Date   : Mon, 21 Aug 2006 22:59:37 +0100
From   : Andrew Benham <adsb@...>
Subject: Hard disk backup - another challenge

I found some old BBC code of mine today which streamed the active
sectors of a hard disk to the serial port using xmodem.  I got
it working OK, and it manages to transfer 4 MegaBytes an hour at
19,200 baud.  So that's a hard disk sitting on a 5 MegaBytes per
second SCSI bus, and 4 MegaBytes an hour to stream it off...  Well,
we are using 20 year-old machines.

Anyway, part way through transferring 17.5 MegaBytes of data, I
looked at the file accumulating on the more modern machine, and
was surprised to see the program was transferring loads of
sequential 'blank' sectors.  The disk was fully *COMPACTed before
I started, and I stop transferring when I hit the start of the free
space at the end of the drive map.  So all the sectors being
transferred should be 'active' ones.

Ah, that'll be the 10 MegaByte $.DOS.DRIVE_C file then - i.e. the
file used by the 80186 Co-Processor to provide the DOS+ file system.
That DOS+ file system is nothing like full, so there's loads of
sectors which I don't need to backup.

Looks like my backup program needs to not only understand the ADFS
free space, but also the free space in the $.DOS.DRIVE_C file too...

-- 
Andrew Benham         adsb@...       
Southgate, London N14, United Kingdom

The gates in my computer are AND OR and NOT, not "Bill"