The Commodore 128 had 2 CPUs, 2 video graphic chips, and 2 I/O systems/chips.

It had 3 modes: Commodore 64 mode, Commodore 128 mode and CP/M. The first shared one of each CPU/chip, the CP/M mode the other.

Was it the original intention to combine them? Or was it intended the way it was launched (two computers, or three in one case and that's it, simly said). I wonder if they could get more out of all those peripherals.

  • 3
    Not a real answer, but if you want to exhaustively research the Commodore 128, here are the players to google. Bil Herd: Designer & Hardware Lead, Dave Haynie: Intricate timing, PLA Emulator and DRAM, Frank Palaia: Z80 Integration and Ram Expansion, Fred Bowen: Programmer and Software Lead- Kernal & Monitor, Terry Ryan: Programmer- Basic V7 including structured language additions, Von Ertwine: Programmer- CPM - Also, this is a great article on hackaday from Bill Herd: hackaday.com/2013/12/09/…
    – Geo...
    Aug 16, 2019 at 10:46
  • @Geo... Thanks for that info ... I never expected so much to be available... The C128 was my first computer (and C128D second, because the first was 'lost during repair'). But to be honest, it was mostly in C64 mode. Aug 16, 2019 at 10:52
  • What should have happened was instead of CP/M mode, it should have been MS-DOS compatible. The 128 would have become a runaway hit.
    – Mook
    Oct 30, 2019 at 12:44
  • At the time of C-128, CP/M was where the business software was. Nov 4, 2019 at 12:55

4 Answers 4


Was it the original intention to combine them?

No. The Z80 was not even part of the original design. It was added out of necessity to meet a "100% C64 compatible" claim of the original marketing. The C64 offered a CP/M expansion cartridge, which for some reason would not run correctly when inserted into early C128 prototypes. So about 2 months into a 5-month design cycle, the engineers decided to throw a Z80 CPU chip right onto the C128 motherboard.

Additional Information

Commodore History Part 5 - The C128 (YouTube video by The 8-Bit Guy)

Why does the Commodore C128 perform poorly when running CP/M? (detailed)

  • Upvoted and accepted; although the shortest answer, it answer my question best in the sense it was not the intention. Aug 15, 2019 at 21:48

The Commodore 128 was intended to be a fully-compatible, more professional, upgrade to the popular Commodore 64. The marketing called for addressing the most widely criticized shortcomings of the C64 that made it unsuitable in competing with more "professional"/business-oriented machines like the Apple //e and //c and the IBM clones.

C64 compatibility was essential. Additionally, the most requested features included:

  1. 80 column text mode for business/productivity applications. This necessitated two video chips since the VDC provided 80 column mode while the VIC-II provided C64 compatibility.

  2. More addressable memory and CPU performance. This resulted in the C128 MMU and the two distinct operating modes for C64 and C128, which have different memory maps, different firmware, and allow different CPU clock speeds (1 MHz or 2 MHz).

  3. Full C64 compatibility also implied working with the C64's Z80 add-on card to support CP/M. According to designer Bil Herd, it was not feasible to get this card working with the C128 because of the cards many timing hacks and power supply issues. That necessitated the second Z80 CPU in the C128 to support CP/M.

  4. Improved floppy disk performance and capacity. This necessitated the additional "Burst" serial port mode and 1571 floppy drive upgrade, which also has a second mode to emulate the 1541.

You can see how the above features are all specifically called out in the advertising for the C128, pictured below.

So, the added hardware basically came from the designers meeting the marketing demands for the machine. It was also done in a rush, as was the case with most Commodore 8-bit machines. So they used the parts that mostly already existed, rather than taking more time to develop a "VIC-III" to support 80 columns and higher resolution, for example. The MMU was the only newly developed part specifically for the C128. The rest of the new stuff was firmware and peripherals.

C128 ad


The original intention was basically to have the functionality of two separate computers: a 6502-based system running Commodore's OS (with some features added beyond what the Commodore 64 offered, such as 80-column display and more memory) and a Z80-based system running CP/M. (These could not be used at the same time.) This is obviously cheaper than having two separate machines for the same functionality, since they can share a lot of hardware. It may or may not be more convenient depending on whether you actually wanted to use them at the same time, how much space you had on your desk, and so on.

From a distance it sounds like a good idea to try to use the CPUs and other dedicated hardware together, but in practice it's far more complex than is worthwhile. Consider that they shared the same memory subsystem and co-ordinating two CPUs sharing memory would have required both extra, fairly complex hardware, significant software changes (likely to both operating systems), and would probably have slowed memory access unless they added more or faster memory, either of which would have increased the cost. (If you want to get an idea of the complexities involved in even making both able to use the same memory and video display, have a look at the answers to this question.)

And yes, there was always at least one more CPU, another 6510 in the disk drive. (That was true from the PET systems onward.) That, too, is dedicated to its task and not shared with other parts of the system.


Was it the original intention to combine them? Or was it intended the way it was launched (two computers, or three in one case and that's it, simly said).

Basically yes. It was a sounding approach to get more revenue from an, at that time already ageing, 8 bit concept without investing much, while running a small risk of failure, as there was no risk of alienating existing customers

  • New Customers could see it as a better 64 they always wanted to buy.
  • Existing C64 customers could see an upgrade path without loosing all investment (read games) thy had spend.

Maybe most important

  • Both got offered a more semi professional, output related usage.

Especially the later was important to bind customers growing out from playing with a (classic) home computer into every day productivity related computer use.

Despite all the work dedicated users have done, the C64 wasn't a real replacement on for professional PET series. Mostly due the lack of a good keyboard and an 80 character display. The 128 did offer both, and with CP/M as OS it could tap a great amount of existing productivity software - from word processing to databases and much more. While CP/M had as well past it's peak, it was still considered a good choice. Neither the PC nor DOS was as all mighty as it became a few years later.

I wonder if they could get more out of all those peripherals.

Not really as the 128 was for most part just a combination of existing enhancements for the C64 - adjusted for better integration, not anything really new.


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