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  Regardless of their data formats, all diskettes and disks are potentially
  bootable; that is, they can contain the information necessary to get an
  operating system running at the time you start your computer. There is
  nothing special about the format of a bootable disk; it's just one that
  contains information that lets the ROM BIOS boot the operating system.
  Here's how it works.

  On all PC and PS/2 diskettes and fixed disks, the first sector on the
  disk--cylinder 0, head 0, sector 1--is reserved for a short bootstrap
  program. (The program has to be short because the size of a sector is only
  512 bytes.) The function of this bootstrap program is to read the bulk of
  the operating system into memory from elsewhere on the disk and then to
  transfer control to the operating system.

  When you start or restart your computer, the last tasks performed by the
  start-up ROM BIOS routines are reading the contents of the disk boot
  sector into memory and checking those contents for a bootstrap program.
  The BIOS does this checking by examining the last 2 bytes of the boot
  sector for a signature (55H and AAH) that indicates that the data in the
  boot sector represents a bootstrap program. If the signature value isn't
  correct, the BIOS assumes there's no bootstrap program in the boot sector
  and, therefore, that the disk isn't bootable.

  The bootstrap program's job is to copy the start-up program for an
  operating system from the disk into memory. There's no restriction on the
  size and location of the operating system's start-up program, so this
  stepwise transfer of control--from ROM BIOS to boot sector to operating
  system--can be used to start DOS, XENIX, OS/2, or even a stand-alone
  application.