Date : Thu, 01 Jul 1999 09:01:10 +0100
From : jgh@... (Jonathan Graham Harston)
Subject: BBC software to grab DFS disc images
Mr M F Scholes <mavhc@...> wrote:
> On Mon, 14 Jun 1999, Steven Flintham wrote:
> > > - you'd need to extend the format to tell any reading tool how many
> > > tracks there are (if you'd like to support 40T).
> > I'm pretty sure this information (more precisely, the number of
> > sectors) is embedded in the first two sectors of track 0 on each side.
> Yes Track 0, Sector 1 Bytes 06 and 07, the high nibble of btye 06 is the
> opt, the low nibble*256+byte 07 = number of sectors on one side a the
> disc, usually &320 for 80 track and &190 for 40 track.
I feel I must clarify a very important point here. No filing system EVER
knows how many tracks there are on a disk or disk drive, and there is no
way it can find out. Also, there is NO WAY a filing system can find out
whether a drive is 3.5" or 5.25" (or even 8"!). The only way a program
can find this information is by asking the user.
What is stored in the catalogue is the size of the dataspace on the disk.
There is absolutely nothing to stop this size being smaller than the
actual space on the disk, or even bigger! Bytes 00:1:06/7 being set to
&190 does not mean that it is a 40-track disk. The ONLY thing it means is
that the dataspace that that catalogue refers to is &19000 bytes long.
Of course, a standard disk formatter will put &190 on a disk formatted
with 40 tracks and &320 on a disk formatted with 80 tracks, but only
because that is the amount of space that fits on a disk formatted to this
size.
There are many instances of programs that for one reason or another set
the disk-size bytes to a value other than the maximum that will fit on the
disk. For instance, information could be 'hidden' from the filing system
by being placed in sectors 'after' the end of the dataspace.
--
J.G.Harston (JGH BBC PD Library) 70 Camm Street, Walkley, SHEFFIELD S6 3TR
jgh@... - Running on BBCs & Masters with SJ MDFS FileServer
Z80+6502/CoPro+Tubes/Econet+SJ -- http://mdfs.net/
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