Deciphering an odd floppy disk format.

Paul Koning paulkoning at
Tue Feb 16 13:43:44 CST 2021

> On Feb 16, 2021, at 2:38 PM, Mattis Lind via cctalk <cctalk at> wrote:
> ...
> What is interesting is that it had variable record length. It was specified
> during the format process which tracks had what number of records and their
> size. The first track always had 40 bytes record since it was the INDEX
> track. File allocation was on a track basis. In the INDEX file each file
> then had a start track and stop track as well as information on how many
> records there were in the file and what size each record was. Also the
> number of records per track were included in the file specification.
> I have never ever heard of such a scheme before.
> /Mattis

I haven't, for floppies.  But flexibility even more than that is found in IBM/360 disk drives, where you get to write sectors of variable length, and you don't even have to pick them ahead of time.  Not to mention that those have keyed sectors, allowing you to do primitive databases directly in hardware.

A similar but more restrictive scheme is the disk drives on the Electrologica X8 (2311 lookalikes, built by CDC I think) where you have a choice of 6 different sector sizes, and you get to make the choice separately for each track, at format time.  You can reformat any track at any time.


More information about the cctech mailing list