[2025-12-03 Wed 22:35] <- After BIOS

Each sector is 512 bytes. A boot sector is looked at by the BIOS and determined by the last two bytes of the sector. That is, it will have 0x55 and 0xAA in the sector. A boot sector is always the first sector.

The BIOS copies the boot sector contents into memory and the boot sector proceeds to boot.

Apart from boot code, the first sector can contain any of the following:

• A partition table describing the partitions of the storage device. In this context it is called the partition sector.
• Bootstrap code, akin to boot code but to chain load.
• Optional 32 bit disk timestamp.
• Optional 32 bit disk signature.

The entire layout of the first sector looks something like this

GENERIC FIRST SECTOR:

Address	Description	bytes
0x0	Bootstrap code area	446
0x01be	Partition entry 1	16
0x01ce	partition entry 2	16
0x01de	partition entry 3	16
0x01ee	partition entry 4	16
0x01fe	boot signature (last two bytes 0x55AA)	2

This is the layout of the partition entry:

Offset	Field Length	Description
0x00	1 byte	Status
0x01	3 bytes	CHS address
0x04	1 byte	partition type
0x05	3 bytes	CHS address of the last sector
0x08	4 bytes	LBA of the first absolute sector of the partition
0x0C	4 bytes	Number of sectors in partition