Hardware

Worth about 18 of the 75 marks on a typical Paper 1, together with Data representation the biggest topic.

1. The CPU and microprocessors

The central processing unit (CPU) processes the instructions and data that are input into a computer so that the result can be output. A microprocessor is a type of integrated circuit on a single chip; the CPU of most devices is a microprocessor.

2. Von Neumann architecture

In a Von Neumann computer, program instructions and data are stored together in the same memory (RAM). The CPU fetches both from memory along the same buses.

Component	Type	Purpose
Arithmetic logic unit (ALU)	Unit	Carries out calculations and logic comparisons
Control unit (CU)	Unit	Decodes instructions and sends control signals to manage the cycle
Program counter (PC)	Register	Holds the address of the next instruction to be fetched
Memory address register (MAR)	Register	Holds the address of the memory location being read from or written to
Memory data register (MDR)	Register	Holds the data or instruction fetched from memory, or about to be written to it
Current instruction register (CIR)	Register	Holds the instruction currently being decoded and executed
Accumulator (ACC)	Register	Holds the result of calculations from the ALU
Address bus	Bus	Carries addresses from the CPU to memory (one direction only)
Data bus	Bus	Carries data and instructions between the CPU and memory (both directions)
Control bus	Bus	Carries control signals, such as read, write and clock pulses (both directions)

3. The fetch–decode–execute cycle

1. Fetch: the address in the PC is copied to the MAR (along the address bus). The instruction at that address is sent to the MDR (along the data bus). The PC is incremented by 1. The instruction is copied from the MDR to the CIR.
2. Decode: the control unit decodes the instruction in the CIR to work out what to do.
3. Execute: the instruction is carried out, for example the ALU does a calculation and stores the result in the accumulator, or data is loaded from or stored in memory.

The cycle then repeats for the next instruction. Step through it below with a three-line program.

4. Cores, cache and clock

Factor	What it is	Effect on performance
Cores	Each core is a processing unit that can run its own FDE cycle	More cores can process more instructions at the same time, if the software can use them
Cache	Small, very fast memory inside or near the CPU that stores frequently used instructions and data	A larger cache means fewer trips to slower RAM, so instructions are fetched faster
Clock	Sends regular pulses that set the pace of the CPU; clock speed is the number of FDE cycles per second (Hz)	A faster clock carries out more cycles, and so more instructions, per second

The effects have limits: doubling the cores does not always double the speed, and a faster clock produces more heat.

5. Instruction sets

An instruction set is the list of all the commands a CPU can process. These commands are machine code. Different CPU designs have different instruction sets, so a program compiled for one may not run on another.

6. Embedded systems

An embedded system is a computer system built into a larger device to perform one dedicated function. A general-purpose computer, such as a PC or laptop, can perform many different functions.

• Examples: domestic appliances (washing machines, microwaves), cars (engine management, airbags), security systems, lighting systems, vending machines.
• Characteristics: small, low power, cheap to mass-produce, often controlled by sensors, and their program (firmware) rarely changes.

7. Input devices

An input device sends data into the computer so it can be processed.

Device	How it works
Barcode scanner	Shines light on the bars; black bars absorb it and white bars reflect it. The reflected pattern is converted to a number
QR code scanner	A camera captures the 2D square pattern, which can hold much more data than a barcode, such as a web address
Digital camera	Light passes through the lens onto a sensor made of millions of light-sensitive pixels, which is converted into a digital image
Keyboard	Each key press completes a circuit; the key’s character code is sent to the computer
Microphone	Sound waves vibrate a diaphragm, creating an electrical signal that is converted to digital
Optical mouse	An LED lights the surface and a sensor takes many images per second; changes between images show the movement
2D scanner	Light scans a flat document; the reflected light is captured to create a digital image
3D scanner	Lasers or light scan an object from many angles to create a digital 3D model of its shape

Touch screens

Type	How it works	Pros and cons
Resistive	Two layers are pressed together; the point of contact is detected	Cheap, works with gloves or a stylus; less clear, no multi-touch
Capacitive	Detects the change in electrical charge when a finger touches the screen	Clear, multi-touch, durable; does not work with ordinary gloves
Infra-red	A grid of infra-red beams across the screen; a touch breaks the beams	Works with any object, good for large screens; can be triggered by accident

8. Output devices

An output device turns processed data into a form people (or machines) can use.

Device	How it works
Actuator	Converts an electrical signal into movement, such as a motor opening a valve
DLP projector	Millions of tiny mirrors tilt to reflect light towards or away from the lens to form the image
LCD projector	Bright light passes through LCD panels that block or let through light to form the image
Inkjet printer	Nozzles spray tiny droplets of ink onto the paper as the print head moves across it
Laser printer	A laser creates an electrostatic image on a rotating drum; toner sticks to it, is transferred to paper and fused with heat
3D printer	Builds a solid object layer by layer from a digital design, for example from melted plastic
LED screen	Each pixel is made of LEDs that give off light
LCD screen	Liquid crystals block or let through light from a backlight (usually LEDs) to form each pixel
Speaker	A digital signal is converted to analogue; it moves a cone that vibrates the air to make sound

9. Sensors

A sensor measures a physical property and sends the data to a computer. In exam answers, choose the sensor that fits the context and say how it is used there.

Sensor	Measures	Example use
Acoustic	Sound	Detecting breaking glass in a security system
Accelerometer	Acceleration and movement	Detecting a phone being tilted, or a car crash for airbags
Flow	Rate of flow of a liquid or gas	Monitoring water flow in pipes
Gas	Level of a gas, e.g. carbon dioxide	Smoke or carbon monoxide alarms
Humidity	Water vapour in the air	Greenhouse climate control
Infra-red	Infra-red radiation (heat)	Detecting a person for automatic doors or alarms
Level	Level of a liquid	Fuel or water tank monitoring
Light	Light level	Street lights turning on at dusk
Magnetic field	Magnetic fields	Detecting a door or window opening
Moisture	Water content, e.g. of soil	Automatic watering of crops
pH	Acidity or alkalinity	Monitoring soil or a swimming pool
Pressure	Pressure or force	Detecting weight on a car seat or in a burglar alarm mat
Proximity	How near an object is	Parking sensors
Temperature	Temperature	Central heating thermostat

10. Primary storage: RAM and ROM

Primary storage is memory that the CPU can access directly.

	RAM (random access memory)	ROM (read only memory)
Volatile?	Yes: data is lost when the power is off	No: data stays when the power is off
Can it be changed?	Read and write	Read only
What it stores	The programs and data currently in use, including the operating system	The start-up instructions (bootstrap) the computer needs to boot

A computer needs both: ROM to start up, and RAM to hold whatever it is working on.

11. Secondary storage

Secondary storage is not directly accessed by the CPU. It is non-volatile, so it is used to store data permanently.

Type	How it works	Examples
Magnetic	Spinning platters divided into tracks and sectors; a read/write head uses electromagnets to magnetise areas. Has moving parts	Hard disk drive (HDD)
Optical	A laser burns pits and lands into the disc surface; reflected laser light is read back	CD, DVD, Blu-ray
Solid-state (flash)	NAND or NOR flash memory: transistors act as control gates and floating gates that trap charge. No moving parts	SSD, SD card, USB drive

12. Virtual memory

When RAM is full, the operating system uses part of secondary storage as virtual memory. Pages of data that are not currently needed are moved from RAM to virtual memory, and moved back when they are needed.

It stops the computer crashing or refusing to open programs when RAM runs out, but it is much slower than RAM, so heavy use slows the computer down.

13. Cloud storage

Cloud storage keeps your data on remote physical servers owned by a provider, accessed over the internet, rather than on local storage.

Advantages	Disadvantages
Access from any device with an internet connection	No internet connection means no access
No need to buy and maintain your own storage hardware	Ongoing subscription cost
Easy to increase capacity when needed	You rely on the provider’s security and could lose data if they are hacked or close
Provider usually handles backups	Uploading and downloading large files can be slow

14. Network hardware

• Network interface card (NIC): needed for a computer to connect to a network.
• MAC address: a unique address given to the NIC when it is manufactured. Usually written in hexadecimal as six pairs (e.g. 00:1A:2B:3C:4D:5E). The first half is the manufacturer code; the second half is the serial code.
• IP address: given to a device by the network it joins. It can be static (stays the same) or dynamic (changes each time it connects).
• Router: sends data to a specific destination on a network, can assign IP addresses to devices, and connects a local network to the internet.

	IPv4	IPv6
Size	32 bits	128 bits
Written as	4 denary numbers (0–255) separated by dots, e.g. 192.168.0.12	8 groups of 4 hex digits separated by colons, e.g. 2001:0db8:85a3:0000:0000:8a2e:0370:7334
Number of addresses	About 4.3 billion: running out	Vastly more, enough for every device

15. Examiner tips

1. Describe the whole fetch stage. “The CPU fetches the instruction” scores little; name the PC, MAR, MDR, CIR and the buses in order.
2. Don’t describe the execute stage when asked about fetch, and vice versa.
3. MDR holds the data fetched from memory, not “the data from the MAR”. The MAR holds an address.
4. Say which way cache changes performance. “A larger cache improves performance because…”, not just “cache affects speed”.
5. Clock speed is cycles per second, not “the speed the CPU runs at”.
6. An embedded system has one dedicated function. A smartphone or laptop is general purpose.
7. Use the context with sensors. In a greenhouse question, say what the sensor measures there and what happens next.
8. RAM is primary storage, not secondary, and it is volatile.
9. Be precise about the cloud. Not “access anywhere”, but “access from any device with an internet connection”.
10. SSD answers: “no moving parts” is the key point for durability, speed and power use.

16. Practice questions

17. Topic test: predict your grade