Citation :
A bootstrap is loaded from the first sector on a disk, track zero, head zero, sector one. Which disk the bootstrap is loaded from is dependent upon the BIOS configuration saved in NVRAM (NonVolatile RAM). This single 512 byte sector is loaded into memory at physical address 0000:7C00. The BIOS will then examine the final two bytes of the bootstrap (offset 1FEh) for the value AA55h. This flags the bootsector as a valid, bootable disk instead of just storing disk information. A bootstrap must be exactly 512 bytes long because of the two byte check and the one sector limitation. After this verification, the BIOS will jump to 0000:7C00 and turn control over to the bootstrap.
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