Why does the IBM PC architecture use 55 AA
magic numbers in the last two bytes of a bootsector for the boot signature?
I suspect that has something to do with the bit patterns they are: 01010101 10101010
, but don't know what.
My guesses are that:
Maybe someone of you could help me answer this nagging question?
I remember I've once read somewhere about these bit patterns but don't remember where. And it migt be in some paperbook, because I cannot find anything about it on the Net.
I think it was chosen arbitrarily because 10101010 01010101 seemed like a nice bit pattern. The Apple ][+ reset vector was xor'ed with $A5 to (10100101) to produce a check-value. Some machines used something more "specific" for boot validation; for PET-derived machines (e.g. the VIC-20 and Commodore 64 by Commodore Business Machines), a bootable cartridge image which was located at e.g. address $8000 would have the PETASCII string "CBM80" stored at address $8004 (a cart starting at $A000 would have the string "CBMA0" at $A004, etc.), but I guess IBM didn't think disks for any other machine would be inserted and have $55AA in the last two bytes of the first sector.