i8080 CPU support

[danboid]

It seems RunCPM currently only emulates a Z80. It would be good if it also had i8080 CPU support.

Whilst we're talking about making RunCPM slower, how can I modify RunCPM to make its Z80 run at 1 Mhz? I've tried low values of CPU_SPEED but it doesn't make it slow enough!

You never thought you'd get a feature request wanting your program to run slower did you?

[danboid]

If you're wondering why I want i8080 support, its thanks to Uzebox PC

https://uzebox.org/wiki/UzeboxPC

I'd be more accurately able to emulate a UzeboxPC if RunCPM had a i8080 core that I could run at 1 Mhz to simulate a Uzebox PC.

I think the slowest i8080 was 2 Mhz but its even slower than that because its being emulated on an Arduino thats also bitbanging the video display.

[MockbaTheBorg]

That Uzebox is interesting.

Regarding 8080: I might be wrong, but as far as I remember the Z80 is fully compatible with the 8080. If you get a program compiled in a 8080 it runs the same on a Z80.
The Z80 though has more instructions than the 8080, but I am unaware of the difference on the single byte ones.
I have recently made the CPUs modular in RunCPM, so you could create a CPU module matching the 8080 instructions and give it a try. Could be just a matter of removing the additional instructions.
I do not have plans of implementing a 8080 any time soon, but I am open to someone submitting a pull request.

Now, about using SPEED to slow down the CPU: SPEED 1 is the slowest you can go. (0 = disabled, fastest) Give it a try. In my laptop (very powerful i9) it still gets pretty slow.
Keep in mind that SPEED is not meant to match a specific MHZ, it just makes the CPU execute a limit of X opcodes per Y milliseconds. So it will never be precise.
You will have to experiment with different numbers there.
Keep in mind that the slowing down is only for the emulated CPU, not for the native emulator code, so when control is passed from the Z80 to the emulator, that runs at native host speed. So, for example, commands of the native internal CCP run really fast, because they are executed by the emulator natively. To test the speed you need to use something that runs on the Z80.
Try SPEED 1 and WS. Then go make a coffee while you wait. 😉😆

Cheers!!!

[MockbaTheBorg]

Ah, and actually, getting it to run slower was actually one of the first requests people made back in the days. 😆

[danboid]

Hi @MockbaTheBorg , thanks for the thoughtful response!

You should consider building a Uzebox, it will only cost you about $40 for the components. Its great fun helping design a platform.

I think I understand better how CPU_CSPEED works now but it seems that, when you start RunCPM it prints:

Estimated Z80 clock speed: 2437 MHz

Yet even if I use CPU_SPEED 1 as I just have now, it still always says

Estimated Z80 clock speed: 2437 MHz

I wasn't running any real CPU intensive programs or tests, I was just rebuilding with different CPU_SPEED values and reloading runcpm and looking at that value but it never changes.

It seems that value doesn't take CPU_SPEED and try to give the user an estimated clock speed taking CPU_SPEED into account.

What programs do you run to test the performance of runcpm?

[MockbaTheBorg]

Ah ok. I see the issue. SPEED setting doesn't remain, it is not saved, but per session. Every time you start RumCPM it measures the Z80 speed as it is without the slowdown. When I set speed I run the INFO.COM, which runs on the Z80 side of things. But even MBASIC or such would serve. Anything that is Z80 native will slow down.

…

On Sun, Dec 14, 2025, 8:34 AM Dan MacDonald ***@***.***> wrote: *danboid* left a comment (MockbaTheBorg/RunCPM#209) <#209 (comment)> Hi @MockbaTheBorg <https://github.com/MockbaTheBorg> , thanks for the thoughtful response! You should consider building a Uzebox, it will only cost you about $40 for the components. Its great fun helping design a platform. I think I understand better how CPU_CSPEED works now but it seems that, whebn you start RunCPM it prints Estimated Z80 clock speed: 2437 MHz Yet even if I use CPU_SPEED 1 as I just have now, it still always says Estimated Z80 clock speed: 2437 MHz I wasn't running any real CPU intensive programs or tests, I was just rebuilding with different CPU_SPEED values and reloading runcpm and looking at that value but it never changes. It seems that value doesn't take CPU_SPEED and try to give the user an estimated clock speed taking CPU_SPEED into account. What programs do you run to test the performance of runcpm? — Reply to this email directly, view it on GitHub <#209 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AD74NC6Q3QZROSTSLKK7BBT4BVRPJAVCNFSM6AAAAACO5Y5DJWVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMZTMNJRGAZTQOJQHA> . You are receiving this because you were mentioned.Message ID: ***@***.***>

[danboid]

Why does ED.COM give a

DISK OR DIRECTORY FULL

error even when its the only file in the dir/disk/"user"?