The Z80 was one of the most popular CPUs of the seventies and eighties. In almost all cases that I know of, the version used was the Z80A, rated for 4 MHz, sometimes clocked a little slower in order to match display hardware.

The Z80B, rated for 6 MHz, was available quite early; Byte, December 1981 page 541, lists the Z80A for $7.25 and the Z80B for $19. Intuitively it seems like the extra twelve dollars should be well worth paying for a fifty percent performance improvement, but I don't remember any computers using it.

Did any computers use the Z80B? If not, why not?

  • 14
    I speculate 3 reasons we didn't hear much about Z80B. The first was that the IBM PC hit the market in 1981, making Z80s and 6502s outgoing tech. The second is price consciousness; if the manufacturer pays $19 for a component, that might well be amplified by 2 to 3 times in the suggested retail price of the computer. Finally, other components, such as memory, would have to be upgraded as well to support the higher CPU speed. This would further increase retail price.
    – RichF
    Jan 10, 2018 at 5:13
  • 7
    @RichF - while this price amplification is definitely true, note that the prices quoted in the question are retail prices for chips being sold in single units to homebrewers. The manufacturers would have been paying substantially less than $19 each for a Z80B -- but availability of affordable RAM that could keep it running with zero wait states is likely the reason we didn't see many "standard" computers using it until later on.
    – Jules
    Jan 10, 2018 at 17:03
  • 3
    You see $12, I see 162% price increase for less than 50% speed gain. So no, price to performance wise alone, it was not even remotely worth it. You would need a marketable use case that the extra clock makes possible to justify the cost. Or pay the cost without justification and flop, just what some did.
    – Agent_L
    Jan 11, 2018 at 9:13
  • 3
    —▶ oldcomputers.net/lobomax80.html
    – user28
    Jan 11, 2018 at 10:50
  • 5
    The TI-85 graphing calculator uses a 6Mhz Z80... Does that count at all?
    – JPhi1618
    Jan 11, 2018 at 16:49

7 Answers 7


One reason you didn't see many Z80B machines is that not much was available in the way of affordable support chips for it. For example, look at the advert you cite: it has prices for PIO, CTC, DMA, and SIO chips to work with either Z80 or Z80A, but for the Z80B there are no DMA or SIO chips listed, and the PIO and CTC chips are much more expensive. You could also use a Z80A with one of Intel's DMA chips, but those only went up to 5MHz, so if you needed DMA, you were only looking at a 25% increase in performance at most, not 50%.

Memory was a problem, too: for a Z80B at 6MHz to run with no wait states, it would need memory with a cycle time of 1.5x the clock rate, which unless you were using page mode (and consequently more expensive support chips) meant you'd need fast RAM chips too (a 4164-120 should do the job, but they were quite a bit more money than the 4164-150s that most computers were using at the time). And you'd still need logic to generate the RAS and CAS lines, which needs to be able to run faster than the CPU clock in order to produce the signals correctly. For example, the Sinclair Spectrum generates these using its ULA, which runs internally at 4 times the CPU's clock rate. So to make a Spectrum-like computer run a Z80B at full speed would need its ULA to run at 24MHz, which was pushing the capabilities of programmable logic at the time, so would likely have required a full custom ASIC, pushing it beyond the capabilities of many manufacturers. It could have been done with TTL logic chips, but would have made the resulting boards quite large, and expensive to produce.

Now, there were manufacturers who had the capabilities to do this, but in reality they had no real desire to: Commodore could have done it, but were completely invested in the 6502 series of CPUs due to their ownership of MOS Technologies. Acorn could, but then by the time the need for a faster mainstream microcomputer became apparent, Acorn were already thinking along the lines that would lead to the ARM processor.

  • I wonder whether the DMA, PIO,CTC ... could be "overclocked" and still work in some cases? Apr 11, 2018 at 23:51
  • 1
    I don't have it any longer (someone threw it away as thrash) but the swedish made Luxor ABC 803 series ? Who was targeted at business and schools so a little less price-sensitive. Dec 7, 2019 at 19:55

A number of computers shipped with the Z80B, including:

  • Brascom BR1000M
  • IPTVT TIM-S (Romanian ZX Spectrum clone)
  • M.G.T. Sam Coupé
  • Osborne Executive
  • Panasonic FS A1 WX (MSX)
  • Sharp MZ-2500 (SuperMZ) series
  • Vector Graphics Vector-3 (VIP)

Upgrades to the Z80B were available for some computers:

  • 5
    The Enterprise was apparently originally planned to use the Z80B, then downgraded for cost reasons. Some of the Australian Microbees apparently used Z80Bs as well
    – tofro
    Jan 10, 2018 at 13:41
  • 4
    I assume the OP is asking about computers that were manufactured with the Z80B in the first place, but of course, many hackers upgraded their Z80- and Z80A-based systems to Z80B on their own.
    – Dave Tweed
    Jan 10, 2018 at 15:21
  • 1
    Props to the Enterprise for being particularly easy to retrofit a different CPU too because it's one of the few memory-mapped display micros of its era in which video output and processor timing are completely decoupled. Contention is achieving by holding the processor clock for as many transitions as it takes to get into the proper window, but it's a completely separate crystal with a non-integral relationship to that of the video.
    – Tommy
    Jul 24, 2018 at 13:36
  • 1
    @Tommy One reason no computer tried connecting the same memory to signals on different clocks: any attempt to arbitrate between two signals that might arrive at nearly the same time could cause metastability, and no physical circuit can prevent that completely.
    – Davislor
    Aug 19, 2018 at 10:17
  • 2
    @Davislor your link is region blocked, but enterprise.iko.hu/schematics/EP64-1~1.jpg works for the Enterprise 64kb (and I can find only one crystal on it, in the lower left); in my defence I defer to enterpriseforever.com/programming/once-more-on-timing/msg71565/… which is a long-time Enterprise hacker responding affirmatively [to my stupid questions] that video and CPU clocks are decoupled albeit with no discussion of whether they're separately sourced.
    – Tommy
    Aug 19, 2018 at 15:38

Funny that this post still doesn't have the right answer. The Commodore 128 of course!

Thanks to Tommy for providing a beautiful picture of the mainboard, which reveals the Z80B very clearly: A beautiful picture provided by Tommy

  • 4
    I’m not sure about the, but to my surprise this is a correct answer. Board photo: upload.wikimedia.org/wikipedia/commons/0/03/C128mobo.jpg . Look about three chips in from the bottom right — the Zilog logo makes it an easy spot in a sea of MOS.
    – Tommy
    Dec 7, 2019 at 19:17
  • 1
    Thank you for your confirmation, the motherboard gives a clear picture of where it is located, as well as a lot of other interesting hardware. I hope a TheC128 comes out next year, Basic V7 seems a little easier to me.
    – Ciel Ruby
    Dec 7, 2019 at 19:29
  • 1
    Hmm, in other words i do have such a machine ... i did a bit of dumpster diving 15 years ago which gave me an 128D. Dec 7, 2019 at 19:59
  • 5
    They must have got a good deal on these chips rated at 6 MHz. They only ran them at 4 MHz. Sort of. The old description was "4 MHz half the time", so effectively 2 MHz. This weird situation had to do with bus logic and the needs of other support chips.
    – RichF
    Dec 8, 2019 at 1:14
  • 1
    As you've seen I added Tommy's beautiful explaining picture of the motherboard to the answer and I made a small change to the C128 Wikipedia page. I found a very useful and fun site where you can learn to program Assembly for Z80/MOS/68K processors called chibiakumas.com . they fully documented the C64, but give some info about the C128 as well. Hopefully they'll cover more of the C128 later.
    – Ciel Ruby
    Dec 10, 2019 at 20:18

I had a homebuilt S100 Z80A system overclocked to 6MHz. It worked very well although I did find it necessary to heat sink the processor. So using a Z80B was not really necessary, and I was able to use standard Z80 family parts.

It is worth considering that $12 around 1980 was worth rather more than $12 today, so that cost comparison is not so simple.

There was also a Z80H which was rated at 8MHz, although I believe that came later.

  • 2
    Of course, while $12 was worth more in 1980 than now, computers in general cost a lot more in 1980 than now. When you wouldn't generally expect to be able to buy anything but the cheapest of computers for much under $1,000, and if you wanted something respectable could plan on spending an easy $2,000, $12 looks somewhat less of an issue. And that, I guess, is where the real difference between the home built systems of the era and the mass market ones: the home builders knew the component pricing, and realised how much money the mass market system builders were making out of each box sold.
    – Jules
    Jan 11, 2018 at 13:18
  • There was also a Z80H which was rated at 8MHz, although I believe that came later. -- and the Z84C00 which came in 10 and 20MHz variants, but that wasn't until the early 90s.
    – occipita
    Aug 25, 2020 at 3:43

Cromemco also made a dual CPU board with the Z80B at 5mhz and MC68010 at 10mhz on the same S100 board. The Cromemco XPU. http://vintagecomputer.ca/files/Cromemco/Cromemco%20XPU%20Manual.pdf


The Lobo Systems Max-80, released in 1982, used a Z-80B, at 6Mhz. Before Lobo ran into financial trouble (CFO skipped town with funds), there was talk of a logic-board & microprocessor upgrade to 8Mhz, or possibly 12Mhz, but it never materialized. Natively, it ran CP/M 2.2, CP/3.0, and LDOS. As I recall, it could also boot TRSDOS. It had 2 banks of 64K RAM, which could be utilized as either 64K of system RAM plus a 64K ram disk, or a full 128K bank of system RAM. I seem to recall that the later required a BIOS-chip update (BIOS was on a chip, and BDOS & CCP were disk-based. I think all 3 were written by Gary Kildall, of Digital Research). I used to work for Lobo, and still have a working Max-80, but the primary 8" drive is bad. Boots from drive B, though.


There were much better things around in the old days of mine, even while not official releases.

In the early 80s I owned a TRS80 Model 1 clone (that Z80-based thing looking like a silver C64, but years before the C64 was even thought of ...), built from a modified ECB-Bus industry computer CPU card originally meant for CP/M.

That thing ran on the original ROMs and FDD OSes, featured 4 FDD drives in DS/DD with 80 tracks and a hand woven (!) graphics logic board, all sitting in a nice ECB-Bus aluminium case.

Its heart was a Z80H, as far as I remember, and it ran at 8MHz (instead of the original 1.77 MHz, which was still available on that box for games ... but switching over usually required a reboot). Was quite quick, that thing. But I sold it to an enthusiast long ago ...

Seems that actually an eZ80 CPU is still around with 50MHz clock ... would be fun to build such a box with that and play "Arcade Bomber Scramble" or "Outhouse" on it ;-)

  • You typed Z80H. Was that a typo? If not, this doesn't really answer the question.
    – wizzwizz4
    Jul 24, 2018 at 6:28
  • 1
    Wow..someone else played Outhouse?
    – Bill K
    Feb 20, 2019 at 23:24
  • eZ80 not only runs on a faster clock, but also it’s a pipelined architecture that is way more cycle efficient. Also got a multiplier IIRC. And some flash and ram too. And gpio :) Mar 13, 2020 at 16:25

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .