Hardware

Architecture: CPU characteristics including Von Neumann

The Central Processing Unit (CPU) is the main processor in a computer. It carries out instructions from programs by performing basic arithmetic, logic, control and input/output operations.

Most modern computers are based on the Von Neumann architecture, proposed by John von Neumann in 1945. Its key principle is that data and instructions are stored together in the same memory and are accessed using the same bus system.

The CPU contains three main components:

• ALU (Arithmetic Logic Unit) — performs all arithmetic calculations (add, subtract) and logical comparisons (greater than, equal to, AND, OR, NOT)
• Control Unit (CU) — directs the operation of the processor. It fetches instructions from memory, decodes them, and coordinates the ALU, registers and memory to execute them
• Registers — tiny, extremely fast storage locations inside the CPU used to hold data and addresses during processing

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Components of CPU in fetch-decode-execute cycle

The CPU continuously repeats three steps to process every instruction:

graph LR
    A[Fetch] --> B[Decode]
    B --> C[Execute]
    C --> A

Registers involved

Register	Full Name	Purpose
PC	Program Counter	Holds the address of the next instruction to be fetched
MAR	Memory Address Register	Holds the address of the memory location being read from or written to
MDR	Memory Data Register	Holds the data that has been fetched from memory or is about to be written to memory
ACC	Accumulator	Stores the result of calculations performed by the ALU
CIR	Current Instruction Register	Holds the instruction currently being decoded and executed

The cycle

1. Fetch

• The address in the PC is copied to the MAR
• The instruction at that address is fetched from RAM into the MDR
• The instruction is then copied from the MDR to the CIR
• The PC is incremented by 1 (to point to the next instruction)

2. Decode

• The Control Unit decodes the instruction in the CIR
• It identifies what operation to perform and what data is needed

3. Execute

• The instruction is carried out — this could be a calculation (ALU), data movement, or a control operation
• Results are stored in the ACC or written back to memory

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Performance factors: Cache size, clock speed, number of cores

Three main factors affect how fast a CPU can process instructions:

Clock Speed

• Measured in hertz (Hz) — typically gigahertz (GHz) for modern processors
• Each “tick” of the clock is one cycle; simple instructions take one cycle, complex ones take more
• A 3.5 GHz processor can perform 3.5 billion cycles per second
• Higher clock speed = more instructions processed per second
• Increasing clock speed generates more heat, requiring better cooling

Cache Size

• Cache is a small amount of very fast memory inside the CPU
• It stores frequently used data and instructions so the CPU doesn’t have to fetch them from slower RAM
• Levels of cache: L1 (smallest, fastest), L2, L3 (largest, slowest of the three)
• Larger cache = fewer trips to RAM = faster performance

Number of Cores

• A core is an independent processing unit within the CPU
• A dual-core processor has 2 cores; a quad-core has 4
• Multiple cores can process different instructions simultaneously (parallel processing)
• More cores = more tasks handled at once
• Not all software can use multiple cores effectively

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Difference between RISC and CISC processors

Feature	RISC	CISC
Full name	Reduced Instruction Set Computer	Complex Instruction Set Computer
Instructions	Small set of simple instructions	Large set of complex instructions
Cycles per instruction	Usually 1 cycle	Multiple cycles
Clock speed	Generally faster	Generally slower
Code length	More lines of code needed	Fewer lines of code needed
Power consumption	Lower (used in mobile devices)	Higher (used in desktops/laptops)
Example	ARM processors (phones, tablets)	Intel x86 processors (PCs)

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Input/output: Use and characteristics of I/O devices

Input devices send data into the computer. Output devices present data from the computer to the user.

Common Input Devices

Device	Use	Characteristics
Keyboard	Typing text and commands	Familiar, accurate for text, slow for large data
Mouse	Pointing and selecting on screen	Intuitive, requires flat surface
Touchscreen	Direct interaction with display	Intuitive, less precise, fingerprint smudges
Microphone	Voice input, recording audio	Hands-free, affected by background noise
Scanner	Digitising physical documents	High quality, slow, bulky
Barcode reader	Reading product codes	Fast, accurate, limited to barcodes
Webcam	Capturing video/images	Real-time, lower quality than dedicated cameras
Sensor	Detecting physical conditions (temperature, pressure, light, motion)	Used in control systems and IoT, provides real-time data, requires ADC to convert analogue readings to digital

Common Output Devices

Device	Use	Characteristics
Monitor	Displaying visual output	High resolution, various sizes
Printer	Producing hard copies	Permanent output, slow, costs per page
Speaker	Audio output	Range of quality, requires amplification
Projector	Large-screen display	Good for presentations, needs dark room
Actuator	Converts electrical signals into physical movement	Used in control systems (motors, valves, servos), essential for robotics and embedded systems

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Primary storage: RAM, ROM, flash memory, cache memory

Primary storage is directly accessible by the CPU and holds data currently in use.

RAM (Random Access Memory)

• Volatile — loses all data when power is turned off
• Stores the operating system, running programs, and data currently in use
• Can be read from and written to
• More RAM allows more programs to run simultaneously

ROM (Read Only Memory)

• Non-volatile — retains data when power is turned off
• Stores the BIOS/boot-up instructions that start the computer
• Can only be read from (not written to in normal use)
• Contents set during manufacture

Cache Memory

• Extremely fast, small, expensive memory inside the CPU
• Stores copies of frequently accessed data from RAM
• Operates in levels: L1 > L2 > L3 (speed vs size trade-off)

Flash Memory

• Non-volatile — retains data without power
• Can be read from and written to electronically
• Used in USB drives, SSDs, SD cards
• No moving parts, durable, but limited write cycles

Type	Volatile?	Read/Write?	Speed	Typical Use
RAM	Yes	Read & Write	Fast	Running programs
ROM	No	Read only	Fast	Boot-up instructions
Cache	Yes	Read & Write	Very fast	Frequently used data
Flash	No	Read & Write	Medium	Portable storage

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Secondary storage: Magnetic, optical and solid state technologies

Secondary storage is non-volatile and used for long-term data storage. It is not directly accessible by the CPU — data must be loaded into RAM first.

Magnetic Storage

• Hard Disk Drive (HDD): spinning platters with a read/write head
• Magnetic tape: sequential access, used for backups and archiving
• High capacity, low cost per GB, but moving parts make it fragile and slower

Optical Storage

• Uses a laser to read/write data on a reflective disc
• Types: CD (~700 MB), DVD (~4.7 GB), Blu-ray (~25 GB)
• Portable, cheap, but low capacity compared to other media and easily scratched

Solid State Storage

• Solid State Drive (SSD): uses flash memory with no moving parts
• USB flash drives, SD cards
• Very fast, durable, silent, lightweight — but more expensive per GB than HDD

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Functional characteristics: Suitability, durability, portability, speed

When choosing storage, consider these characteristics:

Feature	HDD	SSD	Optical	Magnetic Tape
Speed	Medium	Fast	Slow	Very slow
Capacity	Very high	High	Low	Very high
Cost per GB	Low	Medium	Very low	Very low
Durability	Fragile (moving parts)	Durable	Scratches easily	Durable
Portability	Low	High	High	Low
Best for	Desktop storage	OS drive, laptops	Software distribution	Backups, archives

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Storage requirements: Bit, nybble, byte, kilobyte, prefix multipliers

Unit	Size	Equivalent
Bit	1 or 0	Smallest unit of data
Nybble	4 bits	Half a byte
Byte (B)	8 bits	One character
Kilobyte (KB)	1,000 bytes	Short text document
Megabyte (MB)	1,000 KB	A photo or song
Gigabyte (GB)	1,000 MB	A movie
Terabyte (TB)	1,000 GB	Large hard drive
Petabyte (PB)	1,000 TB	Data centre scale

Note: In computing, 1 KB is sometimes treated as 1,024 bytes (using binary prefixes: KiB, MiB, GiB). The WJEC spec uses the 1,000 convention above for simplicity.

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Data capacity and calculating data capacity requirements

Image file size

File size = Width (px) × Height (px) × Colour depth (bits)

• Colour depth is the number of bits per pixel (e.g. 24-bit colour = 16.7 million colours)
• A 1920×1080 image at 24-bit colour: 1920 × 1080 × 24 = 49,766,400 bits ≈ 5.93 MB

Sound file size

File size = Sample rate (Hz) × Bit depth × Duration (s) × Channels

• Sample rate: how many samples per second (e.g. 44,100 Hz for CD quality)
• Bit depth: bits per sample (e.g. 16-bit)
• A 3-minute stereo track at CD quality: 44,100 × 16 × 180 × 2 = 254,016,000 bits ≈ 30.2 MB

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Other hardware: GPU, sound cards, motherboards

GPU (Graphics Processing Unit)

• Specialised processor designed for rendering images and video
• Contains thousands of small cores optimised for parallel processing
• Used for gaming, video editing, 3D modelling, and increasingly for AI workloads
• Can be integrated (built into CPU) or dedicated (separate card)

Sound Card

• Processes audio input and output
• Contains a DAC (Digital-to-Analogue Converter) which converts digital data into analogue signals for speakers and headphones
• Contains an ADC (Analogue-to-Digital Converter) which converts analogue signals from microphones into digital data the computer can process
• Built-in sound is sufficient for most users; dedicated cards offer higher quality for music production

Motherboard

• The main circuit board that connects all components together
• Contains the CPU socket, RAM slots, expansion slots, and connectors for storage and peripherals
• Houses the chipset that manages data flow between the CPU, memory, and peripherals
• Determines what components are compatible with the system

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Embedded systems: Use and examples

Characteristics

• Dedicated purpose — performs one specific function
• Built into a larger device
• Often operates in real time
• Usually has limited resources (memory, processing power)
• Typically no user interface (or a very simple one)
• Often uses ROM to store its program (firmware)

Examples

Device	Embedded System Function
Washing machine	Controls water temperature, spin speed, cycle timing
Traffic lights	Manages signal timing and sequences
Central heating thermostat	Monitors and regulates temperature
Car engine management	Controls fuel injection, emissions, engine timing
Digital watch	Keeps time, manages alarms and display
Smart TV remote	Processes button presses, sends IR/Bluetooth signals
Microwave oven	Controls power levels, timing, and safety interlocks