) and restart the instruction. alternatives are to use the old BBC
processor for this job (which means it'll have less time to pretend being one
of the BBC images, screen updates etc), or perhaps step back an instruction in
software with hardware support (Say, keep a hardware copy of the last
instruction address, on page fault read the instruction that caused it and
reverse it if needs be. Most instructions won't need reversing, but
instructions like ROR will).

Actually, that last method will probably be the best, but that's not a big job,
so that's ok.

BBC<-->SuperBEEB communication I'm not sure about. In actual fact, the best
option would seem to be to cut out the old 6502 and interface our baby directly
to the bus, but assuming you want to keep the original functional by itself
through all this, I guess a small memory transfer program is what's wanted, so
SuperBEEB can say 'write here' or 'read here' type thing. And the original BBC
should probably stick to doing the file handling by itself.

This is all pretty ambitious of course! But the end result would be a pretty
serious machine! Are people interested in following up these (sketchy) ideas?
Anybody got ideas of their own on this? Anybody ever HEARD of anyone
interfacing a true memory manager to a 6502 before?

Dave.