The CPU controls the operation (and determines the configuration) of all other components in the PC, including RAM, buses, and peripheral devices.  Despite its complex design and construction, however, the CPU cannot perform a single operation without directions from a program – either permanently stored in ROM, or temporarily stored in RAM.
The CPU “executes” a program in memory by continuously performing a sequence of operations known as the CPU instruction cycle.  The instruction cycle actually consists of four steps, as follows:
  1. Fetch - get the next instruction from memory.
  2. Decode - translate the instruction into a form that the CPU can understand.
  3. Execute - perform the action as specified in the instruction.
  4. Store - copy the results of the execution back into memory for future use.