[Coco] Re: Re: Re: Disk Basic and 512 byte block floppies.

[Brett K. Heath]

On Wed, 21 Jan 2004 KnudsenMJ at aol.com wrote:

> In a message dated 1/20/04 10:06:38 PM Eastern Standard Time,
> alxevans at concentric.net writes:
>
> > All CP/M disk formats use virtual 128 byte sectors, and a few (very
> >  few) use physical 128 byte sectors.
>
> ISTR that in CP/M, the 128th byte of each sector points to the next sector in
> the file, so you get only 127 bytes per sector, and some real opportunities
> to screw up your file system :-)  Also, this would limit a disk to 256 sectors
> total, so maybe I'm not remembering quite right.  --Mike K.


No, not quite. Most versions of CP/M used the IBM standards for disk
geometry. Track 0 was always single density (128 bytes per sector) the
geometry of subsequent tracks was specified in the boot information found
on Track 0. AIR there were 4 standard formats ranging from 26 sectors of
128 bytes on each track to 8 sectors of 1024 bytes for each track (with
both sides of track 0 being restricted to 128 bytes per sector).

To decrease the size of directory entries the sectors were grouped into
allocation blocks ranging in size from 1k (or less) for single density
single side disks to 2k or even 4k for double sided double density disks
(IIRC 2k allocation blocks were most common).

Each directory entry contained the name of the file, the user number for
this file, the sequential number of the current entry in the list of
entries for this file, various other housekeeping info, and a list of up
to 16 allocation blocks associated with this entry. If this was the last
entry for this file then it would also include information about how many
allocation blocks and how many bytes in the last allocation block listed
would actually be used.

That's probably more than you needed (or wanted) to know but what it means
is that all information about file structure was kept on the directory
tracks.

HTH

Brett K. Heath