The Central Processing Unit (CPU) and the Machine Cycle

Central Processing Unit (CPU) The Central Processing Unit (CPU) is the most important hardware component of a computer system. All the processing takes place in the CPU as, a result it is commonly referred to as the “brain” of the computer.

Components of the CPU The CPU consists of: 1. Control Unit (CU) – controls the operations of the CPU 2. Arithmetic Logic Unit (ALU) – performs operations such as addition, subtraction, multiplication, division and logical operations. 3. High speed memories such as Registers and Cache

Machine Cycle Each time the CPU executes an instruction, it takes a series of steps. The complete series of steps is called a machine cycle.

Stages of the Machine Cycle The series of steps that are carried out during the Machine Cycle are the: 1.Fetch Stage 2.Decode Stage 3.Execution Stage 4.Storing

Fetch Stage In the Fetch Stage, the Control Unit takes the instruction that is store in the Main Memory.

Decode Stage In the Decode Stage, the Control Unit translates the instruction into machine language or binary code.

Execution Stage In the execution stage, the Control Unit or Arithmetic Logic Unit then performs (carry out) the instruction from the decode stage.

Storing The Control Unit or Arithmetic Logic Unit then place the result from the execution stage into memory.

Diagram of the Machine Cycle

Instruction Time (I-Time) The instruction time is the time it takes the Control Unit to fetch (get) and decode (translate) the instruction from memory.

Execution Time (E-Time) The execution time is the time it takes the instruction to be performed (execute) and the result placed in memory.

Diagram showing the relation between Instruction Time and Execution Time

Practice Questions Which part of the Central Processing Unit is responsible for the following: A. Fetch X B. Fetch Y C. Check if X is greater than Y D. Add X and Y and store the result in Z