3.4.1 Hardware — AQA 8525

Von Neumann Architecture

Modern computers are based on the Von Neumann architecture, proposed by John Von Neumann in 1945. Its key idea is a stored-program computer: both the program instructions and the data they operate on are stored in the same memory, and the CPU fetches and executes them sequentially.

Von Neumann Architecture

CPU

ALU

Registers (PC, MAR, MDR, ACC)

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Main Memory

RAM / ROM

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I/O Devices

keyboard, screen…

The CPU — Central Processing Unit

The CPU is the brain of the computer. It fetches, decodes, and executes instructions. It has two main sub-units:

Component	Full name	Role
CU	Control Unit	Directs the operation of the processor. Fetches and decodes instructions, controls data flow around the CPU and between CPU and memory/I/O
ALU	Arithmetic Logic Unit	Performs arithmetic (+, −, ×, ÷) and logical operations (AND, OR, NOT, comparisons)

CPU Registers

Registers are tiny, ultra-fast storage locations inside the CPU. AQA requires knowledge of four:

Register	Full name	Stores
PC	Program Counter	Address of the next instruction to be fetched
MAR	Memory Address Register	Address in memory to be read from or written to
MDR	Memory Data Register	Data that has just been fetched from / about to be written to memory
ACC	Accumulator	Holds the result of the most recent ALU calculation

The Fetch-Decode-Execute Cycle

The CPU works by continuously repeating the Fetch-Decode-Execute (FDE) cycle:

1. Fetch: PC → MAR (copy address); RAM → MDR (load instruction); PC increments (points to next instruction).

2. Decode: CU decodes the instruction in the MDR — determines what operation and what operand.

3. Execute: ALU carries out the operation (e.g. adds two values); result stored in ACC.

Factors Affecting CPU Performance

Factor	What it is	Effect
Clock speed	Number of FDE cycles per second, measured in GHz (billions)	Higher clock speed → more instructions per second → faster
Number of cores	Independent processing units on a single chip	More cores → multiple tasks (threads) run simultaneously → faster for multi-threaded workloads
Cache size	Very fast memory between CPU and RAM (L1, L2, L3)	Larger cache → more data accessible without slow RAM access → faster

Memory — RAM and ROM

	RAM	ROM
Full name	Random Access Memory	Read Only Memory
Volatile?	Yes — loses contents when powered off	No — retains contents without power
Read/Write	Read and write	Read only (normally)
Used for	Storing running programs and data currently in use	Storing BIOS / firmware — startup instructions
Effect of more RAM	More programs / larger programs can run at once without using slow virtual memory

Secondary Storage

Secondary storage is non-volatile (keeps data when power is off) and stores data long-term. The three main types to know:

Type	Technology	Advantages	Disadvantages
HDD	Spinning magnetic disks; read/write head	High capacity; cheap per GB	Slower; fragile (moving parts); louder
SSD	Flash memory (no moving parts)	Fast; silent; shock-resistant; lighter	More expensive per GB; limited write cycles
Optical	CD/DVD/Blu-ray; laser reads pits/lands	Cheap; portable; durable for archiving	Low capacity; slow; easily scratched

Exam tip: Be able to explain why both RAM and ROM are needed — RAM is volatile (programs in use), ROM keeps startup instructions permanently. Also know all four registers: PC, MAR, MDR, ACC. Three CPU performance factors: clock speed, cores, cache.

⚠️ Common Mistakes

Confusing RAM (volatile, temporary, read/write) with ROM (non-volatile, permanent, read-only)
Saying "more cache = more storage" — cache is not the same as storage; it's small, fast memory between CPU and RAM
Confusing the MAR and MDR: MAR holds an ADDRESS; MDR holds the actual DATA
Secondary storage and main memory (RAM) are different: secondary is permanent and much slower

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Key points

Von Neumann: program and data stored together in memory; CPU fetches and executes instructions
CU controls data flow; ALU does arithmetic/logic
Four registers: PC (next address), MAR (current address), MDR (data), ACC (result)
FDE cycle: fetch (copy from memory) → decode (CU interprets) → execute (ALU acts)
CPU performance: clock speed (GHz), number of cores, cache size
RAM: volatile, holds running programs; ROM: non-volatile, holds BIOS
Secondary storage: HDD (cheap, slow), SSD (fast, expensive), optical (archiving)

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Worksheet — 3.4.1 Hardware

8 questions · 23 marks

Q1What is the Von Neumann architecture? State its key principle.[2]

✅ Mark scheme

Mark scheme

A computer architecture in which both program instructions and data are stored together in the same memory [1]; the CPU fetches and executes instructions sequentially (stored-program concept) [1].

Q2State the role of the Control Unit (CU) and Arithmetic Logic Unit (ALU).[2]

✅ Mark scheme

Mark scheme

CU: fetches and decodes instructions and controls data flow around the CPU [1]; ALU: performs arithmetic and logical operations (comparisons, AND, OR, NOT) [1].

Q3Name the four CPU registers and state what each one stores.[4]

✅ Mark scheme

Mark scheme

PC (Program Counter): address of next instruction [1]; MAR (Memory Address Register): address in memory to read from/write to [1]; MDR (Memory Data Register): data fetched from / to be written to memory [1]; ACC (Accumulator): holds result of most recent ALU operation [1].

Q4Describe the Fetch-Decode-Execute cycle in three steps.[3]

✅ Mark scheme

Mark scheme

Fetch: PC value copied to MAR; instruction loaded into MDR; PC incremented [1]; Decode: CU decodes the instruction in the MDR [1]; Execute: ALU carries out the operation; result stored in ACC [1].

Q5State three factors that affect the performance of a CPU.[3]

✅ Mark scheme

Mark scheme

Clock speed (GHz) [1]; number of cores [1]; cache size [1]. (1 mark each)

Q6Explain the difference between RAM and ROM, including whether each is volatile.[3]

✅ Mark scheme

Mark scheme

RAM: volatile — loses contents when powered off; used to store programs and data currently in use [1]; ROM: non-volatile — retains contents without power; used to store the BIOS/firmware [1]; RAM is read/write; ROM is normally read-only [1].

Q7Compare HDDs and SSDs, giving one advantage and one disadvantage of each.[2]

✅ Mark scheme

Mark scheme

HDD: advantage — higher capacity / cheaper per GB [1]; disadvantage — slower / has moving parts so more fragile [1]. OR SSD: advantage — faster / no moving parts (shock-resistant) [1]; disadvantage — more expensive per GB [1]. (Award 2 marks for one complete pair or one mark for each valid point)

Q8A laptop has a dual-core CPU running at 2.5 GHz with 4 GB of RAM. A user upgrades to 16 GB RAM. Explain two ways in which this upgrade could improve the laptop's performance.[4]

✅ Mark scheme

Mark scheme

More programs can be open simultaneously without slowing down [1] because the OS does not need to use slow virtual memory (paging to disk) as frequently [1]; larger programs (e.g. video editing software) can run without data being swapped to the hard drive [1] so operations complete more quickly [1]. (2 × developed points)

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Von Neumann Architecture

The CPU — Central Processing Unit

CPU Registers

The Fetch-Decode-Execute Cycle

Factors Affecting CPU Performance

Memory — RAM and ROM

Secondary Storage

Key points

Worksheet — 3.4.1 Hardware

Mini Test — 3.4.1 Hardware