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Why did manufacturers of home computers avoid using the 6809 CPU? I realize that the Z80 and 6502 had a 3- or 4-year head start in availability. But once it did become available in 1978, I don't understand why designers of new computers didn't choose it. I can only think of the TRS-80 Color Computer (home market) and the Commodore SuperPet (educational market) as mass-produced computers which used it.

It doesn't surprise me that it was not used in any business computers. New = unnecessary risk to many of those potential customers. But most home users didn't care much about having a well-known operating system and large preexisting library of professional applications.

The 6809 had inherent advantages. From the Wikipedia:

The 6809 was, by design, the first microprocessor for which it was possible to write fully position-independent code and fully reentrant code in a simple and straightforward way, without using difficult programming tricks. It was also one of the first microprocessors to implement a hardware multiplication instruction, and it features full 16-bit arithmetic and an especially fast interrupt system.

It seems strange to me that the engineers of manufacturers would not have been attracted to these advantages, and striven to build machines utilizing it.

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    The Dragon 32/64 also; almost exactly like the CoCo but I think because both are based on the support chips provided directly by Motorola (and, especially, the 6847) rather than because the one is based on the other. – Tommy Feb 18 '18 at 23:02
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    "Couldn't run CP/M" was not helping. – Thorbjørn Ravn Andersen Feb 19 '18 at 0:57
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    It was used a lot. By french company Thomson, at least. They sold a lot of computers (MO5, TO7) all using the 6809. Thet were cheap, and found a way in a lot of homes and schools. – dim Feb 19 '18 at 5:17
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    The following is not exactly an answer to your question, but might be of interest. There was a real-time multi-tasking operating system "OS-9" that started with a 6809 version (what gave it its name) and became quite successfull later when ported to the 68000 family. Personally, I only used OS-9/68000, so I have to guess a little what the predecessor looked like. OS-9 as a real-time OS needed low interrupt response times, so the 6809 probably was a good match. The OS made use of position-independent, reentrant code, by having the software organized in "modules" containing code and read-only da – Ralf Kleberhoff Feb 19 '18 at 9:50
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    In comparing to the Z80, you have to consider that most Z80 based systems were treated by at least application software as being 8080's - the hardware design became simpler, but software efforts could continue an already proven and supported heritage. – Chris Stratton Feb 19 '18 at 18:17
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Why did manufacturers of home computers avoid using the 6809 CPU?

I can't really see that anyone 'avoided' it. There have been many successful machines using the 6809. Beside the mentioned Tandy's CoCo there where other computers for the general audience, like

(Not exhaustive, there might be many more, as it's just from memory)

They were quite successful in Japan throughout all of the 1980s, and somewhat strange, the FM7 also in Portugal. The Fujitsu machines even featured two 6809, the second one operating as independent graphics subsystem.

Then there was Thomson as a major French player with the

  • Thomson TO7/8/9 series (starting 1982), and
  • Thomson MO5/6 series, a somewhat low cost version of the TO7(*1).

These machines had good sales and a strong following (still today) in French speaking countries. In other places they were rather rate (*2). The MO6 was also OEM'ed by Olivetti as Prodest PC128.

(For all these machines it might be more useful to read the corresponding French/Italian/Japanese Wiki pages than the English ones :))

Another successful machine was the British Dragon 32/64 series of 1982. They are often attribute as Tandy clones, but that's rather due the fact that both use Motorolas SAM chipset. Compared to the CoCo, they offer a better Keyboard and an on board parallel interface.

And lets not forget the Vectrex (1982), and the fact that the MacIntosh prototype, developed around the same time, was also 6809 based.

I realize that the Z80 and 6502 had a 3- or 4-year head start in availability. But once it did become available in 1978, I don't understand why designers of new computers didn't choose it.

For one, above examples do show that it has been used, but it takes some time to decide, design and market a new machine. So while the raw CPU may have been available in late 1978, above examples show that it took roughly 3 years for computers to show up using this new CPU. Comparable to the Atari Series, only showing up 4 years after the 6502 CPU was available.

It doesn't surprise me that it was not used in any business computers. New = unnecessary risk to many of those potential customers. But most home users didn't care much about having a well-known operating system and large preexisting library of professional applications.

It might be less simple here. Professional users don't care about the machine or its CPU. They care about certain application. If a manufacturer supports its applications after switching the CPU, they gladly buy the new, incompatible one.

Now, with third party software it becomes more complicated. If a manufacturer can convince them about the new system and its future sales, they will support it and users will follow. Otherwise it's playing safe and make the new machine compatible. Back in the 1970s and early 1980s, professional software was rather closely tied to computer manufacturers, so switching CPUs wasn't uncommon. Their decisions were hardware driven and supported by good profit margins, allowing them to spend large amounts on software ports.

On the fast moving home computer market, margins were rather small, and changing a machine design that would result in a complete rewrite was unaffordable. That's why Commodore stayed that long with the 6502. It was less expensive to patch some parts of the Kernel for a new video controller while keeping the same old CPU.

Pagetable has just released a nice work showing how Commodore kept reusing code in the Kernal.

The 6809 had inherent advantages. From the Wikipedia [...]

I guess that and for sure the usage of full 64 KiB where the main reasons for the University of Waterloo Computer Systems Group's development of a 6809 daughterboard for the PET - what later became known as SuperPET after Commodore bought the design in 1981 (*3).

In fact, the SuperPET has been one of many 6809 add on cards for existing machines. Like The Mill for the Apple II or the 6809 Tube Modul for the BBC.

It seems strange to me that the engineers of manufacturers would have been attracted to these advantages, and would have striven to build machines utilizing it.

At the time the 6809 became available the game was no longer played by some lone engineer starting a new computer, but bigger companies and driven by much more than just curiosity for a new chip. Still, a wide usage of 6809 systems as CPU in other systems, from knitting machines to street lights and telephone systems does show that engineers did appreciate the additional abilities.

Also, and maybe even more important, 16-bit CPUs (8086, 68k, 32k) became available about the same time as the 6809. And the Mac is a great example, that switching over to 16-bit did bring even more advantages, especially in terms of memory, than just using a more advanced 8-bit unit. Kind of a 'too little, too late' case.

Conclusion: I don't think the 6809 was avoided. There have been many successful systems. But it was already too late to successfully compete with upcoming 16-bit systems.


*1 - Thomson somewhat screwed their own success by making the MO5 not fully compatible. While the Hardware is quite similar, they mixed up the memory map, thus making exchangeable programs less common.

*2 - Keep in mind, most machines had their home markets and respective companies were rather niche players in other parts of the world. For example Tandy was a big number in the US, but never really a thing in continental Europe. Much like Thomson machines were big in France, Belgium and Italy, but exotic in other parts of Europe. Interestingly they were somewhat successful in Britain. Similarly, Japan had a complete separate ecosystem.

*3 - Reading the SuperPET history reveals that the original 6809 choice even came from IBM(!) as part of the MICROWAT program they developed for the University of Waterloo CSG.

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    Thank you for this well-researched and point-by-point response. With my North American perspective, I was only aware of your list's Dragon computer. I guess the Vectrix was sold in the USA, but I don't remember it, not having much interest in game consoles. I'm glad the rest of the world was more open to a broader spectrum of home/office machines. – RichF Feb 19 '18 at 15:25
  • I wouldn't call it more open. Just different markets with comanies beliving to have a chance. In case of Thomson, try to imagine if GE had come up with a home computer in 1982 to match some government recomendation for a nation wide school system. That might give a comparable picture. – Raffzahn Feb 19 '18 at 18:44
  • Vectrex. I had one of those. :) – Almo Feb 19 '18 at 20:38
  • Nice answer! Shame the question's closed, but The Mill card would've been a great answer for “What early home computers have more than one CPU, where both could be used by the programmer?”. It looks like it was a fully independent 6809 that could interact with the 6502 in the Apple //. – scruss Feb 20 '18 at 2:03
  • @scruss The 6502 wasn't independant, as they could just work interleaved. The same mechanics are true for the Z80. Then again, there have been real independant cards for the Apple II where the additional processors had seperate Memory and could realy work in parallel. – Raffzahn Feb 20 '18 at 6:17
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While I don't know the answer to this, I'll hazard a few guesses:

  • It was quite an expensive CPU. For example, in 1983 retail price in the UK was £6.50 for the 6809 or £12 for the 68B09, versus £3.20 for the Z80A or £5 for a 6502A.

  • Its performance didn't exactly set the world on fire. With most instructions taking 3-7 cycles at 1MHz, the base 6809 would be somewhat slower than either of those alternative processors, except for applications that used a lot of 16-bit operations. The 68B09 at 2MHz may have been a little faster, but probably not enough faster to be worth the cost, especially given that an 8088 was only another 50% more expensive and gave a much larger leap in performance.

  • The advantages cited in the Wikipedia article weren't generally seen as particularly important for PC/home applications at the time. It was only when more demanding applications started to be used on micros that they became relevant, and by then the 8088 had already achieved a dominant position.

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    Also the Motorola 6809 was not a drop-in replacement for the MOS 6502 or even the 6800. Later, the 68000 line experienced similar backwards compatibility issues that Intel's x86 line never had, and I think this contributed to Motorola's demise. – snips-n-snails Feb 19 '18 at 6:28
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    I just found the 6809 difficult to program. It had these combination commands that required an extra level of thinking, very much like Data General Assembler where you had to do a couple of commands in parallel. – cup Feb 19 '18 at 8:45
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    well, for electrical engineering students just learning microcomputer hardware and software and DSP, the 6809 was sexy. it was the first microporicessor CPU with a multiply instruction that i am aware of. and the orthogonal addressing modes with post-increment and pre-decrement were also quite cool. – robert bristow-johnson Feb 20 '18 at 4:44
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    @traal The 68000 was wildly successful. It didn't contribute to Motorola's demise in any way. – JeremyP Feb 20 '18 at 9:29
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    @Blrfl: I think that's over-simplifying quite a bit. Motorola made some pretty major changes in how the MMU worked in the 68040 vs. 68030. Then they changed things again in the 68060. Yes, it was possible to detect the processor and use code that worked on each. It's still a lot different from Intels where code written for a 386 in 1987 will still work just fine on a 2018 Skylake X. – Jerry Coffin Feb 21 '18 at 17:25
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While the 6809 had (and still has) technical applications, it lost out badly in the consumer market in the “mug's eyeful” department: if you were buying your kid a computer for Christmas and didn't know any different, why buy the Dragon which only had 0.89 megahertz when the ZX Spectrum came with 3½ of them?

While folks here know that processor speed isn't remotely comparable, to people buying and selling computers without a technical background, it mattered. That's why Alan Sugar was so fond of his mug's eyeful, the extra bits that make it look to the buyer that they're getting a lot for their money.

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One of the missed opportunities, in the United Kingdom, was the BBC Micro.

In 1979 Acorn was selling 6502-based eurocard kit computers, and quickly brought out a successor based on the 6809 -- often omitted from descriptions of Acorn, but you can see one here http://chrisacorns.computinghistory.org.uk/8bit_Upgrades/Acorn_6809_CPU.html

It was vastly better board: CAD designed while the previous 6502 was hand-drawn, and the 6809 obviously gave access to professional operating systems Flex and OS/9.

The specification for the BBC Micro was being drawn up, and my understanding was that the discussions were between Best (68000 or whatever) and Current (Z80/6502), with not much credence for Good (6809). As the 68000 products would take too long, Acorn won with developments on its existing 6502 experience. I have heard it rumoured that the design was done right up to the deadline and the 6809 team at Acorn lost out for very ordinary internal reasons.

If the BBC had chosen a 6809-based design, I think it would have had enormous impact on the wider deployment of this CPU.

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This is a fascinating subject, IMHO, and a fun one to find searching Google for a Z80/6502/6809 showdown. I think one of the areas that people in here haven't addressed - aside from cost - is the ability to license the 6809 core vs, say, the 6502.

Let's take for example, Atari. Both Atari Inc and later Atari Corp. Atari Inc. did end up using the 6809 somewhat in a few of their arcade games, but their consoles and 8-bit computers remained using the 6502. Atari didn't own their own fab like Commodore did - MOS/CSG - but they worked closely with Synertek, Rockwell, and others. They designed their own 6502 variant, the 6502C SALLY, and had those companies manufacture it for them. Could they have done the same with a 6809? Doubtful. And they would've had to have purchased all of the 6809s from Motorola and possibly later from Hitachi. It's actually a shame Atari did take to manufacturing even further 6502 enhancements they came up with in-house like adding 16-bit instructions to it back in 1979.

Flash forward all the way to 1988/89 when the successor Atari Corp was acquiring what became the Atari Lynx from Epyx. Despite its powerful 16-bit graphics chip, it still had a 6502 as its main CPU. Why? Because the 6809 and 68000 weren't available to license for customized cores, according to the development team. For whatever reason, they also didn't go with the 65816 from WDC either. But that's a different story...

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MC6809 did have a lot of instructions however (to be compatible with 6800 most one byte instruction codes were unavailable) to access them required multiple bytes (multiple instruction fetches) and the internal microcode (Motorola didn't improve the internal microarchitecture of the CPU) took many more CPU cycles to execute those instructions. Basically for the same clock speed other processors executed the same program function faster and with less code (memory was costly).

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    That fits with something I remember. Well IIRC that is. What one would think should be one of the simplest and quickest operations, copy register A to B, was actually executed as Push A, Pop B. Apparently the registers had no internal path, so one of them had to be pushed onto the stack and popped into the other register. That said, I still like the assembly language of the 6809, even if the actual implementation of those instructions was less than ideal. – RichF Aug 1 '18 at 5:40
  • @RichF - While you're correct that it's a surprisingly inefficient implementation, the MC6809 datasheet shows that TFR and EXG generate dummy bus cycles rather than stack accesses, suggesting the use of internal temporary storage of some kind. – Jeremy Nov 2 '18 at 10:02
  • This is mostly wrong. 1) The 6809 did not have "a lot of" instructions, in fact considerably fewer than the 6800 (59 vs. 78). 2) It was not object-code combatible with the 6800. 3) Most opcodes were single-byte, with the usual additional bytes to specify data or addresses. 4) There was no microcode (it was perhaps the last of the combinational logic processors) and this was one of the reasons it often used fewer clock cycles per instruction than competitors. – Curt J. Sampson Jul 23 at 14:12

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