Central Processing Unit (CPU)
It is the part of a computer which controls all the other parts. Designs vary widely but, in general, the CPU consists of the control unit, the arithmetic and logic unit (ALU) and memory (registers and cache).
Control Unit
A control unit performs the following four operations during a machine cycle:
Fetch: Places the program counter contents on the address bus and reads in the word at that location into the instruction register (IR).
Decode: Uses the contents of the IR to determine which gates should be opened between the CPU's various functional units and buses and what operation the ALU(s) should perform.
Execute: A simple instruction will require only an execute cycle whereas a complex instruction may require 3 or 4.
Store: The result of the instruction is written to its destination, either a register or a memory location.
