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OCR J277 · Component 1 · Topic 1.1.1
Architecture of
the CPU
Von Neumann Architecture · CPU Components · Fetch-Decode-Execute Cycle
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OCR GCSE Computer Science J277
Learning Objectives
By the end of this video you will be able to...
Name and describe every component inside the CPU
State the purpose of each register and whether it stores an address or data
Walk through each stage of the Fetch-Decode-Execute cycle
Answer exam questions on this topic with confidence
⚡ This topic appears in every OCR J277 exam
Von Neumann Architecture
The Foundation of Modern Computing
"Programs and data are stored together in the same memory.
The CPU fetches instructions, decodes them, and executes them one at a time."
PROPOSED BY
John Von Neumann
1945 — still used in every computer today
TWO MAIN PARTS
CPU — processes instructions
Main Memory — stores programs and data
CPU Component 1 of 4
ALU — Arithmetic Logic Unit
Where all actual processing happens inside the CPU
LOGICAL OPERATIONS
Comparisons (greater than, equal to)
💡 Every Python calculation — x + 5 or if x == 10 — is processed by the ALU
CPU Component 2 of 4
Control Unit — CU
The Control Unit does not process data itself.
It coordinates and controls all other components.
Manages the Fetch-Decode-Execute cycle
Directs the flow of data between CPU components
✈ Like an air traffic controller — nothing moves without its instructions
⚠ EXAM TRAP:
Never say the CU performs calculations — that is the ALU's job
CPU Component 3 of 4
Registers — Ultra-Fast CPU Storage
The exam tests what each register stores — an address or data. Know this.
| Register | Full Name | Stores | What it holds |
|---|
| PC |
Program Counter |
ADDRESS |
Stores the address of the next instruction to be fetched. Increments after each fetch. |
| MAR |
Memory Address Register |
ADDRESS |
Stores the address of the memory location to be read from or written to |
| MDR |
Memory Data Register |
DATA |
Stores the data or instruction fetched from memory, or to be written to memory |
| ACC |
Accumulator |
DATA |
Stores the result of calculations performed by the ALU |
Key rule:
PC and MAR store addresses · MDR and ACC store data
CPU Component 4 of 4
Cache
Cache is a small amount of very fast memory built into or very close to the CPU.
It stores frequently used instructions and data so the CPU can access them quickly without fetching from main memory every time.
Faster than RAM
Reduces time spent waiting for instructions
Less capacity
Much smaller than main memory
Fetch-Decode-Execute Cycle
The CPU's Continuous Loop
FETCH
→
DECODE
→
EXECUTE
→
repeat...
Repeats billions of times per second — every program ever written goes through this cycle
📥
Retrieve the next
instruction from memory
🔍
CU works out what
the instruction means
⚡
ALU or memory
carries it out
FDE — Stage 1
FETCH
The CPU uses the Program Counter to find the address of the next instruction.
The instruction is fetched from memory.
The Program Counter then increments by one so it holds the address of the next instruction.
REGISTER USED
PC — holds the address of where to find the instruction
REGISTER USED
MDR — holds the data (the instruction itself) once fetched
FDE — Stage 2
DECODE
The instruction is decoded by the Control Unit.
It works out:
• What operation needs to be performed
• What data or address is needed
FDE — Stage 3
EXECUTE
The instruction is executed / processed.
• Arithmetic / logic → ALU performs the calculation
Result stored in ACC
• Memory operation → data read from or written to RAM
🔁 Cycle repeats immediately — billions of times per second
Exam Practice
Let's test your knowledge — pause after each question
Question 1
Describe the purpose of the Program Counter.
2 marks
✓ MARK SCHEME
1 mark
Stores the address of the next instruction to be fetched from memory
1 mark
Increments (by one) after each fetch so it points to the next instruction
⚠ Say what it stores, not what it does — "holds the address" not "fetches the address"
Exam Practice
Question 2
Describe the role of the Control Unit in the CPU.
2 marks
✓ MARK SCHEME
1 mark
The Control Unit coordinates and controls the operation of the CPU
1 mark
It manages the FDE cycle / directs the flow of data between components
You need both what it does and how for a describe question
Exam Practice
Question 3
Explain the difference between what is stored in the MAR and what is stored in the MDR.
2 marks
✓ MARK SCHEME
1 mark
The MAR stores a memory address — the location in memory being accessed
1 mark
The MDR stores the actual data or instruction being read from or written to that location
The spec explicitly tests: address vs data — this is a high-probability question
Exam Practice
Question 4
Describe what happens at each stage of the Fetch-Decode-Execute cycle.
3 marks
✓ MARK SCHEME
1 mark
Fetch: Data/instructions are fetched from memory (using the address stored in the Program Counter)
1 mark
Decode: Data/instructions are decoded — the CU determines what operation is needed
1 mark
Execute: Data/instructions are executed / processed — the ALU performs arithmetic/logical calculations
Common Mistakes
Don't lose easy marks
1
Confusing MAR and MDR — MAR stores an address, MDR stores data. The middle letter is the clue: Address, Data.
2
Saying the Control Unit processes data — it does NOT. The ALU processes data. The CU controls and coordinates only. Writing that the CU calculates will lose marks.
3
Saying the Accumulator stores an address — it does NOT. The ACC stores the result of a calculation. It stores data. Only PC and MAR store addresses.
4
Describing a register's purpose as an action — e.g. "the PC fetches the instruction." The mark scheme warns against this. Say what it stores: "the PC stores the address of the next instruction to be fetched."
Topic Summary — 1.1.1
What You Need to Know
CPU COMPONENTS
ALU — arithmetic and logic processing
CU — coordinates, does NOT process
Cache — fast, small, close to CPU
Registers — PC, MAR, MDR, ACC
REGISTERS — ADDRESS vs DATA
PC — stores an address (next instruction); increments after fetch
MAR — stores an address (memory location to read/write)
MDR — stores data (instruction/data fetched from memory)
ACC — stores data (result of ALU calculation)
FDE CYCLE
Fetch — instruction fetched from memory using PC
Decode — instruction decoded by the CU
Execute — instruction executed/processed; ALU performs calculations
Repeats billions of times per second
VON NEUMANN
Programs and data stored together
CPU fetches, decodes, executes
Foundation of all modern computers
Proposed by Von Neumann, 1945
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Next Video
1.1.2
CPU Performance Factors
Clock speed · Cores · Cache size
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