Why did Acorn use the 6502?

Question

It is an interesting quirk of the British microcomputer industry, that the main vendor of cheap microcomputers, Sinclair, used the better, more expensive CPU (Z80), whereas the main vendor of better, more expensive microcomputers, Acorn, used the cheaper CPU (6502).

I know why Sinclair used the Z80: the DRAM refresh feature could be repurposed to serve as much of the video circuitry in the ZX80, reducing total system cost.

Why did Acorn use the 6502? I can think of several candidate reasons:

1. Historical happenstance: the founding engineers became familiar with it in Cambridge. In a startup under time pressure, there is much benefit to working with technology you already know.

2. Cost: they didn't realize they were going to end up as the premium brand.

3. They didn't have time for a detailed evaluation, so just went with what most home computers seemed to be using.

4. They disagreed with me about the relative technical merits.

Or maybe a fifth reason I have not thought of?

Answer 1

Like with most things, it's almost certainly a combination of a number of different factors.

First of all, Acorn had previously built a number of other machines: the Acorn System 1, the Acorn System 2, the Atom, and the Proton. All of these used the 6502 processor.

When the BBC decided to run a series on microcomputers, and use a computer to demonstrate concepts (and such) they wrote a set of specifications, and invited vendors to submit machines for consideration. Machines were to be considered very soon after the spec was published, which suggests that they expected the submissions to be existing machines.

At least according to rumor, Sinclair had been involved to some degree in drawing up the specification, and had tailored it to fit a new machine they were designing at the time--and assure that other existing machines would not meet the specification.

That included the existing Acorn machines. But (unlike most of the other vendors) some of Acorn's people literally worked day and night for a week or so to build a prototype that would meet the specifications. At the same time, Sinclair dropped out of the competition, and didn't submit an entry at all, because they weren't sure they could produce the necessary machines if they were selected.

That left Acorn's entry as the only one that met the specification. This wasn't exactly a government acquisition, so the BBC probably could have selected some other machine if they'd been convinced that it would really be drastically superior in the long term--but apparently they didn't.

The BBC's discussion of the selection doesn't mention the processor at all, focusing more on the company's implementation of BASIC, and the hardware being expandable:

In February of 1981 it wad decided that the bid from Acorn Computers, of Cambridge, was the most attractive because their machine--of which they had been able to demonstrate a prototype--seemed to offer the best combination of harware and software. The company were also committed to the design philosophy of high expandability, and they had a particularly strong in-house research and development team.

References

https://lowendmac.com/2007/acorn-and-the-bbc-micro-from-education-to-obscurity/

https://www.bbc.com/news/technology-15969065

https://clp.bbcrewind.co.uk/media/Towards%20Computer%20Literacy.pdf

Answer 2

Why did Acorn use the 6502? I can think of several candidate reasons:

5. Market timing

When the 6502 was introduced it ran at 1 MHz, with 2 and 4 "on paper" as the B and C versions. A 6502A running at 1 MHz was about half the overall performance of a Z80A running at 4 MHz in common tasks. Yet it costs less than half as much when bought in bulk.

The 2 MHz B models were somewhat more expensive and more limited in number, but were still cheaper than the 80A and offered similar overall performance. B versions were somewhat common by 1980, and Atari used those in the 400/800. By the time of the Beeb development, B models were more widely available.

6. Video integration

As you note, "the DRAM refresh feature could be repurposed to serve as much of the video circuitry in the ZX80". Ahh, but the opposite case is not true.

If your video circuitry is separate from the CPU and requires access to main memory, then the 6502's cycle timing allows you to implement cycle stealing basically for free. This is why so many earlier 6502 machines ran at 1 MHz: the memory of the era ran at (effectively) 2 MHz, so if you used a 6502A at 1 MHz, the graphics (or other devices) could cycle steal the other 1 MHz worth of throughput by watching a single pin that already existed. In contrast, the Z80 machines generally had more complex timing systems to implement this feature.

So in the end, if you are using the original 2 MHz (150 ns) memories, which everyone used, and you have external video systems, guess what, you're machine is going to run about the same speed in the end. The memory is the throttle, not the CPU. One can greatly simplify the video, say text-only, or complicate it with its own memory, but for a low-cost offering with graphics you're going to share memory and then everyone ends up roughly the same place.

That's not the case for a S-100 machine, but that's because they had separate memory for video and the CPU. Those machines would offer higher performance because the Z80 had the RAM all to itself, but that also took you way off your price point.

The only difference for the BBC compared to, say, the Apple II, is that the BBC used faster memory, basically 4 MHz, which allowed them to cycle steal at 2 MHz. So now they had a machine that was faster than Z80 system, easier to implement, and cheaper. This is all-win.

7. They already had them

All of their previous experience was on the 6502. The 6502 is a perfectly good CPU. They already had a new machine under construction. There are many, many good reasons to stick with it.