The Purpose of the CPU

The Fetch-Execute Cycle

• The purpose of the CPU is to carry out the processing of data on the computer system
• It performs the fetch-decode-execute cycle.
• The CPU fetches, decodes and executes instructions

Three Stages in the Cycle

1. Fetch

1. The address from the PC is copied to the MAR
2. Instruction from MAR is fetched and copied to MDR
3. The instruction in the MDR is copied to the CIR
4. Increment the PC

2. Decode

1. The instruction in CIR is decoded by the control unit
2. Data may be loaded into the MDR

3. Execute

1. The instruction is performed
2. The ALU may be used for any logic or calculations
3. The result is stored in the accumulator