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FDISK creates all partition records (sectors). The primary purpose of a
partition record is to hold a partition table. The rules for how FDISK
works are unwritten but so far most FDISK programs seem to follow the
same basic idea.
First, all partition table records (sectors) have the same format. This
includes the partition table record at cylinder 0, head 0, sector 1 --
what is known as the Master Boot Record (MBR). The last 66 bytes of a
partition table record contain a partition table and a 2 byte
signature. The first 446 bytes of these sectors usually contain a
program but only the program in the MBR is ever executed (so extended
partition table records could contain something other than a program in
the first 466 bytes). For more information, see section The structure of Master Boot Record.
Second, extended partitions are nested inside one another and
extended partition table records form a linked list. We will
attempt to show this in a diagram at section The format of the table entry.
Each partition table entry is 16 bytes and contains things like the
start and end location of a partition in CHS, the start in LBA, the size
in sectors, the partition type and the active flag. Older
versions of FDISK may compute incorrect LBA or size values. And when
your computer boots itself, only the CHS fields of the partition table
entries are used (another reason LBA doesn't solve the >528MB
problem). The CHS fields in the partition tables are in L-CHS format,
see section CHS addressing and LBA addressing.
There is no central clearing house to assign the codes used in the one
byte type field. But codes are assigned (or used) to define most
every type of file system that anyone has ever implemented on the x86
PC: 12-bit FAT, 16-bit FAT, HPFS, NTFS, etc. Plus, an extended partition
also has a unique type code.
In the FDISK program `sfdisk', the following list is assumed:
- 00
-
Empty
- 01
-
DOS 12-bit FAT
- 02
-
XENIX /
- 03
-
XENIX /usr
- 04
-
DOS 16-bit FAT <32M
- 05
-
DOS Extended
- 06
-
DOS 16-bit FAT >=32M
- 07
-
HPFS / NTFS
- 08
-
AIX boot or SplitDrive
- 09
-
AIX data or Coherent
- 0A
-
OS/2 Boot Manager
- 0B
-
Windows95 FAT32
- 0C
-
Windows95 FAT32 (LBA)
- 0E
-
Windows95 FAT16 (LBA)
- 0F
-
Windows95 Extended (LBA)
- 10
-
OPUS
- 11
-
Hidden DOS FAT12
- 12
-
Compaq diagnostics
- 14
-
Hidden DOS FAT16
- 16
-
Hidden DOS FAT16 (big)
- 17
-
Hidden HPFS/NTFS
- 18
-
AST Windows swapfile
- 24
-
NEC DOS
- 3C
-
PartitionMagic recovery
- 40
-
Venix 80286
- 41
-
Linux/MINIX (sharing disk with DRDOS)
- 42
-
SFS or Linux swap (sharing disk with DRDOS)
- 43
-
Linux native (sharing disk with DRDOS)
- 50
-
DM (disk manager)
- 51
-
DM6 Aux1 (or Novell)
- 52
-
CP/M or Microsoft SysV/AT
- 53
-
DM6 Aux3
- 54
-
DM6
- 55
-
EZ-Drive (disk manager)
- 56
-
Golden Bow (disk manager)
- 5C
-
Priam Edisk (disk manager)
- 61
-
SpeedStor
- 63
-
GNU Hurd or Mach or Sys V/386 (such as ISC UNIX)(14)
- 64
-
Novell Netware 286
- 65
-
Novell Netware 386
- 70
-
DiskSecure Multi-Boot
- 75
-
PC/IX
- 77
-
QNX4.x
- 78
-
QNX4.x 2nd part
- 79
-
QNX4.x 3rd part
- 80
-
MINIX until 1.4a
- 81
-
MINIX / old Linux
- 82
-
Linux swap
- 83
-
Linux native(15)
- 84
-
OS/2 hidden C: drive
- 85
-
Linux extended
- 86
-
NTFS volume set
- 87
-
NTFS volume set
- 93
-
Amoeba
- 94
-
Amoeba BBT
- A0
-
IBM Thinkpad hibernation
- A5
-
BSD/386
- A7
-
NeXTSTEP 486
- B7
-
BSDI fs
- B8
-
BSDI swap
- C1
-
DRDOS/sec (FAT-12)
- C4
-
DRDOS/sec (FAT-16, < 32M)
- C6
-
DRDOS/sec (FAT-16, >= 32M)
- C7
-
Syrinx
- DB
-
CP/M or Concurrent CP/M or Concurrent DOS or CTOS
- E1
-
DOS access or SpeedStor 12-bit FAT extended partition
- E3
-
DOS R/O or SpeedStor
- E4
-
SpeedStor 16-bit FAT extended partition < 1024 cyl.
- F1
-
SpeedStor
- F2
-
DOS 3.3+ secondary
- F4
-
SpeedStor large partition
- FE
-
SpeedStor >1024 cyl. or LANstep
- FF
-
Xenix Bad Block Table
The 16 bytes of a partition table entry are used as follows:
+--- Bit 7 is the active partition flag, bits 6-0 are zero.
|
| +--- Starting CHS in INT 13 call format.
| |
| | +--- Partition type byte.
| | |
| | | +--- Ending CHS in INT 13 call format.
| | | |
| | | | +-- Starting LBA.
| | | | |
| | | | | +-- Size in sectors.
| | | | | |
v <--+---> v <--+--> v v
0 1 2 3 4 5 6 7 8 9 A B C D E F
DH DL CH CL TB DL CH CL LBA..... SIZE....
80 01 01 00 06 0e be 94 3e000000 0c610900 1st entry
00 00 81 95 05 0e fe 7d 4a610900 724e0300 2nd entry
00 00 00 00 00 00 00 00 00000000 00000000 3rd entry
00 00 00 00 00 00 00 00 00000000 00000000 4th entry
Bytes 0-3 are used by the small program in the Master Boot Record to
read the first sector of an active partition into memory. The DH,
DL, CH and CL above show which x86 register is loaded
when the MBR program calls INT 13H AH=02h to read the active partition's
boot sector. For more information, see section The structure of Master Boot Record.
These entries define the following partitions:
-
The first partition, a primary partition DOS FAT, starts at CHS 0H,1H,1H
(LBA 3EH) and ends at CHS 294H,EH,3EH with a size of 9610CH sectors.
-
The second partition, an extended partition, starts at CHS 295H,0H,1H
(LBA 9614AH) and ends at CHS 37DH,EH,3EH with a size of 34E72H sectors.
-
The third and fourth table entries are unused.
Keep in mind that there are no written rules and no
industry standards on how FDISK should work but here are some basic
rules that seem to be followed by most versions of FDISK:
-
In the MBR there can be 0-4 primary partitions, OR, 0-3 primary
partitions and 0-1 extended partition entry.
-
In an extended partition there can be 0-1 secondary partition
entries and 0-1 extended partition entries.
-
Only 1 primary partition in the MBR can be marked active at any
given time.
-
In most versions of FDISK, the first sector of a partition will be
aligned such that it is at head 0, sector 1 of a cylinder. This means
that there may be unused sectors on the track(s) prior to the first
sector of a partition and that there may be unused sectors following a
partition table sector.
For example, most new versions of FDISK start the first partition
(primary or extended) at cylinder 0, head 1, sector 0. This leaves the
sectors at cylinder 0, head 0, sectors 2...n as unused sectors. This
same layout may be seen on the first track of an extended partition.
See example 2 below.
Also note that software drivers like Ontrack's Disk Manager depend on
these unused sectors because these drivers will hide their code
there (in cylinder 0, head 0, sectors 2...n). This is also a good place
for boot sector virus programs to hang out.
-
The partition table entries (slots) can be used in any order. Some
versions of FDISK fill the table from the bottom up and some versions of
FDISK fill the table from the top down. Deleting a partition can leave
an unused entry (slot) in the middle of a table.
-
And then there is the hack that some newer OS's (OS/2 and Linux)
use in order to place a partition spanning or passed cylinder 1024 on a
system that does not have a CHS translating BIOS. These systems create a
partition table entry with the partition's starting and ending CHS
information set to all FFH. The starting and ending LBA information is
used to describe the location of the partition. The LBA can be converted
back to a CHS -- most likely a CHS with more than 1024 cylinders. Since
such a CHS can't be used by the system BIOS, these partitions can not be
booted or accessed until the OS's kernel and hard disk device drivers
are loaded. It is not known if the systems using this hack follow
the same rules for the creation of these type of partitions.
There are no written rules as to how an OS scans the partition
table entries so each OS can have a different method. For DOS, this
means that different versions could assign different drive letters to
the same FAT file system partitions.
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