The master boot record is always located on sector 1 of the first track (track 0) on the disk. The following table shows the layout of the various components inside the Master Boot Record. The various components are described below.

┌──────────┬──────────────────────────────┬────────────────────┐
│Offset    │Description                   │Size                │
├──────────┼──────────────────────────────┼────────────────────┤
│+0        │Master Boot Record Program    │446 bytes           │
├──────────┼──────────────────────────────┼────────────────────┤
│+446      │Partition Table               │64 bytes            │
├──────────┼──────────────────────────────┼────────────────────┤
│+510      │Signature (55AAH)             │2 bytes             │
└──────────┴──────────────────────────────┴────────────────────┘

Master Boot Record Program

This code is given control from BIOS during boot. Its function is to load the operating system's boot program from the partition that was marked as being startable and turn control over to the (assumed) code that was loaded.

The Master Boot Record Program may be placed on the disk by individual operating systems. If the signature in the Master Boot Record is valid, then the Master Boot Record Program must not be modified. Operating systems must not place requirements on nor make assumptions about the Master Boot Record Program.

Partition Table
This is a vector of 4 structures that allows the disk to be divided up into four distinct areas or partitions. The following table shows how they are arranged in this vector.

┌──────────────────┬──────────────────┬──────────────────┐
│Offset            │Description       │Size              │
├──────────────────┼──────────────────┼──────────────────┤
│0                 │Partition 1       │16 bytes          │
├──────────────────┼──────────────────┼──────────────────┤
│16                │Partition 2       │16 bytes          │
├──────────────────┼──────────────────┼──────────────────┤
│32                │Partition 3       │16 bytes          │
├──────────────────┼──────────────────┼──────────────────┤
│48                │Partition 4       │16 bytes          │
└──────────────────┴──────────────────┴──────────────────┘

It is up to an individual operating system if one of those parts is to be further sub-divided. For example, DOS Version 3.30 implemented a scheme where an "extended partition" could be used to define logical disks to allow the use of larger hardfiles.

The following table shows the format of the individual entries in the partition table. A description of the individual fields follows.

┌──────┬─────────────┬─────────┬─────────┬─────────┬─────────┐
│Offset│ Description │    0    │    1    │    2    │    3    │
├──────┼─────────────┼─────────┼─────────┼─────────┼─────────┤
│  +0  │Partion Start│  Boot   │  Head   │ Sector  │Cylinder │
│      │             │Indicator│         │         │         │
├──────┼─────────────┼─────────┼─────────┼─────────┼─────────┤
│  +4  │Partion End  │ System  │  Head   │ Sector  │Cylinder │
│      │             │Indicator│         │         │         │
├──────┼─────────────┼─────────┴─────────┼─────────┴─────────┤
│      │ Offset from │                   │                   │
│  +8  │start of disk│     Low Word      │     High Word     │
│      │ (sectors)   │                   │                   │
├──────┼─────────────┼───────────────────┼───────────────────┤
│      │  Partion    │                   │                   │
│ +12  │  Length     │     Low Word      │     High Word     │
│      │ (sectors)   │                   │                   │
└──────┴─────────────┴───────────────────┴───────────────────┘

Partition Start

This 4 byte field identifies the beginning of a partition. It also contains an indicator that flags the partition as being active or bootable. This field is composed of several bytes defined as follows.
Boot Indicator
This byte indicates if the partition is active. If the byte contains 00H, then the partition is not active and will not be considered as bootable by the Master Boot Record Boot Program. If the byte contains 80H, then the partition is considered active. The Master Boot Record Boot Program will then attempt to load the first sector described by this partition table entry and transfer control to it. The Master Boot Record Boot Program should only attempt to boot the first partition it finds that is marked active.
Head
This byte contains the number of the first head of the partition.

All partitions are allocated in cylinder multiples and begin on sector 1, head 0.

EXCEPTION: The partition that is allocated at the beginning of the disk should start at cylinder 0, head 1, sector 1, to leave room for the disk's master boot record and other information used to define the fixed disk type on that system. An operating system should not use any data space on cylinder 0 head 0 of a fixed disk.

Sector
This byte contains the sector number of the first sector of the partition. This value should always be 1 (sector numbers are 1 based) for the Partition Begin field because partitions are defined to start on cylinder boundaries. Note that the sector number byte also contains the high order 2 bits of the cylinder number in the high order 2 bits of this byte. Therefore, this byte can have values other than one, but the sector bits of this byte always contains the value 1.
Cylinder
This byte contains the low order 8 bits of the 10 bit cylinder number that indicates the starting cylinder of the partition.
Partition End
This 4 byte field identifies the end of the partition. It also contains an indicator as to which operating system owns the partition. This field is composed of several bytes that are defined as follows.
System Indicator
This byte indicates what operating system owns the particular partition. The values and what they represent are listed in Fixed Disk Partition ID Assignments. A value of 0 indicates an unused entry.
Head
This byte contains the last head number in the last cylinder occupied by this partition.
Sector
This byte contains the sector number of the last sector on the last cylinder occupied by this partition. It also contains the high order two bits of the cylinder number in the high two bits of this byte.
Cylinder
This byte contains the low order 8 bits of the 10-bit cylinder number that indicates the ending cylinder of this partition.
Offset from Start of Disk
This 4-byte field contains the number of sectors preceding each partition on the disk. The value is obtained by counting the sectors beginning with cylinder 0, sector 1, head 0 of the disk, and incrementing the sector, head, and then cylinder values up to the beginning of the partition. Thus, if the disk has 17 sectors per track and 4 heads, and the second partition begins at cylinder 1, sector 1, head 0, the partition's starting relative sector is 68 (decimal)-there were 17 sectors on each of 4 heads on 1 track allocated ahead of it. The field is stored with the least significant word first.
Partition Length
This 4 byte field contains the number of sectors allocated to the partition. This field is stored least significant word first.
Signature
The last 2 bytes of the boot record (55AAH) are used as a signature to identify a valid boot record containing code that is executable on Intel X86 processors. Both this record and the partition boot records are required to contain the signature at offset 01FEH (510).

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