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  In a PC or PS/2, one of the CPU's essential tasks is to respond to
  hardware interrupts. A hardware interrupt is a signal generated by a
  component of the computer, indicating that component's need for CPU
  attention. For example, the system timer, the keyboard, and the disk
  drive controllers all generate hardware interrupts at various times. The
  CPU responds to each interrupt by carrying out an appropriate
  hardware-specific activity, such as incrementing a time-of-day counter
  or processing a keystroke.

  Each PC and PS/2 has a programmable interrupt controller (PIC) circuit
  that monitors interrupts and presents them one at a time to the CPU. The
  CPU responds to these interrupts by executing a special software routine
  called an interrupt handler. Because each hardware interrupt has its own
  interrupt handler in the ROM BIOS or in DOS, the CPU can recognize and
  respond specifically to the hardware that generates each interrupt. In
  the PC, PC/XT, and PS/2 models 25 and 30, the PIC can handle 8 different
  hardware interrupts. In the PC/AT and PS/2 models 50, 60, and 80, two
  PICs are chained together to allow a total of 15 different hardware
  interrupts to be processed.

  Although the programmable interrupt controller is indeed programmable,
  hardware interrupt management is not a concern in most programs. The ROM
  BIOS and DOS provide nearly all of the services you'll need for managing
  hardware interrupts. If you do plan to work directly with the PIC, we
  suggest you examine the ROM BIOS listings in the IBM technical reference
  manuals for samples of actual PIC programming.