Device Driver

   An interface module between the device-independent portions of
the operating system and an actual hardware device which converts
device-independent requests into the actual sequence of device
operations to perform the requested action.  IO.SYS contains the
standard, built-in MSDOS device drivers such as CON, COM1, AUX, PRN,
etc.  See also INT 21h Function 52h and INT 2Fh Function 0802h.