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Computer Structure

Lesson CS5 of 10 Approx 60 min Processor, memory and buses
Learning intentions
  • Understand the purpose of the main parts of a processor
  • Describe how memory is organised using unique binary addresses
  • Explain how the address bus and data bus link the processor and memory
Success criteria
  • I can identify the ALU, control unit and registers from a description of their role
  • I can explain that every memory location has a unique binary address
  • I can state that the address bus carries addresses from processor to memory, and the data bus carries data both ways
  • I can apply these ideas to simple exam-style scenarios about fetching data or executing instructions
Warm up — what do you already know?

Answer before the lesson begins. These check prior knowledge — it's fine if you're unsure.

1. What is the main job of a processor?

2. Which type of memory can be read from and written to, but loses its contents when power is switched off?

3. What does binary allow a computer system to represent?

Key vocabulary

Processor
The main chip that runs instructions and processes binary data. Also called the CPU.
ALU
Arithmetic and Logic Unit; carries out arithmetic calculations, comparisons and logic operations.
Control unit
Coordinates the processor: fetches instructions, decodes them and controls the movement of data.
Registers
Small, very fast temporary storage locations inside the processor.
RAM
Random Access Memory; read/write memory that stores programs and data currently in use.
ROM
Read Only Memory; memory that stores start-up instructions and is not normally changed.
Memory address
A unique binary number used to identify one location in memory.
Address bus
A one-way set of pathways that carries the address of a memory location from the processor to memory.
Data bus
A two-way set of pathways that carries data or instructions between the processor and memory.

Computer Structure

The stored program computer

A computer system takes input, processes it, stores data where needed and sends output. The key idea behind modern computer structure is the stored program concept: program instructions and data are both stored in memory as binary. The processor fetches instructions from memory, carries them out, and uses memory again when it needs to read or write data.

This matters because the same hardware can run many different programs. A laptop can edit a photo, play a video or run a Python program because the instructions in memory change. The processor is not redesigned each time; it simply follows the next set of binary instructions.

Inside the processor

The processor, or CPU, is the part of the computer that carries out instructions. At National 5, you need to know three main parts: the Arithmetic and Logic Unit (ALU), the control unit and registers.

The ALU performs arithmetic operations such as addition, subtraction and multiplication. It also performs comparison operations such as >, < and =, and logic operations such as AND, OR and NOT. For example, when a program checks whether score > 50, that comparison is carried out by the ALU.

The control unit coordinates the work of the processor. It fetches instructions from memory, decodes what each instruction means, and sends control signals to the correct part of the processor or computer system. If the processor needs a value from RAM, the control unit manages that request.

Registers are tiny, fast storage locations inside the processor. They temporarily hold data, instructions, addresses and calculation results while the processor is working. Registers do not store large amounts of data. Anything needed for longer is stored in main memory.

Processor ALU Control unit Registers Memory 00000000 00000001 00000010 00000011 address bus data bus
The processor uses the address bus to select a memory location, and the data bus to transfer data or instructions.

Memory locations and addresses

Main memory stores the programs and data currently being used by the computer. Each memory location has a unique binary address. This means the processor can ask for one exact location rather than searching randomly. Addressing normally starts at 0 and continues upwards in binary.

Think of a memory address like a house number. The house number is not the person living there; it is the way to find the place. In the same way, a memory address is not the data itself. It identifies where the data or instruction is stored.

RAM can be read from and written to. It is used for the operating system, open programs and current data, but it needs power to keep its contents. ROM can be read from but is not normally changed. It stores instructions needed when the computer starts up.

Buses link processor and memory

A bus is a set of electrical pathways used to transfer binary values between components. The address bus carries the memory address being accessed from the processor to memory. It is unidirectional, meaning it travels one way: processor to memory. If the processor wants memory location 01001110, that address is sent along the address bus.

The data bus carries the actual data or instruction. It is bidirectional, meaning it can transfer data from memory to the processor when reading, or from the processor to memory when writing. This distinction is a common exam point: the address bus identifies where; the data bus transfers what.

ComponentPurposeExam clue
ALUArithmetic, comparisons and logiccalculate, compare, AND/OR/NOT
Control unitCoordinates instructions and data movementfetch, decode, control signals
RegistersTemporary storage inside the processorholds current data/address/instruction/result
Address busCarries memory addresses to memorywhich location?
Data busCarries data between processor and memorywhat value/instruction?

Worked examples

Example 1 — Identifying the processor part

A program contains the instruction SET total TO price * quantity. Which part of the processor carries out the multiplication?

1
Look for the action: the program is multiplying two values.
2
Multiplication is an arithmetic operation.
3
The arithmetic operation is carried out by the Arithmetic and Logic Unit (ALU).
Example 2 — Reading data from memory

The processor needs the value stored at memory address 01001110. Explain how the buses are used.

1
The processor places the address 01001110 on the address bus.
2
The control unit sends a read signal so memory knows the processor wants to read from that location.
3
The data stored at that location is sent back to the processor along the data bus.
Example 3 — Interpreting a memory address

A question says: "Every location in memory has a unique binary address." What does this mean?

1
Memory is split into individual locations.
2
Each location has its own binary number so it can be identified exactly.
3
A clear answer would be: the processor can access a specific memory location by using its unique binary address.
Now you try

A program runs the following instruction:

SET points TO points + bonus

Answer the following:

  1. Which processor part adds points and bonus?
  2. Which processor part coordinates fetching the instruction from memory?
  3. Which processor part temporarily holds the values while the instruction is being processed?
  1. ALU — addition is an arithmetic operation.
  2. Control unit — it coordinates fetching and decoding instructions.
  3. Registers — they temporarily hold values, addresses, instructions or results while the processor works.
Common mistakes
✗
Mixing up the ALU and the control unit. The ALU performs calculations, comparisons and logic. The control unit coordinates fetching, decoding and control signals.
✗
Saying the address bus carries data. The address bus carries the memory address. The data bus carries the actual data or instruction.
✗
Forgetting direction. At N5, the address bus is one-way from processor to memory. The data bus is two-way between processor and memory.
✗
Confusing memory with storage. RAM holds programs and data currently in use. Backing storage keeps files more permanently when power is off.
Exam tip

For a describe question, give the role of the component clearly. For an explain question, link components together. Example: "The address bus carries the address of the required memory location from the processor to memory; the data bus then carries the data from that location back to the processor."

Watch for command words such as state, describe and explain. One-word answers are only enough when the question asks you to identify or state a component.

Task Set

Questions 1–5 are auto-checked. Questions 6–9 are self-marked — write your answer, then reveal the model answer to check your work.

1. Which part of the processor carries out comparisons such as > and <? TYPE 1

2. Which component coordinates the fetching and decoding of instructions? TYPE 1

3. What does a register do? TYPE 1

4. Which bus carries the memory address from the processor to memory? TYPE 1

5. The data bus is bidirectional. What does this mean? TYPE 1

6. Describe the purpose of the ALU, control unit and registers. TYPE 2

The ALU carries out arithmetic operations, comparison operations and logic operations. The control unit coordinates the processor by fetching and decoding instructions and controlling the movement of data. Registers are small, fast temporary storage locations inside the processor that hold data, addresses, instructions or results while processing is taking place.

7. Explain how every memory location can be accessed by the processor. TYPE 2

Memory is divided into individual locations. Each location has a unique binary address. When the processor needs to access a specific location, it sends that address along the address bus to memory. This allows the exact location to be read from or written to.

8. A program executes the line SET answer TO a * b. State which part of the processor would: (a) hold the values of a and b temporarily, (b) multiply the values, and (c) coordinate fetching the instruction from memory. TYPE 3

(a) Registers hold values temporarily while the processor works.
(b) The ALU multiplies the values because multiplication is an arithmetic operation.
(c) The control unit coordinates fetching and decoding the instruction.

9. The processor needs to read data stored at memory address 10110010. Explain how the address bus and data bus are used. TYPE 3

The processor sends the address 10110010 to memory using the address bus. The address bus identifies the memory location to be accessed. The data stored at that location is then sent from memory back to the processor using the data bus. The address bus carries where to look; the data bus carries the data found there.
Teacher notes — Shift+T to hide

Suggested timing: 60 minutes. Warm up 6 min; vocabulary 6 min; notes and diagram 20 min; worked examples 10 min; now you try 5 min; task set 13 min.

Key misconception to address: Pupils often treat "processor", "ALU" and "control unit" as interchangeable. Keep returning to exam clue words: calculate/compare = ALU; fetch/decode/coordinate = control unit; temporarily hold = registers.

Live demo suggestion: Draw a simple memory table on the board with binary addresses and values. Ask all pupils to act as the processor: first point to the address, then retrieve the data. This makes the address/data bus distinction concrete.

Extension question: Ask why increasing address bus width lets a computer access more memory locations. Keep this conceptual at N5: more address bits means more unique binary addresses.

SQA command words covered: "state", "identify", "describe", "explain".