What you'll learn
This revision guide covers the fundamental hardware and software components that make computer systems function. You'll understand how processors execute instructions, how different types of memory and storage work together, and how system software manages hardware resources. These concepts form the foundation of computer science and appear frequently in WJEC GCSE examinations.
Key terms and definitions
Central Processing Unit (CPU) — the hardware component that executes program instructions by performing the fetch-decode-execute cycle
Random Access Memory (RAM) — volatile primary storage that temporarily holds data and instructions currently in use
Read Only Memory (ROM) — non-volatile primary storage containing boot instructions that cannot be easily modified
Operating System (OS) — system software that manages hardware resources, provides a user interface, and enables application software to run
Secondary storage — non-volatile storage devices that permanently retain data when power is removed
Virtual memory — a memory management technique that uses hard disk space to extend available RAM when physical memory is full
Embedded system — a dedicated computer system designed to perform specific tasks within a larger device
Utility software — system programs that perform maintenance tasks such as file compression, disk defragmentation, and antivirus scanning
Core concepts
The Central Processing Unit
The CPU is the brain of the computer, responsible for executing all program instructions. Modern CPUs contain several key components:
Control Unit (CU) — coordinates the fetch-decode-execute cycle, sending control signals to other components
Arithmetic Logic Unit (ALU) — performs mathematical calculations (addition, subtraction) and logical operations (AND, OR, NOT comparisons)
Registers — extremely fast temporary storage locations within the CPU that hold data currently being processed:
- Program Counter (PC) stores the memory address of the next instruction
- Memory Address Register (MAR) holds the address of data being accessed
- Memory Data Register (MDR) temporarily stores data transferred to/from memory
- Accumulator stores intermediate results of calculations
Cache — small, high-speed memory located on or near the CPU that stores frequently accessed instructions and data, reducing access time to main memory
The Fetch-Decode-Execute Cycle
The CPU processes instructions using a continuous three-stage cycle:
- Fetch: The Control Unit retrieves the next instruction from memory using the address stored in the Program Counter
- Decode: The Control Unit interprets the instruction to determine what operation is required
- Execute: The instruction is carried out (e.g., the ALU performs a calculation or data is moved between registers)
This cycle repeats billions of times per second, measured in Hertz (Hz).
CPU Performance Factors
Three main factors affect CPU performance:
Clock speed — measured in Gigahertz (GHz), determines how many cycles the CPU can complete per second. A 3.5 GHz processor executes 3.5 billion cycles per second. Higher clock speeds generally mean faster processing, though this creates more heat.
Number of cores — modern CPUs contain multiple processing units (cores) that can execute instructions simultaneously. A quad-core processor has four cores, enabling parallel processing of tasks. This improves performance for multitasking and software designed for multi-core systems.
Cache size — larger cache memory reduces the time the CPU spends waiting for data from RAM. Level 1 (L1) cache is fastest but smallest; Level 2 (L2) and Level 3 (L3) cache are progressively larger but slightly slower.
Primary Storage (Memory)
Primary storage holds data and instructions that the CPU needs immediate access to.
RAM (Random Access Memory)
RAM is volatile memory that loses all contents when power is switched off. It stores:
- The operating system currently running
- Application programs in use
- Data being actively processed
More RAM allows more applications to run simultaneously without performance degradation. When RAM is full, the system uses virtual memory, swapping data to the hard disk, which significantly slows performance.
ROM (Read Only Memory)
ROM is non-volatile memory containing permanent instructions that cannot be lost when power is removed. It stores:
- Boot instructions (BIOS/UEFI) that run when the computer starts
- Basic input/output operations
- Firmware for embedded systems
ROM is essential because the computer needs instructions to load the operating system from secondary storage into RAM when powered on.
Secondary Storage
Secondary storage provides permanent, non-volatile storage with much larger capacity than primary storage but slower access speeds.
Hard Disk Drives (HDD)
Traditional magnetic storage using spinning platters and read/write heads. HDDs offer:
- Large capacity (several terabytes)
- Low cost per gigabyte
- Slower access times (moving parts create latency)
- Suitable for bulk storage of files, applications, and backups
Solid State Drives (SSD)
Flash memory-based storage with no moving parts. SSDs provide:
- Much faster read/write speeds than HDDs
- More durable (no mechanical failure risk)
- Silent operation
- Higher cost per gigabyte
- Ideal for operating systems and frequently accessed programs
Optical Storage
CDs, DVDs, and Blu-ray discs use laser technology to read/write data:
- Portable and suitable for software distribution
- Relatively slow access speeds
- Limited capacity (CD: 700MB, DVD: 4.7GB, Blu-ray: 25-50GB)
- Declining in popularity due to cloud storage and USB drives
Cloud Storage
Data stored on remote servers accessed via the internet:
- Accessible from any location with internet connectivity
- Automatic backup and file synchronization
- Subscription costs for larger storage amounts
- Dependent on internet speed and reliability
- Raises data security and privacy concerns
Operating Systems
The operating system is essential system software that acts as an interface between hardware and application software.
Key Functions of an OS
Memory management — allocates RAM to running programs, manages virtual memory, and prevents programs from interfering with each other's memory space
Processor scheduling — determines which processes receive CPU time and for how long, ensuring fair resource allocation and preventing any single program from monopolizing the processor
User interface provision — provides either a Command Line Interface (CLI) requiring typed commands, or a Graphical User Interface (GUI) with windows, icons, and pointer-based interaction
File management — organizes data into hierarchical folders/directories, handles file creation, deletion, moving, and copying, and manages file permissions
Device management — controls input/output devices through drivers (specialized software that enables the OS to communicate with specific hardware)
Security — manages user accounts, authentication, and access permissions to protect system resources
Error handling — detects and reports hardware/software errors to prevent system crashes
Virtual Memory
When RAM becomes full, the operating system uses a portion of secondary storage (usually the hard disk) as an extension of RAM. This technique:
- Allows more programs to run than physical RAM would normally permit
- Swaps inactive data from RAM to disk, freeing space for active processes
- Creates significant performance slowdowns because disk access is thousands of times slower than RAM access
- Is managed automatically by the OS without user intervention
Utility Software
Utility programs perform specific system maintenance tasks to optimize performance and protect data.
Disk defragmentation — reorganizes fragmented files on HDDs so data is stored contiguously, reducing read/write head movement and improving access speeds (unnecessary for SSDs)
File compression — reduces file sizes using algorithms that remove redundancy, making files faster to transfer and requiring less storage space. Lossless compression (ZIP, PNG) allows perfect reconstruction; lossy compression (JPEG, MP3) achieves greater reduction by discarding some data
Antivirus software — scans files for malware signatures, monitors system behavior for suspicious activity, quarantines infected files, and updates virus definitions regularly
Backup utilities — automatically copy files to separate storage locations, creating incremental backups (only changed files) or full backups (all data) to protect against data loss
Firewalls — monitor incoming and outgoing network traffic, blocking unauthorized access while permitting legitimate communication based on security rules
Embedded Systems
Embedded systems are specialized computers built into larger devices to perform dedicated functions. Unlike general-purpose computers, they:
- Execute specific, predefined tasks repeatedly
- Run specialized software stored in ROM
- Have minimal user interface (often just simple controls or displays)
- Consume low power and operate reliably for extended periods
- Cannot be easily reprogrammed by users
Common examples include:
- Washing machines (controlling wash cycles and temperature)
- Digital cameras (processing images and managing settings)
- Traffic light systems (managing intersection timing)
- Central heating controllers (maintaining temperature schedules)
- Automotive engine management systems (optimizing fuel injection and ignition)
Worked examples
Example 1: Comparing storage technologies
Question: A school is purchasing new computers. Explain two reasons why solid state drives are more suitable than hard disk drives for storing the operating system. [4 marks]
Model answer: SSDs have much faster read/write speeds than HDDs [1 mark], which means the operating system will load significantly faster when the computer boots up and programs will launch more quickly [1 mark]. SSDs have no moving parts unlike HDDs which use spinning platters [1 mark], making them more reliable and less likely to fail from physical damage if the laptop is moved or dropped [1 mark].
Examiner note: Each reason requires both a feature and its consequence/benefit to earn both marks.
Example 2: CPU performance factors
Question: A laptop has a dual-core 2.4 GHz processor with 4 MB cache. The manufacturer releases a new model with a quad-core 2.0 GHz processor with 8 MB cache. Evaluate whether the new model will have better performance. [6 marks]
Model answer: The new processor has more cores (four instead of two) [1 mark], which allows more instructions to be processed simultaneously and improves multitasking performance [1 mark]. The larger cache (8 MB instead of 4 MB) [1 mark] means more frequently used instructions can be stored closer to the CPU, reducing the time spent fetching data from RAM [1 mark]. However, the clock speed is lower (2.0 GHz instead of 2.4 GHz) [1 mark], which means each core completes fewer cycles per second, potentially reducing performance for single-threaded applications that cannot use multiple cores [1 mark].
Conclusion: The new model will likely perform better for multitasking and modern software designed for multiple cores, but may be slower for older single-threaded applications. Overall, the additional cores and cache probably outweigh the modest clock speed reduction for typical use. [Additional quality marks for balanced evaluation]
Example 3: Operating system functions
Question: Describe how the operating system manages memory when a user opens multiple applications. [4 marks]
Model answer: The operating system allocates portions of RAM to each application [1 mark], ensuring that programs cannot access or overwrite each other's memory space [1 mark]. If RAM becomes full, the OS uses virtual memory [1 mark], moving inactive data from RAM to secondary storage to free space for active programs [1 mark].
Common mistakes and how to avoid them
Confusing primary and secondary storage — Remember: primary storage (RAM/ROM) is directly accessed by the CPU and is limited in size; secondary storage (HDD/SSD) is permanent but slower and accessed through the operating system. Use "volatile/non-volatile" and "fast/slow" to distinguish clearly.
Mixing up RAM and ROM functions — RAM is volatile and holds data currently being used; ROM is non-volatile and contains boot instructions. Don't say "ROM runs the operating system" — it only starts the boot process; the OS loads into RAM.
Assuming more cores always means better performance — Multi-core processors only improve performance for software designed to use multiple cores (multi-threaded applications). Single-threaded programs run on one core regardless of how many are available.
Writing vague explanations without consequences — When explaining why a technology is suitable, state both the feature AND its benefit. For example, "SSDs are faster" is incomplete; add "which reduces boot time and improves application loading speed."
Confusing virtual memory with RAM — Virtual memory is part of the hard disk used as overflow when RAM is full. It doesn't increase actual RAM; it prevents crashes but significantly slows performance.
Ignoring the context in questions — If a question asks about laptops, mention portability or battery life; for servers, discuss reliability and capacity. Tailor answers to the specific scenario provided.
Exam technique for "Computer Systems: Hardware and Software"
Identify command words precisely — "State" needs a brief fact (1 mark); "Describe" requires features plus some detail (2 marks); "Explain" demands reasons or consequences using connecting words like "because," "therefore," "which means"; "Evaluate" requires balanced arguments with a justified conclusion.
Use technical terminology accurately — Write "volatile" instead of "temporary," "fetch-decode-execute cycle" instead of "how the CPU works," and "device drivers" instead of "programs for hardware." Precise language earns marks.
Structure extended answers logically — For 4+ mark questions, use point-evidence-explanation structure. Make a technical point, provide specific detail, then explain the consequence or benefit relevant to the question context.
Calculate marks per minute — With approximately 1.5 minutes per mark, a 6-mark question deserves 9 minutes. Don't spend 15 minutes on a 2-mark definition question or rush a 6-mark evaluation.
Quick revision summary
Computer systems rely on hardware (CPU, RAM, ROM, storage devices) and software (operating systems and utilities) working together. The CPU executes instructions through the fetch-decode-execute cycle; performance depends on clock speed, cores, and cache size. RAM provides fast volatile storage; ROM holds boot instructions. Secondary storage (HDD, SSD, optical, cloud) offers permanent data retention. The operating system manages memory, processes, files, devices, and security. Virtual memory extends RAM using disk space. Utility software maintains system performance and security. Embedded systems perform dedicated tasks within larger devices. Understanding these components and their interactions is essential for WJEC GCSE Computer Science success.