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  The fourth element of the ROM has more to do with the PC's design than
  with the actual contents of its memory. The PC was designed to allow for
  installable extensions to the built-in software in ROM. The additional ROM
  is usually located on a plug-in adapter such as the Enhanced Graphics
  Adapter or a fixed-disk controller card. Computers in the PC/XT/AT family
  also have empty sockets on their system boards to accommodate additional
  ROM chips. Because the original ROM BIOS could not include support
  programs for future hardware, ROM extensions are obviously a necessary and
  helpful addition.

  Several memory areas are reserved for ROM extensions. Addresses C000:0000H
  through C000:7FFFH are reserved for video adapter ROM. The area between
  C800:0000H and D000:FFFFH can be used by nonvideo adapters. (For example,
  the IBM XT fixed-disk adapter occupies addresses starting at C800:0000H.)
  Finally, ROM extensions on chips placed onto the system board of a PC, XT,
  or AT occupy the address range E000:0000H through E000:FFFFH. In the PS/2
  models 50, 60, and 80, you cannot add ROM chips to the system board. The
  system ROM in these computers occupies the entire address range between
  E000:0000H and F000:FFFFH.