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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?

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    I would guess largely 4. In many ways, the 6502 is a better processor. For one obvious example, to get even close to the same performance, a Z80 needs roughly double the clock speed of a 6502. Commented Jun 1 at 1:58
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    "...the better, more expensive, Z80..."; an objectively incorrect summation of the relative merits of the two chips. Commented Jun 1 at 5:58
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    There’s no objectively better in CPUs, really, but: @JerryCoffin cycles are an arbitrary unit of time; the Z80’s approach just gives it greater precision in where in places cycles. You might as well argue that seconds are worse than minutes because you need a much higher second rate to fill the same amount of time.
    – Tommy
    Commented Jun 1 at 11:40
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    @JerryCoffin, "to get even close to the same performance, a Z80 needs roughly double the clock speed of a 6502" But what's the advantage of being able to use a slow crystal for your choice of MCU? Buy the crystal your CPU needs and connect it, job done. There's no crystal price difference or anything else. I've never understood that argument in those Z80 vs 6502 debates, always gets brought up, though.
    – TonyM
    Commented Jun 1 at 16:10
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    @JerryCoffin, I've been designing electronics for many many decades, including Z80/Z180 and 6502 boards and writing their assembler. I can promise you it's nothing to worry about, nothing, especially at these low frequencies. It wasn't back then, either. I Verowired much faster proto boards with zero problems. I do see people online that have a strange allegiance to one or other MPU, and I do see those people using arguments like the clock thing. I'm kinda free from all that guff, they're just MPUs to me. It's liberating :-) And just leaves me technical conclusions, like: it's irrelevant.
    – TonyM
    Commented Jun 1 at 19:07

2 Answers 2

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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

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    And for the final piece of the puzzle, it looks like the System 1 was partly derived from an automated cow feeder Wilson designed while an undergraduate. speleotrove.com/acorn/acornWilson.html Of course, the choice of 6502 for that was perfectly logical; such embedded systems were literally what the 6502 was made for. So it was a chain of events where every decision that was made along the way was entirely reasonable.
    – rwallace
    Commented Jun 2 at 2:35
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    My understanding is that at the time the specification was to include a Z80 and as a result the Acorn offering nearly didn't happen at all Commented Jun 2 at 14:00
  • @ChrisDavies My understanding is that this was indeed so, and the Tube interface was basically a compromise 'look, the handful of users who actually care about CP/M can install a Z80 coprocessor'.
    – rwallace
    Commented Jun 2 at 23:19
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    @ChrisDavies: I've seen rumors to that effect as well--but the official statement from the BBC was that the Acorn design met all the requirements they set, which all but directly states that couldn't have been so. Commented Jun 3 at 5:18
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    I think this answer confusing the Atom with the Proton. The Atom was the production machine that preceeded the BBC micro. The proton was the prototype machine that evolved into the BBC micro. Commented Jun 3 at 14:55
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Why did Acorn use the 6502? I can think of several candidate reasons:

  1. 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.

  1. 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.

  1. 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.

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    the 2 MHz 6502 required more expensive fast RAM, though, which is why the BBC B had only 32 K
    – scruss
    Commented Jun 3 at 22:24
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    @scruss - of course, the BBC was designed for a mid-range price point where the slight increase of the cost in memory was a useful tradeoff versus (1) being one of the fastest machines in the ~$500 price range and (2) having enough bandwidth to implement high resolution graphics (320x256x4 colours was a massive win over what most of the competition was doing in 1980)
    – occipita
    Commented Jun 10 at 19:04
  • Yet interleaving DRAM cycles between CPU and video requires lots of circuitry, as well as doing refresh. Compare that to ZX Spectrum where the refresh was made exclusively by Z80 (when outside of video data fetch time), and the extra 32k of memory was exclusively controlled by Z80 and its intrinsic refresh capability.
    – lvd
    Commented Jun 12 at 9:12
  • Lots of circuity on a Z80, but not a 6502. You have a pin that told you when the bus was free and it was free every other cycle no matter what instruction was running. That was a big deal - Atari didn't use this method and they needed a custom version of the 6502 with latches to achieve Z80-style access. Commented Jun 12 at 17:58
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    @MauryMarkowitz Technically, the bus was free for half of every cycle.
    – JeremyP
    Commented Jun 13 at 17:07

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