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  DOS can work with most common computer devices, such as ordinary disk
  drives, serial communications lines, printers, and, of course, the
  keyboard and display screen. However, many other kinds of devices can be
  attached to PCs and PS/2s. Most of these devices require additional
  software support--device drivers--to connect to DOS and to DOS programs.

  Since the release of version 2.0, DOS has been able to incorporate into
  its own operations any device driver that follows a standard set of
  integration rules. During start-up, a disk file named CONFIG.SYS tells DOS
  when there is a device driver to be loaded. The name and file location of
  each device driver are identified by the command line DEVICE = filespec in
  the CONFIG.SYS file. For each DEVICE = command line, DOS locates the
  program file, loads it into memory, and goes through the series of steps
  necessary to welcome the device driver into the DOS fold.

  Typically, a device driver supports a new kind of device in an old way.
  For example, a device driver that supports a disk drive whose detailed
  control commands are new to DOS but whose overall features are similar to
  other kinds of disk drives will most likely follow the program format laid
  down by its more common predecessors. Likewise, a device driver that
  supports the addition of a mouse or joystick may treat them as
  keyboard-like devices.

  On the other hand, device drivers can perform functions that have little
  or nothing to do with the addition of new hardware devices to the
  computer; witness the ANSI device driver, which we'll be discussing in the
  following section. The ANSI device driver doesn't add new hardware to the
  computer; instead, it modifies the operation of the computer's standard
  hardware (the keyboard and the display screen).

  All the technical details of writing a device driver really belong in a
  book specializing in DOS systems programming, but we can give you the main
  points here.