Re: Slot 'extender' dev cable

["mdj" <mdj.mdj@gmail.com>]

On Feb 4, 1:13 am, pau...@saaf.se (Paul Schlyter) wrote:

> CP/M had a *documented* way of acheiving this, described in the CP/M
> alteration guide: each logical disk had a Disk Parameter Table, where
> you could specify things as the number of logical sectors (or "blocks"
> in ProDOS jargon - a CP/M "block" was only 128 bytes though), the
> number of entries in the directory (there were no subdirectories), the
> interleave factor, the number of system tracks, the cluster size, etc.
> Nowhere on the CP/M disk there was any info about the disk geometry of
> that disk.

The logical structure of CP/M and ProDOS is actually quite similar. CP/
M provides the CBIOS layer which provides basic device abstraction. In
ProDOS this interface is either implemented in firmware on a storage
card, or by ProDOS' built in Disk ][ and RAMDisk drivers.

> ....but what happens if you then insert a 35-track floppy?  I assume
> reading it would be OK, but if you tried to write it, could your patched
> ProDOS then attempt to write beyond the last (35'th) track?

There's no problem. ProDOS volumes contain a VBM (Volume Bit Map)
which tracks the available blocks and ultimately the devices size.

> In CP/M you could set up two different CP/M volumes to refer to the
> same physcial disk - the two volumes could then specify different disk
> geometries for that physical disk.

In the case of 40 track 5.25" there's no need under ProDOS or DOS3.3
to worry about disks created on smaller drives. I've personally never
bothered with utilising the extra space, as the non-portability of the
resulting disks to "real" Apple drives isn't offset by the greater
capacity (IMO)

Matt