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  The PS/2 ROM BIOS start-up routines allocate an additional area of RAM for
  their own use. The BIOS routines use this extended data area for transient
  data storage. For example, the BIOS routines that support the
  pointing-device (mouse) controller hardware use part of the extended data
  area for temporary storage.

  You can determine the starting address of the extended data area by using
  a system service available through ROM BIOS interrupt 15H. (See Chapter
  12.) The first byte in the extended data area contains the size of the
  data area in KB.