Date   : Mon, 21 Aug 2006 23:18:29 +0000
From   : Jules Richardson <julesrichardsonuk@...>
Subject: Re: Hard disk backup - another challenge

Andrew Benham wrote:
> 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.

:-)

Using a SCSI hard disk and the Acorn controller? Or a ST506 disk, Adaptec 
bridge, and the Acorn controller? Or something else entirely?

I bet you'll get a few tens of KB/s at most between a BBC and a ST506 drive 
via an Adaptec bridge...

> 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...

Might be worth scanning the sector on the beeb side prior to transmit; if it's 
all the same data then just send some sort of token to indicate this rather 
than sending an entire sector. That does depend on the nature of your 'empty' 
sectors, though.

cheers

Jules