AltairZ80 SIMH: Can you convert v3.x ".dsk" files to work on v4.x

[Joseph Corda]

Greetings
Trying to use .dsk files created with Windows AltairZ80 (SIMH v3.x) on a Linux  v4.1 instance of SIMH.  When I try I get the error (also see images below):

SIMH interface version SIMH003 required.
HALT instruction, PC: 00B69 (HALT) 

From my research I read that a SIMH developer added some logic to the end of the  v4.x dsk files and I wonder if this is what causing the failures.  I thought I read that dsk files should just be raw track/sector dumps of the original disks (maybe I'm wrong?).

My question is there anyway to convert, modify or trick Linux Altairz80 v4.1 to run v3.x dsk files.  Note:  I did try to compile v3.9 SIMH (Altairz80) on my Linux  system but that currently fails though I can compile v4.1 without issue.

Here is the link of the Zip file with the disk files 
(note this zip file is for windows):

http://www.s100computers.com/My%20System%20Pages/IDE%202CF+SD%20Board/SD%20Card%20Test%20Programs.zip

Any suggestions are greatly appreciated
Thanks
Joe
 

[Patrick Linstruth]

I will look into this further, but just today I was working with Peter on the AltairZ80 “ALTAIRROM”, which isn’t the Altair’s ROM (Disk Boot Loader). In version 4, it is a customized version of the Disk Boot Loader that is incompatible with the “standard” disk images downloaded from places like Deramp. For years I have been loading DBL myself from a binary file.

These CP/M versions seem to be modified to only run under a specific simulator.

I don’t know what you’re trying to accomplish with the files you downloaded from S100computers.com, but if you’re just wanting to boot CP/M 3, you can try these on your Version 4.

[Joseph Corda]

Patrick
I'm trying to modify CP/M code John M includes with a board I built (the Dual SD card).. I have a Z80 master card running with a monitor and I wanted to see how/if they worked together.
As I might have mentioned before I mainly use Linux and the ZIP files John M supplies have with everything needed to run a AltairZ80 SIMH v3.x and dsk files but for Windows.

Now I did find this while searching but not sure if its the possible cause of my issue

simh changes .dsk image files by silently adding signature #1059

https://github.com/simh/simh/issues/1059   It looks to be a older post from 2021 
Let me know if you find anything
Joe

[Patrick Linstruth]

I remember that “SIMH changes .dsk image” debate. It was referring to the PDP11 simulator, not AltairZ80.

The zip should do what you want on Version 4 under Linux. It will boot SIMH4 dependent version on A: and have the SIMH3 dependent version on B: and the hard disk on I:. Use R and W to move files between your Linux machine and B.

It looks like you don’t even need the B drive. It’s just there for CP/M. Everything else is on I:. You will need to use “DIRS” to see all the files on I:.

Let me know if that helps.

I am not sure why they made simulator-dependent versions of CP/M. I am working with Peter Schorn on the AltairZ80 boot rom. It doesn’t use the standard Disk Boot Loader. It has a custom, simulator-dependent, loader that actually modifies itself and boots disks starting at sector 8, not sector 0.

Notes: What I would do is move everything you need to a single 8MB CP/M 2.2 image instead of using all this proprietary stuff and the complexity of CP/M 3.

[Patrick Linstruth]

Never mind on the CP/M 2 plan. John used underscore characters in the filenames, which aren’t compatible with CP/M 2.

So, you could use CP/M 2, but the files will need to be renamed.

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[Joseph Corda]

Patrick
So with your suggestion .  I adjusted my v4 working Altairz80 (SIMH) cpm3 launch file and added John's "I" dsk as  hdsk1.
I was able to boot my working version of CPM3 and then access the "I" drive (i.dsk file) ..so now I can move forward with what I wad trying to do .

So now I still wonder what causes the error I get when booting John cpm3.dsk and if I can fix it ?  I guess I will do more research and play around 
Anyways thanks for the help! 
Joe

[Patrick Linstruth]

From what I can tell, John’s CP/M 3 disk does not have any purpose other than booting CP/M 3. With the exception of the underscores in the file names, you could use any CP/M that runs on the simulator.

I looked at the source code for the boot loader on John’s CP/M 3 disk and it checks to see which version of SIMH is running, and if it isn’t Version 3, it displays the error message and halts. I have not dug in deeper to see why it cares.

It is possible to control the simulator through IO Port 0xFE. You can tell it to return the version number, or switch from 8080 to Z80. Maybe the CP/M BIOS uses some of these features making it dependent on a particular version of SIMH.  It could also be one of those “let’s do it because we can” features.

You could try loading the disk image into a hex editor and change “SIMH003” to “SIMH004” and see if it will pass the version check and run on version 4. But again, I don’t see any value to using the CP/M 3 image from John as it appears that everything needed is on the I: drive.

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[Joseph Corda]

Patrick

What you said makes sense..  and you are correct I can just mount the i.dsk file in a working cpm3 setup and get what I want done!

Thanks 

Joe  

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[Patrick Linstruth]

If you want to boot John’s disk for SIMH V3, this one will boot on V4. Interestingly, SIM V4 return “SIMH005”.

[Joseph Corda]

Thanks Patrick I will give it a try
but honestly your suggestion of using a cpm3 that already works(which I already had) and then attach the other dsk files included in John zip is working fine...

Joe

On Wed, Oct 22, 2025 at 6:10 PM Patrick Linstruth <pat...@deltecent.com> wrote:

If you want to boot John’s disk for SIMH V3, this one will boot on V4. Interestingly, SIM V4 return “SIMH005”.

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