How can I format a higher capacity 3 inch disc on a regular CPC6128?

Question

A standard Amstrad 3 inch disc can be formatted to CP/M Data format on the machine: single sided, single track, 40 tracks, 9 sectors/track, 512-byte sectors, no reserved tracks, 1k blocks, 2 directory blocks, gap lengths 2Ah and 52h, not bootable.This gives around 178Kb usable space.

You typically format these discs using the Amstrad DISCKIT3 under CP/M, though it is possible to do so from AMSDOS using firmware routines: there are simple RSX utilities to do so.

There were also some non-standard formats used by game developers to fit more space onto the disc. For example, I have seen disc images with 10 sectors/track for 38 of the 40 tracks. The gaps between sectors are reduced accordingly. These disks appear to be readable on the Amstrad disc drive. This is distinct from various copy protection schemes that were also used.

Is it possible to format a regular 3 inch diskette to a higher capacity than the standard 180Kb (less directory), with the following restrictions:

• The disc should be readable from the computer using AMSDOS routines, patched if necessary.
• There should be no additional hardware involved, for example additional ROMs.
• Using an Amstrad CPC6128.
• No need to read the disc under CP/M

I am open to solutions that involve creating a disk image on another machine and transfering it to the CPC via a utility such as dsk2cdt2disc, which now copes with non-standard gaps between sectors.

Answer 1

Nemesis' Bonzo Doo Dah and Richard Monteiro's FastForm/BigK in Amstrad Action Issue 16 gave somewhere between 203 – 206 K per side, leaving one or two standard tracks for a small boot stub to load up the custom disk parameters.

Both of these hit all three of your parameters: all they did was modify some settings held in RAM. I was a dedicated user of Bonzo Doo Dah back in the day on my dual-drive CPC464. It was useful, but the custom format made backups a little fiddly.

Answer 2

This is not yet a complete and verified answer, because I haven't tested it on real hardware.

There is a utility by CNGSoft called '2H' (written in 2013) which claims a capacity of 208K, using 41 tracks of 10 sectors each. I found this on the CPCWiki forum and a dsk image of the tool on CPC-Power.