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  The microprocessor cannot control the entire computer without some
  help--nor should it. By delegating certain control functions to other
  chips, the CPU is free to attend to its own work. These support chips
  can be responsible for such processes as controlling the flow of
  information throughout the internal circuitry (as the interrupt
  controller and the DMA controller are) and controlling the flow of
  information to or from a particular device (such as a video display or
  disk drive) attached to the computer. These so-called device controllers
  are often mounted on a separate board that plugs into one of the PC's
  expansion slots.

  Many support chips in the PCs and PS/2s are programmable, which means
  they can be manipulated to perform specialized tasks. Although direct
  programming of these chips is generally not a good idea, the following
  descriptions will point out which chips are safe to program directly and
  which aren't. Because this book does not cover direct hardware control,
  you should look in the IBM technical manuals as well as in the chip
  manufacturers' technical literature for details about programming
  individual chips.