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  The many video subsystems available with the PCs and PS/2s present a
  variety of programmable control interfaces to the video hardware. For
  example, all PC and PS/2 video subsystems have a cathode ray tube (CRT)
  controller circuit to coordinate the timing signals that control the
  video display.

  Although the video control circuits can be programmed in application
  software, all video subsystems have different programming interfaces.
  Fortunately, all PCs and PS/2s are equipped with basic video control
  routines in the ROM BIOS. We'll describe these routines in Chapter 9.