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  In all PCs, the microprocessor is the chip that runs programs. The
  microprocessor, or central processing unit (CPU), carries out a variety of
  computations, numeric comparisons, and data transfers in response to
  programs stored in memory.

  The CPU controls the computer's basic operation by sending and receiving
  control signals, memory addresses, and data from one part of the computer
  to another along a group of interconnecting electronic pathways called a
  bus. Located along the bus are input and output (I/O) ports that connect
  the various memory and support chips to the bus. Data passes through these
  I/O ports while it travels to and from the CPU and the other parts of the
  computer.

  In the IBM PCs and PS/2s, the CPU always belongs to the Intel 8086 family
  of microprocessors. (See Figure 1-4.) We'll point out the similarities
  and differences between the different microprocessors as we describe them.

  +------------------------------------------------------------------------+
  | Figure 1-1 can be found on p.3 of the printed version of the book.     |
  +------------------------------------------------------------------------+

  Figure 1-1.  The IBM PC system board.

  +------------------------------------------------------------------------+
  | Figure 1-2 can be found on p.4 of the printed version of the book.     |
  +------------------------------------------------------------------------+

  Figure 1-2.  The PC/AT system board.

  +------------------------------------------------------------------------+
  | Figure 1-3 can be found on p.5 of the printed version of the book.     |
  +------------------------------------------------------------------------+

  Figure 1-3.  The PS/2 Model 60 system board.

  Model                                Microprocessor
  --------------------------------------------------------------------------
  PC                                   8088
  PC/XT                                8088
  PC/AT                                80286
  PS/2 Models 25, 30                   8086
  PS/2 Models 50, 60                   80286
  PS/2 Model 80                        80386
  --------------------------------------------------------------------------

  Figure 1-4.  Microprocessors used in IBM PCs and PS/2s.