OCR GCSE Computer Science (J277) – Revision Pack + Exam-Ready MCQ Practice

Topics covered: Systems Architecture, Memory & Storage, Operating Systems, Utility Software.

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How to use this pack

1) Read the Revision Notes. 2) Go to MCQ Practice. 3) Submit → see your score + explanations.

Topics included

1.1 Systems Architecture: CPU, FDE, registers, cache, performance, embedded
1.2 Memory & Storage: RAM/ROM, virtual memory, storage types, comparisons
4.3 Operating Systems: Six OS functions + how to recognise them
4.4 Utility Software: Encryption, compression, defrag + comparisons

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Revision Notes

Exam-ready summaries

1.1 Systems Architecture

Know what each CPU component does, the Fetch–Decode–Execute cycle (with registers), and the 3 performance factors OCR expects.

CPU purpose + components

CPU executes program instructions and processes data.
ALU: arithmetic (add/subtract) + logic comparisons (==, >, <).
Control Unit (CU): decodes instructions and sends control signals.
Registers: tiny, ultra-fast storage inside the CPU.
Cache: very fast memory storing frequently used instructions/data to reduce RAM access.

Fetch–Decode–Execute (FDE) cycle

Stage	What happens
Fetch	PC stores the address of the next instruction. PC → MAR (address being accessed). Instruction fetched into MDR. PC increments to the next address.
Decode	CU decodes the instruction (works out what it means and what is needed).
Execute	Instruction is carried out (ALU calculates / data moved / result stored). ACC commonly holds the current result/data being processed.

Common mistake

The PC stores an address, not “how many instructions have happened”.

Registers (easy meanings)

PC: address of current/next instruction.
MAR: address in memory being accessed.
MDR: data/instruction fetched (or to be written).
ACC: current result / data being processed.

Von Neumann (GCSE meaning)

Instructions and data are stored in memory and processed via the FDE cycle.

CPU performance (OCR expects these 3)

Clock speed: more cycles per second → more instructions per second.
Cache size: more frequent items near CPU → less waiting for RAM.
Number of cores: can run tasks in parallel if software can split the work.

Embedded systems

Computer system inside a larger device, designed for a dedicated task.
Examples: washing machine controller, microwave, car engine control, sat nav.

1.2 Memory and Storage

RAM vs ROM, virtual memory, storage types, and comparisons using the 6 characteristics OCR mark schemes reward.

Primary storage: RAM vs ROM

Feature	RAM	ROM
Volatility	Volatile (lost when off)	Non-volatile (kept when off)
Writable?	Read/write	Read-only (not normally changed)
Main use	Running programs + data in use	Boot program (e.g. BIOS) to start the system

Virtual memory

If RAM is full, OS uses secondary storage as extra “pretend RAM”.
Moves less-used data into a page file and swaps it back when needed.
Advantage: more programs can stay open.
Disadvantage: slow because storage is much slower than RAM.

Secondary storage types + comparisons

Magnetic (HDD): large capacity, cheaper per GB, moving parts.
Solid-state (SSD/USB/SD): no moving parts, durable, fast access, often higher cost per GB.
Optical (CD/DVD/Blu-ray): portable, cheap to distribute, lower capacity, can scratch, needs a drive.

Use these 6 comparison points:

Capacity • Speed • Portability • Durability • Reliability • Cost

Exam-style precision for “speed” answers

Write: “faster at reading/writing/accessing data”.
Avoid: “it’s faster” (too vague).

Units + quick conversions

bit (0/1), nibble (4 bits), byte (8 bits)
bits → bytes: ÷ 8 | bytes → bits: × 8

File size calculations (core formulas)

Images

pixels = width × height
size(bits) = pixels × colour depth
size(bytes) = bits ÷ 8
colours possible = 2^(colour depth)

Sound

size(bits) = sample rate × bit depth × duration
Higher sample rate/bit depth → better quality + bigger file

Compression (lossy vs lossless)

Type	What happens	Best for	Not for
Lossy	Data removed permanently → smaller, lower quality	Images/audio/video	Text/program files
Lossless	No data lost → perfect restore	Text/program/important data	When quality loss is acceptable

Storage choice “sentence template”

Use for 4–6 mark evaluation questions:

State the storage type.
Give 2–4 reasons using the 6 characteristics.
Link each reason to the scenario.

Example: “Solid-state is suitable because it is durable (no moving parts) and fast at reading/writing data, which is important when transferring large files.”

4.3 Operating Systems

Learn the six OS functions and how to recognise them quickly in exam questions.

What an OS does

Manages hardware/resources and provides services for applications and users.

The six OS functions (OCR list)

Function	What it does	Example
User interface	Lets user interact (GUI/CLI).	Desktop, menus, command line
Memory management	Allocates RAM; manages swapping/virtual memory.	Giving a program RAM to run
Multitasking	Shares CPU time between processes.	Music + document at once
Peripheral management	Controls I/O devices using device drivers.	Printer driver
File management	Organises files/folders; save/open/delete.	Creating folders
User management	Accounts, logins, permissions/access levels.	Admin vs standard user

Memory management (more detail students can use in exam answers):

Allocates RAM to each program/process so it can run.
Deallocates/frees RAM when a program closes (RAM becomes available again).
Manages virtual memory (swapping/paging): moves less-used data from RAM to a page file on secondary storage and brings it back when needed.
Protection/isolation: keeps each process’ memory separate so one process does not overwrite another’s data.

Memory fragmentation (RAM) – different from disk fragmentation: when programs are loaded/unloaded, free RAM can be left as small gaps between used blocks (external fragmentation), or a process may be given a block that is slightly larger than it needs (internal fragmentation). Some systems reduce this using fixed-size blocks (e.g. paging) or compaction.
Note: Disk fragmentation/defragmentation refers to secondary storage (HDD) and is covered under utility software.

Spot the function (quick rules)

Mentions drivers → peripheral management.
Mentions accounts/permissions → user management.
Mentions RAM allocation/swapping → memory management.
Mentions running multiple programs → multitasking.
Mentions folders/files → file management.

4.4 Utility Software

Utility software = housekeeping tools. Includes key comparisons (e.g. defrag: HDD yes, SSD no).

Utility software (definition)

Performs housekeeping/maintenance to keep the system running efficiently.
Examples: encryption, compression, defragmentation, backup, system cleanup, updates.

Encryption (key exam point)

Uses an algorithm + key to scramble data so it is unreadable without the key.
Does not stop interception — it stops understanding/using the data.

Defragmentation – the comparison students need

What is disk fragmentation (HDD)?

A file is stored in non-contiguous blocks (separate parts) across the disk.
This happens over time as files are saved/edited/deleted and gaps appear.

Why defrag helps an HDD (magnetic)

HDDs can store files in fragments across the disk.
Read/write head must move to each fragment → slower access.
Defrag stores fragments together → less movement → faster access.

Why defrag is not suitable/needed for SSD/flash

No moving parts → access time does not depend on head movement.
Defrag causes unnecessary writing → wastes time and can reduce lifespan over time.

Misconception: Defrag does not increase capacity. It mainly improves access speed on HDDs.

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