How come the boot sector signature of IBM PC are them?

[gapry]

An IBM PC boot loader for x86-CPUs in the boot sector is by convention indicated by a two-byte hexadecimal sequence 0x55, 0xAA. Why do them use 0x55 and 0xAA, not others? Do the values mean something which is special?

[Antti]

I cannot say for sure but maybe it is just a magic number sequence. If looking at single bits, it has some symmetry.

Code: Select all

	55h == 01010101b
	AAh == 10101010b

[sortie]

It's a magic number, so it probably depends on the whatever the developers thought was useful to put there. It's a pretty bad magic number since it's pretty small and there is no checksum, so detecting whether a sector contains a MBR is pretty risky of collisions with other formats.

[Combuster]

0x55 and 0xAA are not typical numbers you would find as data in a program, nor the does the opcode combination push bp;stosb that it would otherwise represent.

Therefore, if you find it in that particular place the chances it's not been put there on purpose is negligible.