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boot sector

Kiril Panayotov Blagoev

The first 512 bytes of a bootable drive, which the BIOS can read into memory and execute immediately after a [[power-on self-test|POST]] check.

Things to note about the boot sector:

  • At this time, the CPU will operate in [[16-bit-real-mode]]. In NASM assembly, this can be signified with [bits 16].

  • The BIOS will load the boot sector onto RAM with offset 0x7C00. This can be signified in NASM assembly as [ORG 0x7c00].

  • The boot sector must end on the magic number 0xaa55.

Example Boot Sector in NASM assembly

[bits 16]                   ; Work in 16-bit mode
[ORG 0x7C00]                ; Set memory offset for where BIOS will load this

loop:
    jmp loop                ; Endless loop

times 510-($-$$) db 0       ; Fill remainder of boot sector with zeroes
dw 0xAA55                   ; End with magic number