How to read Osborne 1 Floppies?

Fred Cisin cisin at
Tue May 25 15:12:43 CDT 2021

On Tue, 25 May 2021, Peter Coghlan via cctalk wrote:
> Early on, floppy disks on the BBC Micro were 5.25in FM with 10 sectors
> per track and 256 bytes per sector.  However, BBC Micros are probably not
> common where you are and some programming in 6502 assembly or BBC Basic
> and a knowledge of the Osbourne filesystem would be required.
> I might be persuaded to offer the use of a BBC Micro and to do the
> programming if it is feasable to send me the disks and details of the
> filesystem involved, assuming a more convenient solution doesn't come
> up.

That should be very suitable for the FM SIngle Density ones, as that is 
the same physical format.

Osborne was an ordinary CP/M, which is fairly well documented.  I don't 
have convenient access right now to the parameters.

Basically, there is a boot sector and system track(s) (How many 
"reserved"/system tracks varies).
Then there are sectors with the directory (how many sectors varies)
In the Directory, each file entry is 32 bytes, with the file name and a 
list of blocks that the file occupies on the disk.
If there are more blocks than will fit in the list in the directory entry, 
there is a second entry for the file.
Each block is 8 or 16 128 byte records (which are often called "sectors" 
to confuse you.

On old versions of CP/M, if the machine is available, STAT DSK: will tell 
you most of the parameters.  What it doesn't tell you is the physical 
sector size (how many LOGICAL "sectors" are in each physical sector), 
interleave, and, on double sided disks, whether the second side acts as 
continuation of the first side tracks, or whether the second side tracks 
come after the first side tracks starting at the inner OR outer tracks.
And, there can be numerous other oddities, . . .

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