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  PCs and PS/2s have several input/output subsystems with specialized
  control circuitry that provides an interface between the CPU and the
  actual I/O hardware. For example, the keyboard has a dedicated
  controller chip that transforms the electrical signals generated by
  keystrokes into 8-bit codes that represent the individual keys. All disk
  drives have separate controller circuitry that directly controls the
  drive; the CPU communicates with the controller through a consistent
  interface. The serial and parallel communications ports also have
  dedicated input/output controllers.

  You rarely need to worry about programming these hardware controllers
  directly because the ROM BIOS and DOS provide services that take care of
  these low-level functions. If you need to know the details of the
  interface between the CPU and a hardware I/O controller, see the IBM
  technical reference manuals and examine the ROM BIOS listings in the PC
  and PC/AT manuals.