BBC Micro ARM7 co-processor available

By Chris Williams. Published: 8th Mar 2006, 00:13:15 | Permalink | Printable

BBC Micro and Master users bowled over by speed boost

The ultimate accessory for 8 bit Acorn users, an ARM7 co-processor, is now available to order. Robert Sprowson's ingenious project to produce a second processor sees a 64MHz ARM7TDMI from OKI running alongside a BBC Master's dinosaur 6502 CPU. The box of tricks uses an Altera FPGA as digital glue to bind together the second processor, 16M of RAM, 512K of Flash ROM, and optional serial port and EEPROM chip. The ARM7 chip includes a 8K cache, and the kit took over six months to develop.

Robert told fellow Beeb users this week that the co-processor runs approximately "128 times faster than the host BBC micro which is left to do I/O chores such as reading the keyboard and floppy discs".

He explained: "The coprocessor is a general purpose application processor, but it could be used for running a fast Econet fileserver; emulating other hard-to-find coprocessors such as a Z80; writing BASIC programs where memory is short such as databases; complex multilevel games; as an ARM7 learning tool or development platform; and so on."

As to whether he found the task of designing the electronics tricky, he added: "Not especially difficult, the worst point is where the 6502 is sitting looking at the ARM expecting something to happen and the ARM is sitting looking back at the 6502 expecting something to happen and they just deadlock. Fortunately there's a spare serial port on the coprocessor which can be made to spew trace information via an assembly time switch."

The system uses Acorn's Tube interface - a proprietary method of hooking a 'parasite' processor to the host 6502 chip. in a twist of fate, Acorn used the Tube in the development of the first ARM cores back in the late 1980s. The company would later design the RiscPC, which could juggle an ARM and Intel x86 compatible processor at the same time. Robert also developed a simple operating system that runs on the ARM7 processor and is stored in the Flash ROM: it provides various interfaces for applications wishing to access the kit's hardware, and it implements some of the software interfaces provided by RISC OS - namely, the OS SWIs.

He said: "When you take out the VDU drivers, as handled by the 6502; memory management, as there's no memory management unit; and module handler, as there's no module chain, the RISC OS kernel doesn't actually do that much. So implementing the remaining few SWIs wasn't too much pain with a trusty set of programmer's manuals. All those pesky OS_Convert calls were a bit dull though."

According to Robert, the custom 'ARM Tube OS' is made up of 4884 lines of ARM assembler source code, and 560 lines of Verilog to implement Acorn's custom Tube chip. There are 65 components in the basic electronic design, with a few extra connectors depending on whether the co-processor unit is fitted internally or in an external box.

Links

Co-processor project website - prices, ordering details, plus PDFs of early Acorn multi-processor documents

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Discussion

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Interestingly the website mentions the ARM7 is fast enough to emulate the other now extremely rare co-pros. You could end up with the equivelent of a whole row of biege wedges next to the Beeb (which would never work as there was only one tube interface, and each box only came with a miserly length of ribbon cable, so it had to sit right next to the machine).

druck on 8/3/06 9:34AM
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I am surprised that there are enough BBC micro still in use, let alone the subset of those who would want to invest in an ARM second processor. to justify the development effort. Good luck to Robert

Jwoody on 8/3/06 9:55AM
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Full marks to Robert, an ingenious project indeed.

Around 1Million BBC Model B's (and variants) were sold (and a smaller number of Master, Master Compacts etc.,). I could envisage a market for people who *have* an old Model B and want to delve into ARM development while still having a full keyboard, screen, storage and so on (the sort of things more expensive development boards often lack).

Hope you have every success with this Robert - well done!

AMS on 8/3/06 1:35PM
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AMS states "Around 1Million BBC Model B's (and variants) were sold" YES but how many are still in use. Intel sold a lot chips for 386 and 486 machines a lot more than 1 Million, but I bet 99.9% of them are in land fills or not used.

Jwoody on 8/3/06 9:34PM
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The availability of an attractive new co-processor might persuade some people that have BBCs but don't currently use them (I, for one, am never getting rid of mine) to get them out of the attic/garage/cupboard.

thesnark on 9/3/06 1:40AM
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Even if there are a lot still around how many of them still work? There was something decidely sick with mine when it was last dragged out of the cupboard a few years ago.

Pity that the million sold can't even be dreamed of with current machines :-(

SimonC on 9/3/06 10:15AM
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If only we had kept our BBC

SimonC: I agree, our BBC was always having trouble with the Solid State Disc board

But I amased my friends by 'fixing' it by reseating it :P

I miss our BBC

em2ac on 10/3/06 11:11AM
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It's something rather wrong with the display (at least those are the symptoms, although I can't remember exactly what it did do wrong). It appeared to work for a short while, then went funny. I tried reseating everything I could poke, without much luck, which is where I stopped, my electronic knowledge being pretty minimal. I'd like to get Elite A on it, only ever played that version on an emulator.

Hmm, there's a thought. Could an emulator be written for this board to run native Beeb stuff rather faster than it's supposed to go? :-)

SimonC on 10/3/06 11:53AM
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SimonC: "Hmm, there's a thought. Could an emulator be written for this board to run native Beeb stuff rather faster than it's supposed to go?"

I like your thinking! :-)

But you'd surely only want to emulate the processor, and then you'd still have to send I/O and screen calls over to the BBC micro itself, possibly causing serious bottlenecks.

A BBC virtualisation solution running on a second processor system? It's so devious that you just have to get working on it! Once you've got your Beeb working again, of course. ;-)

guestx on 10/3/06 4:14PM
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Now with this MMC (MultiMedia Card) filing system designed for the Beeb/Master etc as well, by J Kortink of Viewfinder fame things are looking good for the BBC Micro. [link]

Pete on 10/3/06 4:25PM
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And when you can't get a goMMC, you can try this: [link] Now in windows version too.

egel on 10/3/06 4:52PM
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