What's so special about 0x55AA?

Question

I have encountered 0x55AA in 2 scenarios:

• the final 2 bytes of boot sector in the legacy booting process contains 0x55AA.
• the first 2 bytes of the Option ROM must be 0x55AA

So what's special about 0x55AA?

The binary version of 0x55AA is 0101010110101010. Is it because it is evenly interleaved 0 and 1? But I don't see that's a strong criteria.

Answer 1

0x55AA is a "signature word". It is used as the "end of sector" marker in the last 2 bytes of a 512 byte boot record. This includes MBR and it's extended boot records and in the newer GPTs protective MBR.

References:

Image from Master Boot Record - microsoft.com.

How Basic Disks and Volumes Work - microsoft.com.

Answer 2

Reference : https://www.techtarget.com/whatis/definition/Master-Boot-Record-MBR

If the final signature is 0x55AA or 0xAA55 of MBR at 511th and 512th bytes respectively BIOS transfers control to the MBR to boot the OS. If the final signature does not match, the BIOS looks for additional bootable devices. If no devices are found, the OS does not boot, and the user receives an error message.

Question : Addition boot devices include "extended boot partitions" also, containing "extended boot records", having the same final signature 0x55AA or 0xAA55 at the end of the record.