OxfordAQA IGCSE Computer Science: Computer Systems — Revision Notes

Computer systems (topic 3.4 of OxfordAQA International GCSE Computer Science 9210) covers hardware and software, Boolean logic with NOT, AND and OR gates, system and application software, and the systems architecture – the CPU, the fetch-execute cycle, main memory and secondary storage, cloud storage and embedded systems. It is assessed on Paper 2 with definitions, truth tables, logic circuits and “compare and recommend” questions.

Condensed for the final weeks. For the full explanation, use the Computer systems study guide. Practise with the Computer systems practice questions.

Syllabus coverage

OxfordAQA 9210 specification (Version 3.4), section 3.4:

• 3.4.1 Hardware and software – definitions and the relationship between them
• 3.4.2 Boolean logic – NOT, AND, OR truth tables; circuits with up to three inputs; Boolean expressions
• 3.4.3 Software classification – system and application software; operating systems and utilities
• 3.4.4 Systems architecture – Von Neumann architecture; main memory, ALU, control unit, clock, buses; performance; fetch-execute cycle; RAM and ROM; secondary storage; cloud storage; embedded systems

NAND, NOR and XOR are not required for 9210.

Hardware and software

• Hardware: the physical components of a computer system.
• Software: the programs that run on the hardware and tell it what to do.
• They depend on each other: hardware needs software to do anything useful, and software needs hardware to run on.

Boolean logic

Notation used in 9210: NOT A is written as A with a bar over it (Ā), A AND B as A · B, A OR B as A + B.

A	B	NOT A	A · B	A + B
0	0	1	0	0
0	1	1	0	1
1	0	0	0	1
1	1	0	1	1

Worked example: a three-input circuit

Q = (A · B) + C̄ – A and B go into an AND gate; C goes into a NOT gate; both outputs go into an OR gate.

A	B	C	A · B	C̄	Q
0	0	0	0	1	1
0	0	1	0	0	0
0	1	0	0	1	1
0	1	1	0	0	0
1	0	0	0	1	1
1	0	1	0	0	0
1	1	0	1	1	1
1	1	1	1	0	1

Three inputs give 2³ = 8 rows. Add a column for each intermediate gate output – it prevents errors.

System and application software

Type	Purpose	Examples
System software	Manages the computer’s resources and acts as a platform to run application software	Operating system, utility programs
Application software	Performs end-user tasks	Word processor, web browser, game
Utility programs	Maintenance tasks that keep the computer working well	Antivirus, backup, disk defragmenter, compression

The operating system manages the processor(s), memory, I/O devices, applications and security (for example user accounts and access permissions).

Von Neumann architecture and the CPU

In the Von Neumann architecture, program instructions and data are both stored in the same main memory, and the CPU fetches and executes the instructions one at a time.

Component	Role
ALU (arithmetic logic unit)	Carries out arithmetic (e.g. addition) and logical operations (e.g. comparisons)
Control unit	Decodes instructions and sends control signals to coordinate the other components
Clock	Sends regular electrical pulses that keep the CPU’s components working in step
Buses	Collections of wires that carry data, addresses and control signals between components
Main memory	Memory directly accessible by the CPU (except cache and registers) holding running programs and data

Fetch-execute cycle: fetch – the next instruction is fetched to the CPU from main memory; decode – the instruction is decoded to work out what it is; execute – the instruction is carried out, which may include reading from or writing to main memory. This repeats continually.

CPU performance:

Factor	Effect
Clock speed	Higher speed -> more fetch-execute cycles per second
Number of cores	More cores -> more instructions processed at the same time (if the task can be split)
Cache size	Larger cache -> more frequently used data and instructions held in very fast memory, so fewer slow trips to main memory

Memory and storage

• Volatile memory loses its contents when the power is off; non-volatile memory keeps them.
• RAM: volatile, read and write, holds the programs and data currently in use. ROM: non-volatile, read-only, holds the start-up (boot) instructions.
• Secondary storage is any non-volatile storage not directly accessible by the CPU. It is needed because RAM is volatile and too small to hold all programs and files permanently.

Type	How it works	Strengths	Weaknesses
Magnetic (hard disk)	Read/write heads magnetise areas of spinning platters	High capacity, low cost per GB	Moving parts can be damaged; slower than SSD
Solid state (SSD, USB stick)	Electrical circuits store data persistently, with no moving parts	Fast access, robust, low power, silent	Higher cost per GB
Optical (CD, DVD, Blu-ray)	A laser reads pits and lands on the disc’s surface	Cheap, portable	Low capacity, slow, scratches easily

• Cloud storage: files stored on remote servers (using magnetic and increasingly solid state storage) and accessed over the Internet. Advantages: access from anywhere, no local hardware to buy, provider handles backups, easy sharing. Disadvantages: needs an Internet connection, ongoing fees, less control over security and privacy of data.
• Embedded system: a computer built into a larger device to perform one dedicated function, e.g. a washing machine controller, a car’s engine management unit. A non-embedded system, such as a laptop, is general-purpose and runs many different programs.

Common mistakes / exam traps

• Drawing NAND, NOR or XOR gates – use only NOT, AND and OR.
• Missing rows in a three-input truth table: there must be 8.
• Describing ROM as “memory you cannot change” without saying it is non-volatile and holds start-up instructions.
• Saying more cores always make a program faster – only if it can be split into parallel tasks.
• Calling cache “secondary storage” – cache is very fast memory inside or near the CPU.
• Recommending a storage type without linking it to the scenario (capacity, speed, portability, durability, cost).

Frequently asked questions

What is the difference between main memory and secondary storage? Main memory (RAM) is directly accessed by the CPU and holds what is currently running; most of it is volatile. Secondary storage is non-volatile, is not directly accessed by the CPU and keeps files permanently.

Why does a larger cache improve performance? Cache is much faster than main memory. A larger cache holds more of the instructions and data the CPU uses often, so it waits less often for main memory.

Give an example of an embedded system. A microwave oven’s controller is an embedded system: a small computer that performs one dedicated job inside a larger device.

Related resources

Computer systems study guide | Computer systems practice questions | Data representation revision notes

Official syllabus

OxfordAQA International GCSE Computer Science (9210) specification, Version 3.4, section 3.4 – oxfordaqa.com/9210.