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  High-capacity fixed-disk systems present some special problems and
  opportunities. Fixed-disk formats vary much more than diskette formats do
  (Figure 5-4). Still, data is organized on fixed disks by cylinder, head,
  and sector numbers, just as it is on diskettes.

                                                   Sectors
  Disk                     Capacity    Cylinders   per Track    Heads
  --------------------------------------------------------------------------
  Typical PC/XT            10 MB       306         17           4
  fixed disk
  PC/AT fixed disk         30 MB       733         17           5
  type 20
  PS/2 Model 30            20 MB       612         17           4
  fixed disk, type 26
  PS/2 Model 60            44 MB       732         17           7
  fixed disk, type 31
  --------------------------------------------------------------------------

  Figure 5-4.  Some typical fixed-disk formats. All use 512 bytes per
  sector.

  Because the storage capacity of a fixed disk is relatively large, some PC
  users prefer to use only part of the disk space for DOS and to use other
  portions of the disk for other operating systems. To facilitate this, the
  available space on a fixed disk can be split into as many as four logical
  partitions, each of which is accessed separately. Each partition's data
  can be kept completely separate from the data in the other partitions.
  Each partition can contain its own boot sector and operating system.

  The first sector on a fixed disk contains a 64-byte partition table
  (Figure 5-5) and a disk bootstrap program. The partition table indicates
  where each partition is located on the disk. The table also designates one
  bootable partition. The first sector in the bootable partition is a
  partition boot sector that the ROM BIOS can use to load an operating
  system.

  The disk bootstrap program examines the partition table to determine which
  one of the partitions is bootable. It then reads the partition's boot
  sector from the disk into memory. The partition boot sector contains a
  bootstrap program that reads the operating system from the disk into
  memory and transfers control to it.

  Because bootable partitions are indicated in a table, you can select among
  fixed-disk partitions simply by updating the table and restarting the
  computer. All operating systems capable of supporting fixed disks provide
  a utility program that lets you update the partition table. (The DOS
  utility FDISK is such a program.)

  Offset from
  Start of Entry Size (bytes)   Meaning
  --------------------------------------------------------------------------
  00H            1              Boot indicator (80H = bootable, 0 = not
                                bootable)
  01H            1              Starting head number
  02H            2              Starting cylinder number (10 bits) and
                                sector number (6 bits)
  04H            1              System indicator:
                                   1 = primary DOS, 12-bit FAT
                                   2 = XENIX
                                   4 = primary DOS, 16-bit FAT
                                   5 = extended DOS
                                   8 = other non-DOS
  05H            1              Ending head number
  06H            2              Ending cylinder and sector numbers
  08H            4              Starting sector (relative to beginning of
                                disk)
  0CH            4              Number of sectors in partition
  --------------------------------------------------------------------------

  Figure 5-5.  The format of an entry in a fixed-disk partition table. The
  table consists of four such 16-byte entries, starting at offset 1BEH in
  the disk boot sector.

  --------------------------------------------------------------------------
  NOTE:
    Be very careful if you access a fixed disk's boot sector. The
    information contained there is intended only for use by the ROM BIOS
    bootstrap loader. Should the data in a disk's boot sector be erased or
    corrupted, the entire contents of the disk may become inaccessible.
  --------------------------------------------------------------------------