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Date: Thu, 28 Sep 1995 01:09:52 +1000
From: Bruce Evans <bde@zeta.org.au>
Message-Id: <199509271509.BAA28013@godzilla.zeta.org.au>
To: bde@zeta.org.au, nate@rocky.sri.MT.net
Subject: Re: Diskslice naming convention?
Cc: bde@FreeBSD.org, hackers@FreeBSD.org
Sender: owner-hackers@FreeBSD.org
Precedence: bulk

>Ok.  How do the 'fdisk' slices (of which there can only be 4) relate to
>the 30 slices available in FreeBSD.  Excuse me for using DOS terminology

The `fdisk' slices (primary DOS partitions) are the first 4 of the 30.

>for a minute, but if I understand correctly, we can use 'extended'
>partitions inside of 'primary' partitions.  However, 30/4 isn't a real
>number, so where does the magic number 30 come from?

Extended partitions aren't inside primary partitions.  Each primary partition
may be an extended partition.  Each extended partition begins with a
secondary boot record with a partition table in it.  The 4 entries in the
partition table may be `logical drives' or extended partitions.  Each
extended partition begins with a secondary boot record...

I decided that 5 bits in the minor number were enough to reserve for the
slice number.  2^5 = 32 and 2 slice numbers are special.

>> `dd if=/dev/rxd#' reads the entire disk.  /dev/rxd# is a completely
>> different device from /dev/rxd#c.  These devices are often confused
>> because disklabel automatically translates from `xd#' to /dev/rxd#c'.

>Ahh, so these are equivalent

># dd if=/dev/sd0c of=/dev/null
># dd if=/dev/sd0  of=/dev/null

>(Read the entire slice)

No, the devices are completely different, as was just explained.

>> You can't create a FreeBSD partition which accesses a DOS slice.  Just
>> access the DOS slice directly.

>Ok.  How do I know which slice is the DOS slice?  (This get's back to
>the determination of the numbering scheme 1-30)

Look at sysinstall or libdisk/tst01 output for slices labeled as `fat".
The numbering corresponds to a particular linearization of the tree of
extended partitions so the relative order may vary with the OS.

Bruce