Computer Systems: Hardware

What you'll learn

This guide covers all hardware topics required for Pearson Edexcel International IGCSE Computer Science. You will understand the purpose and operation of key hardware components including the CPU, memory, storage devices, and input/output peripherals. You will also learn how these components work together to form a functioning computer system and be able to explain their characteristics and limitations.

Key terms and definitions

Central Processing Unit (CPU) — the electronic circuitry that executes program instructions by performing arithmetic, logic, control and input/output operations specified by the instructions

Random Access Memory (RAM) — volatile primary storage that temporarily holds data and programs currently in use; contents are lost when power is removed

Read-Only Memory (ROM) — non-volatile primary storage containing permanent instructions (firmware) that cannot be easily modified and are retained when power is removed

Secondary storage — non-volatile storage used to permanently store programs and data when not in active use; examples include hard disk drives, solid state drives, and optical media

Clock speed — the frequency at which the CPU executes instructions, measured in hertz (Hz); one hertz equals one cycle per second

Cache memory — very fast, small-capacity memory located on or near the CPU that stores frequently accessed data and instructions to reduce access time

Fetch-Execute cycle — the sequence of operations performed by the CPU to retrieve an instruction from memory, decode it, execute it, and store the result

Peripheral — an external hardware device connected to a computer system to provide input, output, or storage functionality

Core concepts

The Central Processing Unit (CPU)

The CPU is the 'brain' of the computer system, responsible for executing all program instructions. It consists of three main components:

The Arithmetic Logic Unit (ALU)

• Performs arithmetic operations (addition, subtraction, multiplication, division)
• Carries out logical operations (AND, OR, NOT, comparisons)
• Processes data according to instructions received from the Control Unit

The Control Unit (CU)

• Coordinates and controls all operations within the CPU
• Fetches instructions from memory
• Decodes instructions to determine required actions
• Sends control signals to other components
• Manages the flow of data between CPU components

Registers

• Very fast, small memory locations within the CPU
• Temporarily store data, addresses, and instructions during processing
• Key registers include:
  • Program Counter (PC): holds the address of the next instruction to execute
  • Accumulator (ACC): stores results of calculations performed by the ALU
  • Memory Address Register (MAR): holds the address of the memory location being accessed
  • Memory Data Register (MDR): temporarily stores data being transferred to or from memory
  • Current Instruction Register (CIR): holds the instruction currently being decoded and executed

CPU performance factors

The performance of a CPU depends on several characteristics:

• Clock speed: Higher clock speeds allow more instructions per second. Measured in gigahertz (GHz) — billions of cycles per second
• Number of cores: Multiple cores allow parallel processing of instructions. A quad-core processor has four independent processing units
• Cache size: Larger cache reduces the need to fetch data from slower RAM, improving performance
• Word length: The number of bits processed in a single operation. Common word lengths are 32-bit and 64-bit

The Fetch-Execute Cycle

The CPU repeatedly performs the fetch-execute cycle to process instructions:

Fetch phase

1. Address from Program Counter copied to Memory Address Register
2. Instruction at that address copied from RAM to Memory Data Register
3. Instruction copied from MDR to Current Instruction Register
4. Program Counter incremented to point to next instruction

Decode phase

1. Instruction in CIR is decoded by Control Unit
2. Control Unit determines what operation is required
3. Control Unit identifies which data is needed

Execute phase

1. Control Unit sends signals to appropriate components
2. ALU performs calculations if required
3. Results stored in accumulator or memory
4. Cycle repeats from fetch phase

This cycle occurs billions of times per second in modern processors.

Primary Storage (Memory)

Primary storage provides fast access to data and programs currently being used by the CPU.

Random Access Memory (RAM)

RAM serves as the computer's working memory:

• Volatile: contents lost when power removed
• Stores currently running programs and data in use
• Directly accessible by CPU
• Read and write operations both possible
• Faster access times than secondary storage
• More RAM allows more programs to run simultaneously
• Typical capacities: 4GB to 32GB in modern systems

Read-Only Memory (ROM)

ROM contains permanent instructions required to start the computer:

• Non-volatile: retains contents when powered off
• Stores boot instructions and firmware (BIOS/UEFI)
• Cannot be easily modified by user
• Read operations only during normal use
• Small capacity compared to RAM (typically megabytes)
• Contents set during manufacture or with special equipment

Virtual Memory

When RAM becomes full, the operating system uses secondary storage as an extension:

• Section of hard disk/SSD used as additional memory
• Slower than physical RAM
• Allows more programs to run than RAM alone would permit
• Data swapped between RAM and virtual memory as needed
• Excessive use causes system slowdown (thrashing)

Secondary Storage

Secondary storage provides permanent, non-volatile storage for programs and data.

Hard Disk Drives (HDD)

• Use rotating magnetic platters and read/write heads
• Large capacities (500GB to several terabytes)
• Relatively slow access times due to mechanical movement
• Lower cost per gigabyte than SSD
• Suitable for bulk storage of files, media libraries
• Fragile — can be damaged by physical shock

Solid State Drives (SSD)

• Use flash memory chips with no moving parts
• Faster access times than HDD
• More resistant to physical shock
• Lower capacities than HDD at equivalent price point
• Higher cost per gigabyte
• Limited number of write cycles (though very high in practice)
• Increasingly common as primary drives in laptops

Optical Media

• CD-ROM: 700MB capacity, read-only
• CD-R: writable once, 700MB capacity
• CD-RW: rewritable, 700MB capacity
• DVD: 4.7GB single-layer, 8.5GB dual-layer
• Blu-ray: 25GB single-layer, 50GB dual-layer
• Portable and durable for distribution
• Slow access times compared to HDD/SSD
• Declining use due to internet downloads and streaming

USB Flash Drives

• Use flash memory technology
• Portable, compact, robust
• Capacities from 8GB to 1TB+
• Convenient for transferring files between systems
• Moderate speed compared to internal drives

Storage selection criteria

• Capacity: amount of data to be stored
• Speed: access time requirements
• Portability: whether device needs to move between systems
• Durability: resistance to physical damage
• Cost: budget constraints per gigabyte
• Reliability: expected lifespan and failure rates

Input and Output Devices

Input devices convert real-world data into digital format the computer can process:

• Keyboard: text and command entry; QWERTY layout standard
• Mouse: pointing device for graphical interfaces; optical or laser tracking
• Touchscreen: direct manipulation of on-screen elements; capacitive or resistive technology
• Scanner: converts physical documents to digital images; uses light sensors
• Microphone: captures audio input; converts sound waves to electrical signals
• Webcam: captures video input; used for video conferencing, streaming
• Barcode reader: reads product codes; common in retail environments
• Sensors: measure physical properties (temperature, light, pressure, motion)

Output devices convert digital data into human-readable or usable form:

• Monitor: displays visual output; LCD, LED, or OLED technology
• Printer: produces physical documents
  • Inkjet: sprays liquid ink; good quality photos; slower; higher running costs
  • Laser: uses toner and heat; fast; high volume; lower running costs
• Speaker: outputs audio; converts electrical signals to sound waves
• Actuator: produces physical movement based on computer signals; used in robotics and control systems

Device selection depends on:

• Purpose and application requirements
• Required input/output format
• Speed and performance needs
• Accuracy and quality requirements
• Budget and running costs
• Environmental considerations

Embedded Systems

An embedded system is a computer system designed to perform dedicated functions within a larger mechanical or electrical system.

Characteristics:

• Purpose-built for specific task
• Often real-time operation requirements
• Limited user interface or no interface
• Low power consumption
• Compact size
• Reliable operation over long periods

Examples:

• Digital cameras: process images, control settings
• Washing machines: control wash cycles, temperature, timing
• Microwave ovens: control heating time, power levels
• Automotive systems: engine management, anti-lock brakes, entertainment
• ATMs: manage cash dispensing, card reading, transaction processing
• Smart home devices: thermostats, security systems, lighting control

Embedded systems typically use simpler processors than general-purpose computers, with just enough power for their specific function, reducing cost and power consumption.

Worked examples

Example 1: CPU architecture (4 marks)

Question: Describe the role of the Control Unit and Arithmetic Logic Unit in the CPU.

Mark scheme answer:

The Control Unit:

• Coordinates all CPU operations (1 mark)
• Decodes instructions fetched from memory (1 mark)

The Arithmetic Logic Unit:

• Performs arithmetic operations such as addition and subtraction (1 mark)
• Carries out logical operations such as comparisons and Boolean operations (1 mark)

Example 2: Storage comparison (6 marks)

Question: A photographer needs to choose storage for 500GB of high-resolution images. Compare the suitability of a hard disk drive (HDD) and solid state drive (SSD) for this purpose.

Mark scheme answer:

HDD advantages:

• Lower cost per gigabyte for the large capacity required (1 mark)
• 500GB HDD significantly cheaper than equivalent SSD (1 mark)

SSD advantages:

• Faster access to image files, improving workflow efficiency (1 mark)
• More reliable with no moving parts, reducing risk of data loss (1 mark)
• Better for laptop use if photographer travels, as more shock-resistant (1 mark)

Conclusion:

• SSD preferred if budget allows due to speed and reliability (1 mark)

Example 3: Fetch-Execute cycle (5 marks)

Question: Explain what happens during the fetch phase of the fetch-execute cycle.

Mark scheme answer:

• Address from Program Counter is copied to Memory Address Register (1 mark)
• Instruction at that memory address is fetched from RAM (1 mark)
• Instruction is copied to Memory Data Register (1 mark)
• Instruction is transferred from MDR to Current Instruction Register (1 mark)
• Program Counter is incremented to point to the next instruction (1 mark)

Common mistakes and how to avoid them

• Confusing RAM and ROM: Remember RAM is volatile (loses data) and used for active programs; ROM is non-volatile and stores boot instructions. Don't say ROM is for permanent file storage — that's secondary storage.

• Mixing up capacity units: Be precise with units. 1GB = 1000MB (or 1024MB). Don't confuse gigabytes with gigabits. Storage uses bytes; network speeds often use bits.

• Vague CPU performance explanations: Don't just say "higher clock speed means faster." Specify that higher clock speed allows more instructions per second to be executed, improving processing performance.

• Incomplete fetch-execute descriptions: Always include all phases and the role of specific registers. Don't omit the Program Counter increment or the role of MAR/MDR.

• Confusing storage types: HDD has moving parts (magnetic); SSD has no moving parts (flash memory). Don't mix up their characteristics or advantages.

• Saying cache is slower than RAM: Cache is actually faster than RAM — that's why it's used. It stores frequently accessed data for rapid retrieval.

Exam technique for "Computer Systems: Hardware"

• "Describe" questions: Provide characteristics and features. For 4 marks, give four distinct points. Example: "Describe the features of RAM" requires four separate characteristics like volatile, stores current programs, directly accessible, read-write capability.

• "Explain" questions: Give reasons and consequences, not just facts. Use connectives like "therefore," "because," "which means." For "Explain why SSD is faster than HDD" — state no moving parts, therefore no seek time, which means faster data access.

• Comparison questions: Use comparative language. Say "faster than," "more expensive than," "larger capacity than." Present advantages and disadvantages of both options. Structure: Option A advantages, Option B advantages, conclusion if asked.

• Mark allocation guides depth: 1 mark = simple fact; 2-3 marks = explanation with reason; 4-6 marks = detailed comparison or multi-step process. Allocate your time accordingly — don't write paragraphs for 1-mark questions.

Quick revision summary

The CPU executes instructions through the fetch-execute cycle using the Control Unit, ALU, and registers. Performance depends on clock speed, cores, and cache. RAM is volatile primary storage for active programs; ROM stores boot instructions. Secondary storage (HDD, SSD, optical media) provides permanent storage with different capacity, speed, and cost characteristics. Input devices convert real-world data to digital; output devices convert digital to human-readable formats. Embedded systems are dedicated computers within larger devices. Always explain with specific technical details and compare using precise criteria.