What Is the Role of the CPU?
The CPU is the part of the computer that processes data and instructions.
Whenever you open a program, type a character, perform a calculation or ask a computer to carry out a task, instructions must be processed. The Central Processing Unit (CPU) is responsible for that processing.
Students are often introduced to names such as PC, MAR, MDR, CIR, CU, ALU and ACC before they understand the bigger picture. In this lesson, we will do the opposite: understand the job first, then attach the correct names to the parts that perform it.
Before we begin...
You do not need to memorise every register immediately. First understand the CPU as a system: it repeatedly gets an instruction, works out what it means, and carries it out.
Once that mental model is secure, the component names become much easier to remember.
By the end of this lesson, you will be able to:
What does the CPU actually do?
The CPU processes the data and instructions needed by a computer system. Programs are made from instructions, and those instructions must be processed in the correct order for the program to work.
A useful mental model is to think of the CPU as the computer's worker. It does not permanently store all of your files. Instead, it works with the instructions and data needed for the task currently being performed.
Check Your Understanding 1
What is the main role of the CPU?
Check the answer
The CPU processes data and instructions.
The Fetch–Decode–Execute cycle
Instructions are processed using a repeating sequence called the Fetch–Decode–Execute cycle.
| Stage | Big idea |
|---|---|
| Fetch | Get the next instruction from memory. |
| Decode | Work out what the instruction means. |
| Execute | Carry out the instruction. |
The cycle then repeats for the next instruction. This simple sequence is at the heart of how the CPU processes programs.
This article introduces the whole cycle. The next two articles in this mini-series will slow the process down and examine what happens inside the CPU in much more detail.
What are CPU registers?
The CPU contains very small, very fast storage locations called registers. Each register has a specific job while instructions are being processed.
For now, learn the names and the broad purpose. The detailed movement between them belongs in the Fetch lesson.
| Register | Full name | Purpose |
|---|---|---|
| PC | Program Counter | Stores the address of the next instruction to be processed. |
| MAR | Memory Address Register | Holds the address of data or an instruction being located in memory. |
| MDR | Memory Data Register | Holds data or an instruction fetched from memory. |
| CIR | Current Instruction Register | Holds the current instruction being processed. |
| ACC | Accumulator | Stores interim results produced during calculations. |
Check Your Understanding 2
- Which register stores the address of the next instruction?
- Which register holds the current instruction?
- Which register stores interim calculation results?
Check the answer
PC stores the next instruction address. CIR holds the current instruction. ACC stores interim calculation results.
Control Unit, ALU and accumulator
Three names are particularly important when understanding how the CPU coordinates and performs work.
| Component | Role |
|---|---|
| Control Unit (CU) | Controls CPU operations and sends control signals to coordinate components. |
| Arithmetic Logic Unit (ALU) | Performs arithmetic and logical operations. |
| Accumulator (ACC) | Stores interim results produced by calculations. |
The distinction between the ALU and ACC is especially important. The ALU performs the calculation. The ACC stores the result.
Exam Focus
Precise component roles matter. Do not say that the accumulator performs calculations, and do not describe the MAR as holding data. Match each component to its exact job.
How does RAM fit into the picture?
Programs and data currently in use are held in RAM. During the Fetch stage, the CPU obtains the next instruction from RAM so that it can be decoded and executed.
This gives us the bridge into the next article: the CPU knows an instruction is needed, but how does it actually locate that instruction and move it into the CPU?
We will unpack that sequence properly in Part 2 rather than trying to memorise it here.
Common Exam Mistakes
The CPU processes data and instructions. Permanent file storage is a different role.
MAR holds an address. MDR holds data or an instruction.
The ALU performs arithmetic and logical operations. The ACC stores interim results.
First secure the overall CPU mental model. Then learn the detailed register sequence.
Quick Quiz
- What is the main role of the CPU?
- Name the three stages of the Fetch–Decode–Execute cycle in order.
- What does the Program Counter store?
- What is the difference between the MAR and MDR?
- What is the difference between the ALU and ACC?
Quick Quiz Answers
- To process data and instructions.
- Fetch, Decode, Execute.
- The address of the next instruction to be processed.
- MAR holds an address; MDR holds data or an instruction.
- The ALU performs arithmetic/logical operations; the ACC stores interim results.
Final Summary
CPU
Processes data and instructions.
FDE Cycle
Fetch the instruction, decode its meaning, execute the action.
Registers
Small, fast CPU storage locations with specific jobs.
CU
Controls and coordinates CPU operations.
ALU
Performs arithmetic and logical operations.
ACC
Stores interim calculation results.