How It Works:
Partition Tables

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All About Partition Tables and Sectors and FDISK

Here is the most complete description of how partition tables work.

PARTITION SECTOR/RECORD/TABLE BASICS

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 (DOS, OS/2, WinNT, etc) 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). See Master Boot Record.

Second, extended partitions are "nested" inside one another and extended partition table records form a "linked list". I will attempt to show this in a diagram below.

PARTITION TABLE ENTRY FORMAT

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. Warning: older versions of FDISK may compute incorrect LBA or size values. And note: 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 "How It Works—CHS Translation".

There is no central clearing house to assign the codes used in the one byte "type" field, however, there is at least one person at both Microsoft and IBM that attempt to keep track of the type codes. Type codes are 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. Refer to Partition Type Codes for the complete list of known partition type codes is contained in part 2 of this document.

The 16 bytes of a partition table entry are used as follows:


    +--- Bit 7 is the active partition flag, bits 6-0 are
    |    zero (when not zero this byte is also the drive
    |    number of the drive to boot so the active
    |    partition is always found on drive 80H,
    |    the first hard disk).
    |
    |    +--- 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.
   DL DH CL CH TB DH CL CH 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. See Master Boot Record.

These entries define the following partitions:

  1. 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.
  2. 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.
  3. The third and fourth table entries are unused.

PARTITION TABLE RULES

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:

  1. In the MBR there can be 0-4 "primary" partitions, OR, 0-3 primary partitions and 0-1 extended partition entry.
  2. In an extended partition there can be 0-1 "secondary" partition entries and 0-1 extended partition entries.
  3. Only 1 primary partition in the MBR can be marked "active" at any given time.
  4. 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 1. 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.
  5. 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.
  6. 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.

PARTITION NESTING

What do I mean when I say the partitions are "nested" within each other? Lets look at this example:

M = Master Boot Record (and any unused sectorson the same track)

E = Extended partition record (and any unused sectors on the same track) pri = a primary partition (first sector is a "boot" sector) sec = a secondary partition (first sector is a "boot" sector)


   |-----------------the entire disk---------------|
   |                                               |
   |M[pri]                                         |
   |      E[sec]----rest of 1st ext part-----------|
   |            E[sec]----rest of 2nd ext part-----|

The first extended partition is described in the MBR and it occupies the entire disk following the primary partition. The second extended partition is described in the first extended partition record and it occupies the entire disk following the first secondary partition.

PARTITION TABLE LINKING

What do I mean when I say the partition records (tables) form a "linked" list? This means that the MBR has an entry that describes (points to) the first extended partition, the first extended partition table has an entry that describes (points to) the second extended partition table, and so on. There is, in theory, no limited to out long this linked list is. When you ask FDISK to show the DOS "logical drives" it scans the linked list looking for all of the DOS FAT type partitions that may exist. Remember that in an extended partition table, only two entries of the four can be used (rule 2 above).

And one more thing... Within a partition, the layout of the file system data varies greatly. However, the first sector of a partition is expected to be a "boot" sector. A DOS FAT file system has: a boot sector, first FAT sectors, second FAT sectors, root directory sectors and finally the file data area. See OS2 Boot Sector.

EXAMPLE 1

A disk containing four DOS FAT partitions (C, D, E and F):


  |----------------------the entire disk--------------------|
  |                                                         |
  |M[---C:---]                                              |
  |           E[---D:---]--rest of 1st ext part-------------|
  |                      E[---E:---]--rest of 2nd ext part--|
  |                                 E[---------F:----------]|

EXAMPLE 2

So here is an example of a disk with two primary partitions, one DOS FAT and one OS/2 HPFS, plus an extended partition with another DOS FAT:


  |-------------------the entire disk-------------------|
  |                                                     |
  |M[pri 1 - DOS FAT]                                   |
  |                  [pri 2 - OS/2 HPFS]                |
  |                                     E[sec - DOS FAT]|

Or in more detail (‘n’ is the highest cylinder, head or sector number number allowed in the indicated field of the CHS)...


            +-------------------------------------+
 CHS=0,0,1  | Master Boot Record containing       |
            | partition table search program and  |
            | a partition table                   |
            | +---------------------------------+ |
            | | DOS FAT partition description   | | points to CHS=0,1,1
            | +---------------------------------+ | points to CHS=a
            | | OS/2 HPFS partition description | |
            | +---------------------------------+ |
            | | unused table entry              | |
            | +---------------------------------+ |
            | | extended partition entry        | | points to CHS=b
            | +---------------------------------+ |
            +-------------------------------------+
CHS=0,0,2   | the rest of "track 0" -- this is    | :
to          | where the software drivers such as  | : normally
CHS=0,0,n   | Ontrack's Disk Manager or Micro     | : unused
            | House's EZ Drive are located.       | :
            +-------------------------------------+
CHS=0,1,1   | Boot sector for the DOS FAT         | :
            | partition                           | : a DOS FAT
            +-------------------------------------+ : file
CHS=0,1,2   | rest of the DOS FAT partition       | : system
to          | (FAT table, root directory and      | :
CHS=x-1,n,n | user data area)                     | :
            +-------------------------------------+
CHS=x,0,1   | Boot sector for the OS/2 HPFS       | :
            | file system partition               | : an OS/2
            +-------------------------------------+ : HPFS file
CHS=x,0,2   | rest of the OS/2 HPFS file system   | : system
to          | partition                           | :
CHS=y-1,n,n |                                     | :
            +-------------------------------------+
CHS=y,0,1   | Partition record for the extended   |
            | partition containing a partition    |
            | record program (never executed) and |
            | a partition table                   |
            | +---------------------------------+ |
            | | DOS FAT partition description   | | points to CHS=b+1
            | +---------------------------------+ |
            | | unused table entry              | |
            | +---------------------------------+ |
            | | unused table entry              | |
            | +---------------------------------+ |
            | | unused table entry              | |
            | +---------------------------------+ |
            +-------------------------------------+
CHS=y,0,2   | the rest of the first track of the  | : normally
to          | extended partition                  | : unused
CHS=y,0,n   |                                     | :
            +-------------------------------------+
CHS=y,1,1   | Boot sector for the DOS FAT         | :
            | partition                           | : a DOS FAT
            +-------------------------------------+ : file
CHS=y,1,2   | rest of the DOS FAT partition       | : system
to          | (FAT table, root directory and      | :
CHS=n,n,n   | user data area)                     | :
            +-------------------------------------+

EXAMPLE 3

Here is a partition record from an extended partition (the first sector of an extended partition). Note that it contains no program code. It contains only the partition table and the signature data.


   OFFSET 0 1 2 3  4 5 6 7  8 9 A B  C D E F  *0123456789ABCDEF*
   000000 00000000 00000000 00000000 00000000 *................*
   000010 TO 0001af SAME AS ABOVE
   0001b0 00000000 00000000 00000000 00000001 *................*
   0001c0 8195060e fe7d3e00 0000344e 03000000 *.....}....4N....*
   0001d0 00000000 00000000 00000000 00000000 *................*
   0001e0 00000000 00000000 00000000 00000000 *................*
   0001f0 00000000 00000000 00000000 000055aa *..............U.*

NOTES

Thanks to Kenneth C. Yue for pointing out that in V0 of this document I did not properly describe the unused sectors normally found around the partition table sectors.

Thanks to Marcus Better for pointing out that in V1a-c of this document I did not properly describe the x86 registers that the partition table entry data is loaded into when INT 13 is called.

PARTITION TYPE CODES

The following table of partition type codes was compiled from many sources including information from kind people at IBM and Microsoft plus Ralf Brown’s list.

Both IBM and Microsoft keep the partition type code lists and both have until recently assigned type codes. However, there apparently is no formal agreement between these two companies and their lists are not always in sync or up to date. It should be noted that the lists I obtained from IBM and Microsoft are fairly short and show most of type type codes as "available" or "reserved". My guess is that many type codes have been used without the knowledge of the IBM or Microsoft. This is probably the case since neither IBM or Microsoft have published a phone number or email address to contact if you wanted to request a type code. Chaos is the keyword here.

It now appears that neither company is assigning new type codes. I have also been told that it is now recommended that anyone defining a new partition type, or more correctly stated, anyone defining a new file system type, should use partition type 07 and use the first block(s) of the partition to fully define the file system type. Of course, the appearance of a new type code could cause problems for older versions of FDISK, various older operating system device drivers and disk utility programs. (This new use of type code 07 doesn’t explain the recent "assignment" of type codes 0E and 0F.)

[If you know of a type code assignment that is not listed here please let me know about it ASAP. Thanks, Hale]

Note that several type codes have multiple uses (for example, see code 08). Also note that there is some question about the use of some codes as denoted by a ‘?’ in the description.


   Code     Description

   00       Unused partition table entry
   01       DOS, 12-bit FAT
   02       XENIX root
   03       XENIX user
   04       DOS, 16-bit FAT
   05       Extended partition (includes other partition types)
   06       DOS, 32+MB support, up to 64K Allocation unit
   07       See partition boot record(s) for file system type:
               could be QNX, OS/2 HPFS, Windows NT NTFS, Unix, ...
   08       OS/2 (thru Version 1.3 only)
   08       DELL partition spanning multiple drives (array)
   08       Commodore DOS
   08       AIX boot? or file system?
   09       AIX boot? or file system?
   09       Coherent swap
   0A       OS/2 Boot Manager
   0A       OPUS
   0A       Coherent swap
   0B       FAT32
   0C       FAT32 LBA (SEE NOTE BELOW)
   0D       ? (perhaps a type 07 LBA, SEE NOTE BELOW)
   0E       FAT16 LBA (SEE NOTE BELOW)
   0F       Extended partition LBA (SEE NOTE BELOW)

   NOTE:  Partitions types 0C, 0E and 0F (perhaps 0D too)
   REQUIRE that the system’s INT 13 BIOS support the
   IBM/Microsoft/Phoenix extended/enhanced functions calls
   (AH=4x).  In these partition table entries the CHS fields are
   NOT used and are generally set to maximum values (all 1 bits)
   in each CHS field.  What this means is that some day there
   will be an LBA type partition for all the other partition
   types that are listed here and still in use by some system.
   This will certainly use up many of the currently unused type
   codes!

   10       OPUS
   11       OS/2 Boot Manager: Inactive type 01
   12       Compaq diagnostics
   13       Available for assignment
   14       OS/2 Boot Manager: Inactive type 04
   14       Novell DOS 7.0 FDISK (result of bug in FDISK?)
   15       Available for assignment
   16       OS/2 Boot Manager: Inactive type 06
   17       OS/2 Boot Manager: Inactive type 07
   18       AST Windows swap file
   18       Compaq System Diagnostics
   19 - 1F  Available for assignment

   20       Available for assignment
   21       Reserved
   22       Available for assignment
   23       Reserved
   24       NEC version of MS-DOS
   25       Available for assignment
   26       Reserved
   27 - 2F  Available for assignment

   30       Available for assignment
   31       Reserved
   32       Available for assignment
   33       Reserved
   34       Reserved
   35       Available for assignment
   36       Reserved
   37 - 3B  Available for assignment
   3C       PowerQuest PartitionMagic recovery partition
   3D - 3F  Available for assignment

   40       VENIX :Venix 80286
   41       Personal RISC Boot
   41       PowerPC boot
   42       Secure File System (Peter Gutmann)
   43 - 4E  Available for assignment
   4F       Oberon boot and data

   50       OnTrack Disk Manager (read-only)
   51       OnTrack Disk Manager (write-only)
   51       Novell
   51       OnTrack Disk Manager (read-only)
   52       CP/M
   52       Microport
   53       OnTrack Disk Manager (write-only)
   54       OnTrack Disk Manager (DDO)
   55       Available for assignment
   56       GoldenBow VFeature
   57 - 5F  Available for assignment

   60       Available for assignment
   61       SpeedStor
   62       Available for assignment
   63       UNIX System V/386
   63       Mach, MtXinu BSD 4.3 on Mach
   63       GNU HURD
   64       Speedstore
   64       Novell
   65       Novell 286 Netware
   66       Novell 386 Netware
   67       Novell
   68       Novell
   69       Novell
   6A - 6F  Available for assignment

   70       DiskSecure Multi-Boot
   71       Reserved
   72       Available for assignment
   73 - 74  Reserved
   75       PC/IX
   76       Reserved
   77 - 79  Available for assignment
   7A - 7F  ? (probably "available for assignment", these codes
               are not shown in the IBM or Microsoft lists!)

   80       Minix (ver. 1.4a and earlier)
   81       Minix (ver. 1.4b and later)
   81       Mitac Advanced Disk Manager
   81       Linux
   82       Prime
   82       Linux swap
   82       Solaris UNIX
   83       Linux ext2fs
   84       OS/2 hiding a type 04
   85       Available for assignment
   86       Reserved
   87       HPFS FT mirrored partition
   88 - 8F  Available for assignment

   90 - 92  Available for assignment
   93       Ameba file system
   94       Ameba bad block table
   95 - 98  Available for assignment
   99       Mylex EISA SCSI
   9A - 9F  Available for assignment

   A0       Available for assignment
   A1       Reserved
   A2       Available for assignment
   A3 - A4  Reserved
   A5       FreeBSD
   A6       Reserved
   A7 - A8  Available for assignment
   A9       NetBSD
   AA - AF  Available for assignment

   B0       Available for assignment
   B1       Reserved
   B2       Available for assignment
   B3 - B4  Reserved
   B5       Available for assignment
   B6       Reserved
   B7       BSDI file system or secondarily swap
   B8       BSDI swap or secondarily file system
   B9 - BD  Available for assignment
   BE       Solaris boot partition
   BF       Available for assignment

   C0       Available for assignment
   C1       DR-DOS LOGIN.EXE-secured 12-bit FAT
   C2       BlueSky Innovations boot manager
   C3       BlueSky Innovations boot manager
   C4       DR-DOS LOGIN.EXE-secured 16-bit FAT
   C5       Available for assignment
   C6       DR-DOS LOGIN.EXE-secured Huge
   C7       HPFS FT disabled mirrored partition
   C7       Cyrnix Boot
   C8 - CF  Available for assignment

   D0 - D7  Available for assignment
   D8       CP/M 86
   D9 - DA  Available for assignment
   DB       Concurrent DOS, CP/M and CTOS
   DC - DF  Available for assignment

   E0       Available for assignment
   E1       Speedstore
   E2       Available for assignment
   E3       Storage Dimensions
   E4       Speedstore
   E5 - E6  Reserved
   E7 - EA  Available for assignment
   EB       BeOS BFS
   EC - EF  Available for assignment

   F0       Available for assignment
   F1       Storage Dimensions
   F2       DOS 3.3+ second partition
   F3       Reserved
   F4       Speedstore
   F4       Storage Dimensions
   F5       Available for assignment
   F6       Reserved
   F7 - FD  Available for assignment
   FE       IBM PS/2 IML, LANstep
   FF       Xenix(?) Bad Block Tables

This page was last updated on 05 October 1999.