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  As we mentioned, the PC family of computers links all internal control
  circuitry by means of a circuit design known as a bus. A bus is simply a
  shared path on the main circuit board to which all the controlling parts
  of the computer are attached. When data is passed from one component to
  another, it travels along this common path to reach its destination.
  Every microprocessor, every control chip, and every byte of memory in
  the computer is connected directly or indirectly to the bus. When a new
  adapter is plugged into one of the expansion slots, it is actually
  plugged directly into the bus, making it an equal partner in the
  operation of the entire unit.

  Any information that enters or leaves a computer system is temporarily
  stored in at least one of several locations along the bus. Data is
  usually placed in main memory, which in the PC family consists of
  thousands or millions of 8-bit memory cells (bytes). But some data may
  end up in a port or register for a short time while it waits for the CPU
  to send it to its proper location. Generally, ports and registers hold
  only 1 or 2 bytes of information at a time and are usually used as
  stopover sites for data being sent from one place to another. (Ports and
  registers are described in Chapter 2.)

  Whenever a memory cell or port is used as a storage site, its location
  is known by an address that uniquely identifies it. When data is ready
  to be transferred, its destination address is first transmitted along
  the address bus; the data follows along behind on the data bus. So the
  bus carries more than data: It carries power and control information,
  such as timing signals (from the system clock) and interrupt signals, as
  well as the addresses of the thousands or millions of memory cells and
  the many devices attached to the bus. To accommodate these four
  different functions, the bus is divided into four parts: the power
  lines, the control bus, the address bus, and the data bus. We're going
  to discuss the subjects of address and data buses in greater detail
  because they move information in a way that helps to explain some of the
  unique properties of the PC family.