Do TRS-80 Model I and Model III load and execute /CMD files using specific memory addresses?

Question

/CMD files on the various TRS-80 Model I and III OSes are machine code command files.

They seem to have some internal format but not really a file header.

They often start with 01 02 and then a 16-bit address that looks like where the file should be loaded, and then Z80 code. But one starts with 05 06 followed by a 6-character string, but another starts with 01 02 followed by a sensible address but then followed by a 7-character string. So I can't make out the pattern.

I can't find either the format of these files or the method they are loaded and executed by the machine documented on the internet. I also can't find source code of any of the emulators that support running them directly from the host computer's file system.

Does somebody know how these /CMD commands work?

Answer 1

/CMD files are a series of tagged records. Some record types have meaning only for certain features of some of the TRS-80 DOSes. Only two seem important for loading and executing them.

byte:        type
byte:        size* in bytes
bytes[size]: data

*size 0 → 256 bytes
*size for type 1 only, size 1 → 257 bytes, size 2 → 258 bytes

type 1:     object code (load block)
data[0..1]: load address, 16-bit little-endian
data[2...]: the actual program code & data

type 2:     transfer address (entry address)
data[0..1]: address execution begins, 16-bit little-endian

There should be one type 2 field, it should be the last field, and it indicates the end of the file.

All types up to 0x1F can be otherwise skipped by using the size field.

All types above 0x1F are invalid. Perhaps the file is not really a /CMD file or is corrupt.

There is often garbage following the type 2 field. (Perhaps they were written to disk in sizes rounded to the next exact sector size?)

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I think I might have found it. There's a "hex2cmd" command on Ubuntu:

hex2cmd - convert Intel hex format to TRS-80 CMD format

And it goes on to say

The LDOS Quarterly, April 1, 1982 (Vol 1, No 4) ⟨http://www.tim-mann.org/misosys.html⟩, has documentation of the TRS-80 DOS CMD file format.

While I didn't find it in the link they gave, I did find the LDOS Quarterly they mention. It's in a column called Roy’s Technical Corner.

(ninjalj found another copy much faster than than me while I was writing up my answer.)

And now I've found a much more recent post on this topic by the blog of Jim Lawless: Understanding TRS-80 CMD Files

The post even includes the C source for Jim's own tool for scanning the CMD files. He also mentions some confusing aspects that the documentation didn't make clear.