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From: Robert Nordier <rnordier@iafrica.com>
Message-Id: <199608291405.QAA00992@eac.iafrica.com>
Subject: Re: Determining disk type & size
To: bde@zeta.org.au (Bruce Evans)
Date: Thu, 29 Aug 1996 16:05:18 +0200 (SAT)
Cc: hackers@freebsd.org
In-Reply-To: <199608290758.RAA01914@godzilla.zeta.org.au> from "Bruce Evans" at Aug 29, 96 05:58:54 pm
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Bruce Evans wrote:
> 
> >>./fdtest /dev/fd0
> >>sectors per cylinder: 158 [<-- 30 expected]
> 
> >>   if (ioctl(fd, DIOCGDINFO, &dl) == -1)
> >>      err(1, argv[1]);
> >>   printf("sectors per cylinder: %lu\n", dl.d_secpercyl);
> 
> I wrote:
> >DIOCGDINFO for floppies just returns whatever happens to be in the label
> >area on the disk (*).  The label is garbage unless the disk actually has
> >a label.  There is no way to tell from the ioctl() return value whether
> >the label is garbage.
> 
> Oops, DIOCGDINFO checks the label magic number and checksum, so
> d_secpercyl is probably correct if the label is valid.  However,
> it's extremely unlikely that a DOS floppy would contain a valid label.
> Sector 2 normally contains part of the FAT, and it is very unlikely that
> the bytes in the FAT form a valid label.

That clarifies it.  Thanks.

-- 
Robert Nordier