ARM creators Sophie Wilson and Steve Furber
Part Two: the accidental chip
Unsung Heroes of Tech The Story so Far At Acorn, Sophie Wilson and Steve Furber have designed the BBC Micro, basing the machine on the ageing MOS 6502 processor. Their next challenge: to choose the CPU for the popular micro's successor. Now read on...
While Sinclair attempted to move upmarket with the launch of the QL in early 1984, Acorn was trying to invade Sinclair's lucrative low-end territory with a cut-down version of the BBC Micro, to be called the Electron. UK resellers assumed the Electron would be as big a success as the BBC Micro. They ordered in hundreds of thousands, but as the units stacked up in the warehouse the market collapsed.
MOS' 6502: good enough for the Electron, not the BBC Micro's successor
While Acorn was heading into cash-flow problems, Wilson and Furber had already become engaged in what would prove to be the biggest adventure of their lives. This wasn't just the next Acorn computer, although that's how it had started, two years earlier.
By 1983, the 8-bit 6502 processor had outlived its shelf life, and Wilson and Furber had been experimenting with available 16-bit processors to power their next machine. The Motorola 68000, the NatSemi 32016 and the many others they'd evaluated all had one flaw in common: not being able to make anything like full use of the memory bandwidth that was becoming available.
“If we were going to use 16- or 32-bit wide memory, we could build a system that would run up to 16, 20 or even 30Mb per second," says Wilson. But the processor was the bottleneck.
With the success of the BBC Micro, Hauser and Curry had been persuaded that an investment in chip design might be a good next move — not necessarily with the idea of Acorn getting into the chip business itself, but to help the company become more informed about this fundamental component of its business.
Hauser had even gone to the lengths of hiring three integrated-circuit designers and buying in chip design tools and workstations. And he’d dumped a bunch of papers on the desks of Wilson and Furber, relating to a novel chip design idea that originated from IBM. Reduced Instruction Set Computing (Risc) meant creating processors that used a limited set of simple instructions rather than the increasingly complex instructions that tended to slow down processors like the 32016.
Sophie Wilson talks ARM
Wilson and Furber started visiting processor manufacturers. Typically they’d find, as Wilson says of a trip to NatSemi’s plant in Israel, “a huge building full of thousands of engineers”. Wilson’s affection for the 6502 also took them, in October of 1983, to the Western Design Centre in Phoenix, Arizona, where Bill Mensch was working on a version of the chip that would support 24-bit addressing.
Next page: Breakfast in America
Almost brought a tear to my eye. A beautiful article about a beautiful piece of technology. My jaw dropped when I read the ARM worked without applying Vcc :)
IBM ROMP vs. ARM
The IBM ROMP chip (aka the 801) was never intended to be a general purpose RISC processor. It was intended to power an office automation product (think of a hardware word-processor like WANG used to sell).
As a result, although it could function as a General Purpose CPU, it was not really that suited for it. It was never a success because at the time, IBM could not see justification for entering the pre-Open Systems UNIX world. RT 6150 and the 6151 were intended as niche systems mainly for education, although they did surface as channel attached display front ends for CADAM and CATIA run on mainframes (and could actually run at least CATIA themselves). This changed completely with the RIOS RISC System/6000 architecture, where IBM was determined to have a creditable product, and invested heavily.
In comparison, the ARM was designed from the ground up as a general purpose CPU. Roger Wilson (as he was then) greatly admired the simplicity and orthogonality of the 6502 instruction set (it is rather elegant IMHO), and designed the instruction set for the ARM in a similar manner. Because the instruction set was orthogonal (like the 6502, the PDP11, and the NS320XX family), it makes the instruction decoding almost trivial. It also made modelling the ARM on an econet of BBC micro's (in BBC Basic, no less) much easier, which allowed them to debug the instruction set before committing anything to silicon.
They had to make some concessions on what they wanted. There was no multiply-add instruction, which appeared to be a hot item in RISC design at the time, and to keep it simple and within the transistor budget, all they could do was a shift-add, (the barrel shifter), which although useful, was a barrier to ultimate performance, but great for multi-byte graphics operations.
It was also simple enough so that they could design the interface and the support chips (MEMC, VIDC and IOC) themselves, achieving early machines with low chip counts.
This is all from memory of articles in Acorn User, PC World, Byte and other publications. Feel free to correct me if my recollections are wrong.
This is why I come to the Reg, so much nicer to read a well informed structured article as opposed to the usual "my dads bigger than your dad" fanboy rantings on other sites
I would just like to congratulate the author of this article. Beautifully written. A real credit to yourself, and The Register, sir. Thank you very much.
It struck me that Hauser, despite not being the technical guy, is as bright as they come. He clearly recognised the talent he had with Wilson and Furber.
I completely riviting read. Actually, the story of Acorn & ARM would make a very good book. Ditto Inmos IMHO.
Have a good day all.
Re: Great article
I agree, it wasn't Acorn and the ARM which failed.
But there is something about the British industrial and financial environment which seems to let the winnings from these works of genius drift away out of reach.
Its not just globalisation, and some factory that is so expensive to build that there can only be one on the entire planet. And we can't expect to spot the right investment choice every time. But what is it about this country which turns a successful entrepreneur into somebody fronting a TV show that tests how people can run a market stall in Essex?