0xAA55

0xAA55 is a 16-bit magic value used as the boot sector signature on IBM PC compatible computers. Also known as the boot signature, it marks the end of the Master Boot Record (MBR) and signals to the BIOS that the sector contains a valid bootable program. On disk, the bytes at offsets 510 and 511 (the last two bytes of the 512-byte sector) are 0x55 and 0xAA, respectively. When the BIOS loads the first sector of a boot device into memory, it checks this signature; if present, it transfers control to the boot loader within the sector. If the signature is missing or incorrect, the BIOS will typically continue searching other bootable devices or halt with an error.

The two-byte value is commonly expressed as 0xAA55, reflecting the little-endian arrangement of the bytes on disk. This signature has been part of PC-compatible boot processes since the early days of DOS and remains a de facto standard for legacy BIOS booting. While modern systems often use UEFI in place of BIOS, some dual-boot configurations and older operating systems still rely on the MBR boot path and thus on the 0xAA55 signature remaining intact.

In practice, 0xAA55 is considered a critical marker for bootability. Corruption of the MBR, including the signature, can render a system unbootable, and recovery tools frequently verify or repair this value as part of the boot repair process. Related concepts include boot loader stages, partition table recovery, and magic numbers in file formats.