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I am familiar with the various accelerator products that were developed over the years to improve the performance of popular 6502 home micros. A short list of the particularly well known devices would include:

  1. SuperCPU for the C64, based on 20MHz 65816.
  2. Zip Chip for Apple ][, based on 8MHz 65C02.
  3. Various Apple ][gs accelerators, which simply used a faster 65816 (up to 20 MHz) than was on the motherboard.
  4. BBC micro 4 MHz 6502 for the Tube interface.

I have not heard of any after-market, plugin accelerators for the popular Z80 micros of the era, like the ZX Spectrum, MSX, or TRS-80. Did any such accelerator products exist, based on either a higher clocked Z80 or a Z8000? If not, is there an obvious technical or marketplace reason for this?

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    I wonder if the Intel 8088 CPU took on that role, with backwards source compatibility being sufficient for developers, something the 6800/68000 line lacked. retrocomputing.stackexchange.com/questions/6647/… – snips-n-snails Jan 8 at 20:27
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    @snips-n-snails I think that could apply to CP/M users. As long as PC-DOS had a port of their favorite CP/M application, they could easily just dispose of their old Z80 or 8080 CP/M machine and buy a PC. – Brian H Jan 8 at 22:02
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    The clone ZX Spectrums were often faster than the original; might there be something in that machine being so simple that if you’re going to sell a replacement CPU you might as well throw the rest of the machine in for free? – Tommy Jan 9 at 1:10
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    @snips-n-snails It wasn't binary compatible with an 8-bit CPU so probably not. – Thorbjørn Ravn Andersen Jan 9 at 12:06
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The main TRS-80 line (Model I, III and 4) had several third party Z-80 accelerator boards.

The Archbold board could bring the Model I up to 5.3 MHz from 1.77 MHz.

The Holmes Sprinter boosted the Model I up to 5.32 MHz. It came in a Model III version to boost it from 2.027 MHz to 5.07 MHz.

The Model 4 had several speedup board options. Incidentally, the Model 4 was compatible with the Model III and featured a Z-80 running at 4.055 MHz which could be software switched to match the Model III CPU speed.

The Alpha Technology Super 4 could boost the Z-80 up to 6 MHz. The H. I. Tech XLR8er did 6.144 MHz and the Seatronics Super Speed-Up gave a whopping 8 MHz.

The XLR8er was the one most commonly seen. The Seatrontics was available for the Model I and III but it came out in 1986, well after the heyday of the Model I and III introduced in 1977 and 1980 respectively.

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    "XLR8er": what a great name! – Brian H Jan 9 at 18:24
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I'm not familiar with the C64, and didn't do much with the Apple ][ back in the day, but I did spend a lot of time under the hood of my TRS-80.

There wasn't a lot of room for plugin accelerators in the TRS-80 Model I. I did put in a CP/M daughtercard, which remapped system memory to get ROM out of the low address space, but didn't replace the processor.

There was a nominal expansion slot in the Expansion Interface, usually used for an RS232 interface board. Its connector was extremely twitchy and unreliable, and as I recall the system's architecture limited its utility.

There was also a further expansion card-edge on the Expansion Interface. I used this to connect a home-built video display board. I believe it did expose the signals you'd need to take over from the Z-80, but it was at the end of a fairly lengthy and not very clean bus, and I doubt you could have run a reliable daughtercard through it, at least if you wanted access to system RAM and the like.

The refresh timing for system RAM was tightly tied to processor clock speed. There were various hacks to increase clock speed, and they generally required some fiddling with the refresh generation circuitry. I don't remember details. I think it would have been challenging, especially with 1980 technology, to tie in a faster processor while maintaining DRAM refresh.

Bottom line: technology was marching quickly at that time, the TRS-80 wasn't very robust for expansion, and there probably just wasn't enough time or market demand to produce accelerator daughtercards.

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For the Amstrad PCW, there was the Sprinter card - containing an 8MHz Z80 CPU that replaced the 4MHz original, a memory expansion, and cache RAM so that the processor wasn't restricted to the speed of motherboard memory. PCWs were largely used for word processing and DTP rather than gaming; in these applications it's useful to have more memory and a faster CPU.

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    Especially the memory was what made the PCW such a great office machine. – Raffzahn Jan 9 at 10:49
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Yes, accelerators did exist - but they were usually niche products, or handled completely differently (see below).

It's more of a market-driven issue than ability to speed up.

Home computers never really had a big need for speed improvement. After all, any speed up would not only break games, but also be rather expensive, as the host system wasn't really made to be extended in a fundamental way (*1).

More so, manufacturers didn't have much interest in selling a CPU upgrade, when they could do as well by selling a new, faster machine instead. Cases were accelerators were successful are rather exceptions.

  • The Apple II because, its incarnations didn't offer any speedup for almost 10 years (until the IIgs and IIc+). This rare combination of a huge user base in a rather professional area, plus a manufacturer not upgrading the system, opened a huge window of opportunity for third party offerings of accelerators. Users taking this road usually had a professional need for higher speed -like crunching ever increasing VisiCalc sheets (*2). Much the same way as this group needed larger memory, making a fortune for Saturn Systems. These users were a quite large percentage on the Apple II (*3).

  • The C64 gives the perfect counter-example. Despite its quite high distribution, only a small minority saw any professional use. As a result, neither Commodore supplied memory expansions (17xx REU), nor did the Super CPU find much use.

  • The BBC Micro is again an exception, as it was intentionally designed to be expandable. Adding subsystems with other CPUs was intended. Doing the same with a faster 6502 came naturally.

The fact that all of them are 6502 systems is rather by accident.

Z80 systems were basically only used in two large non-intersecting groups: S100-based professional systems and home computers, with MSX being eventually the biggest share.

  • On the home computer side, manufacturers had no interest in upgrade sales, but unlike Apple, they did offer compatible follow-up models. It's interesting that in this segment it was much more important to add capabilities like better video than speeding up the CPU.

  • On the professional side, Z80 systems were usually S100 based, enabling easy swap of the CPU board for a new, faster one. Interestingly, the same worked for CP/M on the Apple, where the basic 2 MHz Z80 cards soon were swapped for boards with local memory and Z80 up to 12 MHz.

A special group to be mentioned as well are dedicated (office) systems. Here speed was as well of no concern in itself, but all about the ability to keep up with the user. The Amstrad PCW is a great example here as it sold quite well for more than a decade (1985-1996), making it one of the longest selling machines. A 4 MHz Z80 is more than capable of keeping up with typing text, handling addresses and printing (*4).


*1 - Mostly due the rather tight coupling of CPU and memory/I/O timing.

*2 - We all know how users in management and marketing bloat their spreadsheets way past a point were any rational user would build a custom software, don't we?

*3 - By 1984, less than 1.5 million Apple II were sold - and 700,000 copies of VisiCalc.

*4 - In some way the PCW is the antithesis to the Apple II. Whereas professional Apple II users were at the forefront, willing to spend a lot of money to get top-end tasks done, PCW users were rather the 'working' folks, needing a reliable and performing tool for work that didn't change much.

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    "Z80 systems were basically only used in two large groups:" - Sinclair Spectrum was not in either of the large groups you mention but sold 5,000,000 units. However, it's architecture is best described as simplistic and there wasn't even a hope of accelerating it. – JeremyP Jan 9 at 10:17
  • @JeremyP You mean, the Spectrum was not a Homecomputer, but professional installed in offices and alike? – Raffzahn Jan 9 at 10:31
  • It wasn't an MSX home computer. – JeremyP Jan 9 at 13:45
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    Lots of CP/M computers which were not necessarily S-100 based. – Thorbjørn Ravn Andersen Jan 9 at 14:48
  • I ran CP/M on an Apple II back in the 80's using mini Z-80 "daughterboard" that was contained on an Apple expansion board called "Z-80 Softcard". en.wikipedia.org/wiki/Z-80_SoftCard – Chris Wolf Jan 10 at 22:21

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