Support for creation of DOS partitions (extended/logical) 4/17/96
=========================================================
This file contains some informal documentation about the changes that I
introduced in mkfp in order to support creation of DOS partitions on various
devices. Earlier versions of mkfp merely supported creation of one large
DOS partition on floppies and flopticals. Now, with the changes that I've
introduced, one can:
(1) Create DOS Type 6 (Huge partitions).
(2) Create DOS Type 5 (Extended partitions, with logical volumes inside).
on a variety of devices such as PC cards, syquest, zip, hard-drives. My
addition doesn't override any previously existing functionality, but rather
adds to it. Currently, floppies and flopticals are still formatted by the
old code (that doesn't allow partitions to be placed on them).
What this program is not
========================
mkfp is not a partition table manipulator. Each invocation of mkfp will lead
to the device being reformatted, and new partitions being placed on it. It is
not possible to modify the sizes/types of existing DOS partitions using mkfp.
Indexing of partitions
======================
A user could specify an arbitrary number of DOS partitions that are to be
created (with the -p option). Each partition that is created has an index,
which is also used by mount_dos to identify partitions.
The first three partitions that are requested to be created result in regular
primary partitions. They have indices 1, 2, 3. The fourth and subsequent
partitions that are created, are actually logical volumes that are created
inside an extended partition. The index of the extended partition is 4, and
the logical partitions inside this would be 5, 6, 7....
Hence a mkfp request to create (say) 5 DOS partitions would result in:
partitions with index: 1, 2, 3, 5, 6 being created.
partition: 1, 2, 3 - Regular primary partition.
partition: 5, 6 - Logical volumes inside extended partition.
partition: 4 - Extended partition, Implicitly created.
Since DOS has exactly four partition table entries in the primary partition
table, it is necessary to create an extended partition and logical volumes
within this, when the user requests more than four partitions to be created.
Implementation Notes
====================
Initially, a DOS formatting/partitioning utility was developed as a standalone
tool and then it was merged with mkfp. The bulk of this can be found in the
files:
(1) part.c - Routines to handle partition table creation.
(2) device.c - Routines to perform SCSI query.
(3) dos.c - Routines to create dos file system inside partitions.
(4) misc.c - Routines for other miscellaneous purposes.
When primary partitions are created, entries are placed in the primary
partition table. The starting head/sector/cylinder and the ending head/sector/
cylinder for the partition needs be stored in the partition table entry. Along
with this, information about the starting logical sector number as well as the
total number of sectors in the partition have got to be stored in partition
table entry. Partitions are cylinder aligned.
DOS doesn't use the physical disk geometry. Instead, it *assumes* a fake
geometry, that depends upon the capacity of the device in question. The
logic used in Adaptec is:
Capacity >= 1Gbyte => heads = 255, sectors = 63, compute C = cyl.
Capacity < 1Gbyte => heads = 64, sectors = 32, compute C = cyl.
Starting head number for:
Primary partition #1: starting head = 1.
Primary partition #2, #3, #4: starting head = 0.
Logical partition #5, #6, ...: starting head = 1.
Logical partitions
==================
In order to get around the fact that can be only four primary partitions,
one could create an extended primary partition, within which any number
of logical partitions can be created. Logical partitions are slightly
different from primary partitions. They form a linked list of sorts.
Each logical partition has a partition table that has two meaningful
partition table entries. The first entry has information about the start
of the current partition (with respect to the beginning of this partition
table) as well as the number of sectors in it. The second entry has some
information about the next logical partition (Its offset with respect to
the beginning of the extended partition).
Below is a sample extended partition, with three logical volumes, with
indices: 5, 6, 7.
Here
p1, p2 are the first two partition table entries, in Log. Vol (5)
q1, q2 are the first two partition table entries, in Log. Vol (6)
r1, r2 are the first two partition table entries, in Log. Vol (7)
<-------------------- Extended Partition ------------------------>
<---- Log Vol (5)------><---- Log Vol (6)---><---- Log Vol (7)--->
---+-------+-------+------+------+------+------+------+------+------+------
|Partn | |Data |Partn | |Data |Partn | |Data |
|Tbl | |Area |Tbl | |Area |Tbl | |Area |
| | | | | | | | | |
|-------| | |------| | |------| | |
|p1 | | |q1 | | |r1 | | |
|-------| | |------| | |------| | |
|p2 | | |q2 | | |r2 | | |
---+-------+-------+------+------+------+------+------+------+------+------
| | | | | | |
|<------------->|<---->|<----------->|<---->|<----------->|<---->|
| p1->start p1->num| q1->start q2->num| r1->start r1->num|
| | |
|<-------------------->| |
| p2->start |
|<----------------------------------------->|
| q2->start
p1, q1, r1 contain information about the partitions: 5, 6, 7.
p2, q2, r2 contain information about the "next" logical partition: 6, 7, 8.
p1->start = starting sector of partition 5, with respect to partn. tbl 5.
p1->num = numbr of sectors in partition 5.
p2->start = starting sector of partition 6
with respect to start of extended partition.
q1->start = starting sector of partition 6, with respect to partn. tbl 6.
q1->num = numbr of sectors in partition 6.
q2->start = starting sector of partition 7
with respect to start of extended partition.
r1->start = starting sector of partition 7, with respect to partn. tbl 7.
r1->num = numbr of sectors in partition 7.
r2->start = zero (last logical volume)
The offset of the data portion of each logical partition, with respect to
the beginning of its partition table, is always one track size. Please note
that there can be only one extended partition on any media.
