Technical information

10.5.2 Example of layout with DOS/FAT extended partitions

In the following example we have a hard disk with 3 primary partitions and an extended partition that

contains two embedded primary partitions. The Master Boot Record contains 3 pointers to the 3

standard partitions and a pointer to the extended partition. The first logical sector of the extended

partition contains the first Extended Master Boot Record. This EMBR contains in turn a pointer to the

primary partition and a pointer to the next Extended Master Boot Record. This second EMBR

contains in turn a pointer to a primary partition.

10.6 DOS/FAT Partition Structures

10.6.1 DOS/FAT Boot sector

The boot sector is the first logical sector of a logical drive and it occupies one logical sector. The

grayed areas are read from the boot sector and stored in the BIOS Parameter Block (BPB).

Name

Offset

Length

Description

BRA

0x00

Jump instruction. This instruction will be executed and will skip past the rest of the

(non-executable) header if the partition is booted from.

OEM

0x03

OEM Name (padded with spaces). This value determines in which system disk

was formatted. MS-DOS checks this field to determine which other parts of the

boot record can be relied on.

BPS

0x0b

Bytes per Sector. A common value is 512, especially for file systems on IDE (or

compatible) disks. The BIOS Parameter Block starts here.

SPC

0x0d

Sectors per Cluster. Allowed values are powers of two from 1 to 128. However,

the value must not be such that the number of bytes per cluster becomes greater

than 32KB.