patch_cpm3.py is a Python script to patch NABU PC CP/M 3 disk images to work from simple sector disk images.

NABU CP/M 3 disks store the Disk Parameter Block (DPB) out-of-band at the start of the raw track, where it is read using the WD179x FDC READ_TRACK command:

This information is lost if the disk is converted to a simple sector-based disk image. This has led to many NABU disk images being distributed in HFE format, which work fine with floppy emulators (Gotek/HxC2001), but don't play well with most CP/M tools.

There's plenty of spare space at the end of the boot sector so the DPB can be stored there instead, moving it back in-band so it's preserved in all disk images:

There are two places that read the DPB: the second stage loader (CPMLDR) and the main CPM3.SYS program. Code in both can be patched to read the DPB from this new location, if needed.

The patch inserts a hook to catch when the raw track reading fails to find a DPB. This happens when it encounters an IDAM (FE byte) after an A1 sync byte instead of the expected 4E filler that leads up to the DPB data. If this happens the hook leads to new code that reads the boot sector to look for a DPB in the last 32 bytes.

If a DPB is not found in either location the behaviour depends on the program. CPMLDR just hangs the system after a failed attempt to display an error message. CPM3.SYS has another try at finding a DPB, matching the format of the first track against a table with 4 entries: 5x1024, 16x128 (FM), 9x512 (DOS), 10x512. If a match is found a DPB stored with the matched entry is used for the disk, allowing some non-NABU disks to be used.

The DPB data starts with a run of 4E bytes, used to indicate the DPB is present. This is followed by a fixed 04 05 06 07 pattern as written by the format.com program, which aren't actually checked by anything. The next byte is a disk type, which is one of 00, 01 or 02, representing 40x1 / 40x2 / 80x2 disk formats. The final 17 bytes are the actual DPB containing CP/M parameters.

The script updates the boot sector to add a DPB block to the end, and attempts to patch CPMLDR and the CPM3.SYS file if they are found. The DPB used depends on the size of the raw disk image, so make sure that is correct. The patched image is written out to a new file with a _patched suffix.

usage: patch_cpm3.py [-h] [-f] [-d {s4,d8,d16}] file

Patch DPB reading in NABU PC CP/M 3 disk images.

positional arguments:
  file

options:
  -h, --help            show this help message and exit
  -f, --force           Force patching boot sector (default: False)
  -d {s4,d8,d16}, --default {s4,d8,d16}
                        Replace fallback DPB for NABU disks: s4, d8, d16. (default: None)

python patch_cpm3.py disk.img

Reading: disk.img
Boot sector: added DPB
CPMLDR: code patched
CPM3.SYS: code patched
Written to disk_patched.img

If the end of the boot sector contains unexpected data it will not be overwritten. If you're happy it's safe you can force it to be updated with the --force option.

CP/M 3 allows access to NABU format disks that don't have an embedded DPB, using a built-in default DPB. This is s4 format by default, which is 40 tracks single-sided. Use the -d option to set this to a different format. For example, -d d16 selects 80 tracks double-sided as the default. Note: -d s4 set the same DPB as the format.com program, which is slightly different to the CP/M 3 internal default.

Special thanks to @alitel, who provided much needed NABU technical information and testing. Thanks also to @brijohn for the NABU MAME core, which made debugging much easier.

patch_cpm3.py is released under the MIT license.

Simon Owen
https://simonowen.com