fetch-execute cycle

    The sequence of actions that a
   central processing unit performs to execute each machine
   code instruction in a program.

   At the beginning of each cycle the CPU presents the value of
   the program counter on the address bus.  The CPU then
   fetches the instruction from main memory (possibly via a
   cache and/or a pipeline) via the data bus into the
   instruction register.

   From the instruction register, the data forming the
   instruction is decoded and passed to the control unit which
   sends a sequence of control signals to the relevant function
   units of the CPU to perform the actions required by the
   instruction such as reading values from registers, passing
   them to the ALU to add them together and writing the result
   back to a register.

   The program counter is then incremented to address the next
   instruction and the cycle is repeated.

   The fetch-execute cycle was first proposed by John von
   Neumann.

   (1998-06-25)