Hand clocking CPUs

Dave McGuire mcguire at neurotica.com
Wed Jul 21 14:08:36 CDT 2010

On 7/21/10 3:03 PM, Bill Sudbrink wrote:
>>> I was reading something the other day about being able to hand-clock a
>>> Z80, that it was so stable (due to not using dynamic registers,
>>> apparently) that with the appropriate debounce circuit you could literally
>>> manually step it through instructions.
>>> Is this as rare as it sounds?
>>> Has anyone -tried- hand-clocking a Z80?
>>   Umm yes, it's called single-stepping, and it's very common.  The first
>> Z80 system I had that had support for that built-in was my IMSAI 8080.
>> I used single-stepping to debug my BIOS modifications in the mid-1980s.
>>   And yes my IMSAI "8080" is actually a Z80 system...It has a CCS 2810
>> CPU board which is compatible with the IMSAI front panel circuitry. :)
> Um, no.  You don't understand how the front panel of the IMSAI
> works.

  Actually yes I do. :)  I've had to dig into it several times over the

> It uses memory wait to stop the processor.  It doesn't
> have anything to do with the CPU clock.

  No, I didn't say it did, I was talking about single-stepping in
general in the context of the IMSAI system.  It is indeed possible to
single-step a Z80 by manually pulsing the clock.  One of my SBCs does
exactly that, based on a circuit in one of Steve Ciarcia's books.


Dave McGuire
Port Charlotte, FL

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