.IMG File Format – Master boot record

Sector 0

Partition entry

CHS addresses

Three bytes (Brouwer, section 2.2): the head, 0 to 255; then a byte whose low 6 bits are the sector, 1 to 63, and whose top 2 bits are bits 8–9 of the cylinder; then the low 8 bits of the cylinder, 0 to 1,023:

h = byte1; s = byte2 & 0x3f; c = ((byte2 & 0xc0) << 2) + byte3;

The values depend on a made-up geometry of H heads and S sectors per track. util-linux’s fdisk fills them in as cylinder = LBA ÷ (H × S), head = (LBA ÷ S) mod H, sector = LBA mod S + 1, so LBA = (cylinder × H + head) × S + sector − 1. When the cylinder would pass 1,023 it uses the last addressable sector instead (H × S × 1,024 − 1), which with 255 heads and 63 sectors gives the bytes FE FF FF. Brouwer: as 24 bits cannot address more than 2²⁴ sectors, such CHS addresses should be ignored and the LBA fields used.

Extended partitions

To have more than four partitions, a primary entry is an extended partition: Brouwer lists types 05, 0F and 85 as extended (EDK2 defines the first two as EXTENDED_DOS_PARTITION and EXTENDED_WINDOWS_PARTITION). Its first sector is another partition table sector, with the same table at 446 and the same 55 AA signature, often called an extended boot record (EBR). Brouwer’s rules (sections 2.4–2.6):

• A partition table sector other than the MBR has at most two used entries.
• An entry whose type is not extended is a logical partition; its starting LBA is an offset from the partition table sector holding the entry.
• An entry whose type is extended points to the next partition table sector; its starting LBA is an offset from the first sector of the primary extended partition. Only its start matters; its size need only be non-zero.

So the first sector of each primary extended partition heads a linked list of partition table sectors, usually one per logical partition.

Partition types

No body assigns MBR partition types; the IDs grew by custom, and many have been used for more than one thing. The table gives every type util-linux’s fdisk names (include/pt-mbr-partnames.h), with the description from Brouwer’s List of partition identifiers for PCs where it adds to it. Brouwer’s list records many more uses of each ID, and IDs fdisk does not name.

Example

The end of sector 0 of an SD card image with a FAT32 boot partition and a Linux root partition.

000001B0  00 00 00 00 00 00 00 00 4D 2C 1B 0A 00 00 00 20  ........M,..... 
000001C0  21 00 0C C2 22 20 00 08 00 00 00 00 08 00 00 C2  !..." ..........
000001D0  23 20 83 A7 7E E9 00 08 08 00 00 00 70 00 00 00  # ..~.......p...
000001E0  00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
000001F0  00 00 00 00 00 00 00 00 00 00 00 00 00 00 55 AA  ..............U.

Reading it: disk signature 0x0A1B2C4D. Entry 1 at 0x1BE: not active; starts at head 32, sector 33, cylinder 0; type 0C, FAT32 (LBA); ends at head 194, sector 34, cylinder 32; starting LBA 0x800 (2,048, 1 MiB in); size 0x80000 sectors (256 MiB). Entry 2 at 0x1CE: type 83, Linux; starting LBA 0x80800 (526,336), right after the first; size 0x700000 sectors (3.5 GiB); its ending CHS byte 7E carries cylinder bit 8, so the cylinder is 0x1E9, 489. Entries 3 and 4 are empty; the sector ends 55 AA.