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ARM creators Sophie Wilson and Steve Furber

Part Two: the accidental chip

Internet Security Threat Report 2014

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

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: Source: Wikimedia

Sophie Wilson talks ARM
Source: Wikimedia

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.

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