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  Regardless of the type of disk you use, all DOS disks are logically
  formatted in the same way: The disk's sides, tracks, and sectors are
  identified numerically with the same notation, and certain sectors are
  always reserved for special programs and indexes that DOS uses to manage
  disk operations. Before we describe how DOS organizes space on a disk, we
  need to briefly cover the conventional notation used by DOS and the ROM
  BIOS to locate information.

  Diskette cylinder numbers start from 0 at the outside edge of the disk
  surface and increase toward the center of the disk. Read/write heads are
  also numbered from 0, but sector numbers start with 1. Any location on the
  disk can thus be described by a unique combination of cylinder, head, and
  sector numbers. This in fact is how the ROM BIOS services access disk
  data.

  DOS, however, does not recognize cylinders, heads, and sectors. Instead,
  DOS sees a disk as a linear sequence of logical sectors. The sequence of
  logical sectors begins with the first sector on a disk: Sector 1, cylinder
  0, head 0 (the boot sector) is DOS logical sector 0.

  Logical sectors are numbered from track to track in the same cylinder, and
  then are numbered from cylinder to cylinder. Thus the last sector in
  cylinder 0, head 0, is followed by the first sector in cylinder 0, head 1;
  the last sector in a cylinder is followed by the first sector in the next
  cylinder. See page 300 for information on converting DOS notation to ROM
  BIOS notation and vice versa.

  The use of logical sector numbers lets DOS avoid having to deal with
  cylinder, head, and sector numbers that vary among different types of
  disk-drive hardware. However, this same feature means that DOS is limited
  in the amount of disk space it can access on a particular disk drive.
  Because DOS maintains logical sector numbers as 16-bit integers, it can
  recognize, at most, 65,536 logical sectors on a disk. Because the default
  size of a disk sector is 512 bytes, the largest disk DOS can manage is
  65,536 x 512, or 32 MB. This certainly is no problem on diskettes, but
  it's an unwelcome limitation for the many PC/AT and PS/2 users who have
  fixed disks larger than 32 MB.

  To get around this restriction, DOS version 3.3 introduced the notion of
  the extended DOS partition. With DOS 3.3, you can use the DOS utility
  program FDISK to allocate a fixed-disk partition as an extended DOS
  partition. You can format the extended partition as one or more separate
  logical drives. Thus, for example, you could use both a primary and an
  extended DOS partition on a fixed disk, with the primary partition as
  drive C and the extended partition as drives D and E.