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  Much of the operation of the PCs and PS/2s is controlled by data stored in
  low-memory locations, particularly in the two adjacent 256-byte areas
  beginning at segments 40H and 50H (addresses 0040:0000H and 0050:0000H).
  The ROM BIOS uses the 256 bytes from 0040:0000H through 0040:00FFH as a
  data area for its keyboard, video, disk, printer, and communications
  routines. The 256 bytes between 0050:0000H and 0050:00FFH are used
  primarily by BASIC, although a few ROM BIOS status variables are located
  there as well.

  Data is loaded into these areas by the BIOS during the start-up process.
  Although the control data is supposed to be the private reserve of the
  BIOS, DOS, and BASIC, your programs are allowed to inspect or even change
  it. Even if you do not intend to use the information in these control
  areas, it is worth studying because it reveals a great deal about what
  makes the PC family tick.