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  From the programmer's point of view, all members of the PC family consist
  of a processor, memory chips, and several smart, or programmable, circuit
  chips. All the main circuit components that make the computer work are
  located on the system board; other important parts are located on
  expansion boards, which can be plugged into the system board.

  The system board (Figures 1-1 through 1-3) contains the microprocessor,
  which is tied to at least 64 KB of memory; some built-in ROM programs,
  such as BASIC and the ROM BIOS; and several very important support chips.
  Some of these chips control external devices, such as the disk drive or
  the display screen, and others help the microprocessor perform its tasks.

  In this section, we discuss each major chip and give a few important
  technical specifications. These chips are frequently known by more than
  one name. For example, some peripheral input/output hardware is supervised
  by a chip known as the 8255. This chip is also referred to as the 8255A
  and the 8255A-5. The suffixes A and 5 refer to revision numbers and to
  parts rated for operation at different speeds. For programming purposes,
  any Intel chip part number that starts with 8255 is identical to any other
  chip whose part number starts with 8255, regardless of the suffix.
  However, when you replace one of these chips on a circuit board, note the
  suffix. If the suffixes are different, the part may not operate at the
  proper speed.