What limited the use of the 6809 CPU in personal computers?

Question

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.

Answer 1

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)

• Hitachi MB6890 of 1980 and the later
• Hitachi S1 series.
• Canon CX-1 business system of 1981. (MCX Operating System)
• Fujitsu FM8 and FM-11 series (1981) as professional desktop machines and
• Fujitsu FM7 series (1984) for the semi professional/home market

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 rare (*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 Motorola's 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 pre-existing 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 applications. 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) were 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 Module 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 by 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 brought 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.

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*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 completely 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.

Answer 2

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.

Answer 3

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.

Answer 4

I think the ultimate reasons were high cost, late introduction, and poor marketing by Motorola (a constant problem with this otherwise technically superior company).

Manufacturers ultimately look at the bottom line of their BOM and the 6809 was at least 2x as much as Motorola was inherently more expensive and did not offer the same level of volume discounts that were available for the 6502 and Z-80.

By time the 6809 was readily available other 8 bit CPUs were already well established and going downward in price. From an ISA perspective the 6809 walked all over any CPU available - but concepts of clean code structure were minor considerations still in the PC world. Remember that FORTH was still a significant language at that time and the dual hardware stacks made the 6809 an ideal FORTH environment which could have helped improve the traction of stack based computing models rather than register oriented.

Had Motorola properly demonstrated the development superiority of their ISA the quality of systems we could have gotten had the 6809 received the same market penetration as the Z-80 was have had a profound impact on our industry by at least one decade. Imagine if IBM had picked up the 6809 instead of that nasty little 8088. The 6809E was designed specifically to operate in a multi-CPU environment.

Sadly, just like DEC, the best tech did not win out because the parent company's mindset was oblivious of where the market was actually going. Had this system gained wide adoption the entire history of the PC revolution would look completely different and our system architectures would be more secure and scalable than they are today. Now we're having to rethink and re-engineer all this technical debt built up over the decades since Moore's law has run its course for the most part.

Answer 5

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.

Answer 6

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...

Answer 7

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).

Answer 8

Don't forget the Intel 8086 came on the scene at the same time; The major difference there was the memory management chip was built into the 8086. This allowed the development of software tools that utilized extended memory without having to deal with the differences from multiple implementions of memory management. While (imho) the 6809 was a superior processor, the easy access to development tools gave a distinct advantage to the 8086 family at a critical time during the development of the industry.

Answer 9

Why did manufacturers of home computers avoid using the 6809 CPU?

To add the excellent responses above, I will add the one that is likely the most significant: price.

The 6809 had about 9,000 gates, whereas the 6502 had about 3,200. Given an equal fab, the 6809 will cost more than the 6502. Not three times, as the packaging is significantly the same, but the overall yield is going to be lower and that goes right to the bottom line.

Now the 6809 offers more functionality, but that functionality has to justify a price/performance ratio that was being offered by other platforms. And simply put, it didn't offer either price or performance.

On the price side, when it was introduced single units were $37, while the Z80 was $9 and the 6502 was about $6. These are single unit prices, which means volume purchases will be even more spread out due to multiple suppliers in the case of the 6502 (consider that Atari got a 6507 and RIOT for $12 at introduction).

You can see the other side of the equation here. Although it was relatively quick in theory, in practice it was slower than most existing systems. Yes, this bench is in BASIC, but of course, that was what would normally end up being used by these processors in the home computer space. The BASIC in question was, at the runtime level, identical to the one on the PET, but as you can see, the PET outran the Dragon in spite of being five years older.

They started the project at the same time as the 68000, because their customers who told them they didn't need 16-bit systems. So they designed an updated 8-bit ISA for a market that, by this time, was moving to dedicated microcontrollers and using 8-bit standalone CPUs largely only in legacy roles. The project seems like it was doomed from the start.

Answer 10

As I remember the 6809 was a great processor in many ways with a great memory paging system that allowed fast IRQ and many tasks to be be swapped efficiently. The merged data/code space (vs 8031) made programing simpler BUT also resulted in quickly running out of address space as data, code and peripherals all had to share that little 64k address space. Paged code and/or data space was needed for any but small embedded 6809 systems and that created it's own software troubles. The 68008 made all the code space issues go away and saved the programmers from endless paged memory problems at the price of being slower in IRQ performance. A few years time quickly solved the speed issue and then 8 bit became only low cost low performance processors and the 6809 was never a low cost part. Pity as the 6809 instruction set was good but there was just never enough address space the make full use of the parts features.