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  The video buffer memory is connected directly to the display circuitry so
  that the data in the video buffer can be repeatedly read out of the buffer
  and displayed. However, the video buffer is also logically (to the CPU) a
  part of the computer's main memory address space. A full 128 KB of the
  memory address space is set aside for use as video buffers, at addresses
  A000:0000H through B000:FFFFH, but the two original display adapters use
  only two small parts of this memory area. The Monochrome Display Adapter
  (MDA) provides 4 KB of display memory located at segment B000H. The
  original CGA provides 16 KB of display memory located at segment B800H.

  With the other IBM video subsystems, the address at which video memory is
  located isn't fixed--it depends on how the subsystem is configured. For
  example, when an EGA is used with a monochrome display, its text-mode
  video buffer is placed at B000H, just as with an MDA. When an EGA is
  attached to a color display, its video buffer can be addressed at B800H.
  And when you use an EGA in non-CGA graphics modes, the starting buffer
  address is A000H. Like the EGA, the MCGA and the VGA also support this
  chameleon-like method of buffer addressing.