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From: jcmorris@mwunix.mitre.org (Joe Morris)
Subject: Re: Partition table
Message-ID: <jcmorris.670772819@mwunix.mitre.org>
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References: <k1DTZ1w163w@unixland.uucp>
Date:  4 Apr 91 13:46:59 GMT

achilles@unixland.uucp (David Holland) writes:

>Owing to some recent computer trouble I've been poking somewhat deeper into 
>my system than usual, and I noticed the following interesting oddity about 
>the partition table: there are only two partitions listed, one of which is 
>twice as large as the other, and only one of them is marked DOS. The disk 
>contains three equal-sized partitions, all of which are DOS; where is this 
>information stored? Or does Norton's partition-table editor mislead me?

The documentation for Norton (at least at 4.5; I haven't had to look it up
in the 5.0 doc) isn't clear on the structure of a (physical) disk with
multiple partitions.  If you have a single physical disk with three DOS
partitions filling it, the partition table as shown by Norton will display
only the first DOS partition and a second partition marked "EXTENDED".
If you read the starting location of the EXTENDED partition from the
display and then display *that* record (using F6 to display it as a
partition table) you'll find that this partition table has one DOS
partition and one extended partition.  Chain to the start location
of this second extended partition and display it; you'll see a partition
table with only a DOS partition.  The three partition tables marked as
DOS partitions are the C, D, and E drives; the extended partitions cover
the space occupied by all lower partitions.

I don't recall ever seeing it documented but apparently you can't have two
DOS partitions in the same partition table.

Here's what your disk should look like:

  Master Partition                                  Real Disk Drive
  Table (cyl 0 head 0
  record 1):           < - - - - - - - - - - - -
        |                                        \
        V                                          \   Start real disk
                                                     \ C0:H0:R1
       DOS-16    - - - - - - - - - - - - - - - - - ->  Start C:
       EXTENDED                \ \ \ \ \ \ \ \ \ \ \      .
              \                  \ \ \ \ \ \ \ \ \ \      .
                \                  - - - - - - - - ->  End C:
                  \
                     - - - - - - - - - - - - - - - ->  Start First
                                                       Extended Partition

                Second Partition       - - - - - - ->  Start D:
                Table (-> by MPT)    / / / / / / / /
                       |            // / / / / / / /
                       V           / / / / / / / / /
                                  // / / / / / / / /
                    DOS-16 - - - - - - - - - - - - ->  End D:
                    EXTENDED - - - - - - - - - - - ->  Start Second
                                                       Extended Partition

                          Third Partition        - ->  Start E:
                          Table (-> by 2nd PT)  // /
                                    |          / / /
                                    V         // / /
                                             / / / /
                                  DOS-16 - - - - - ->  End E:
                                                       End second ext. part.
                                                       End first ext. part
                                                       End real disk

Note that the four last lines (the end of E:, the end of the two extended
partitions, and the end of the real disk) all occur at the last track of the
last cylinder of the physical disk.

Note also that the partition table in each partition is located at the
first record on the first track of the first cylinder in that partition.
In effect this is the same rule as is used for the master partition table,
which is always on the first record of the physical disk.  Also, note
that the master partition table occupies the same record (0/0/1) as the
master boot record.

Quiz on Friday.

Joe
