Date   : Fri, 09 Jan 2004 13:43:10 +0000
From   : Jules Richardson <julesrichardsonuk@...>
Subject: BBC floppy / drive formats

Hi all,

I've just put together a C library on the Linux machine to access floppy
drives via a SCSI floppy controller that I have, the intention being to
hopefully use it to backup floppies from the various old machines that I
have (not all of them Acorn)

Can somebody summarise for me the various known BBC disk formats in
terms of the following:

  Sectors per track
  Data bytes per physical sector
  Cylinders
  Transfer rate (125 / 250 / 500Kbps)
  Modulation (FM / MFM)
  Write precompensation (none also being a valid option!)    

I assume different vendors' DFS code did different things... although
possibly not and the BBC's floppy controller hardware really dictated
things.

I also have the option to set the following physical paramters for the
drive - I expect the defaults will work here however...

  Head step rate (controller minimum us 3ms)
  Head settle delay (ms)
  Motor on delay
  Motor off delay

I'm going to be doing a little fiddling about as the controller has the
"data bytes per physical sector" parameter as well as a "block length"
parameter (the latter being fixed at one of 128, 256, 512, or 1024
bytes). The spec for the board doesn't mention how the two differ.
Possibly the former is how many bytes are essentially pulled off the
drive per sector, which as then padded with zeros up to the nearest
block boundary - but there's also a "block count" parameter which is
unexplained. Hopefully some trial and error will sort that one out...

cheers

Jules