Cambridge A Level Computer Science (9618): Hardware – Revision Notes

Topic 3 Hardware covers the parts of a computer system (memory, storage, input and output devices, embedded systems, and monitoring and control systems) and the logic gates they are built from. In Cambridge International AS & A Level Computer Science 9618 it is examined in Paper 1 Theory Fundamentals, where you must explain how named devices work, compare memory types, and construct truth tables, logic circuits and logic expressions without error.

Related: Hardware study guide · Hardware practice questions · Computer Science course hub

Syllabus coverage

These notes follow the 9618 syllabus subject content for Topic 3 (the 2027-2029 syllabus states it has no significant changes from 2026).

Section	Candidates should be able to
3.1	Show understanding of the need for input, output, primary memory and secondary (including removable) storage
3.1	Show understanding of embedded systems, including their benefits and drawbacks
3.1	Describe the principal operations of: laser printer, 3D printer, microphone, speakers, magnetic hard disk, solid state (flash) memory, optical disc reader/writer, touchscreen, virtual reality headset
3.1	Show understanding of the use of buffers
3.1	Explain the differences between RAM and ROM, and between SRAM and DRAM, with their uses in devices
3.1	Explain the difference between PROM, EPROM and EEPROM
3.1	Show an understanding of monitoring and control systems: sensors (temperature, pressure, infra-red, sound), actuators and feedback
3.2	Use the symbols for, and define the functions of, NOT, AND, OR, NAND, NOR and XOR gates (two inputs except NOT)
3.2	Construct a logic circuit, a truth table and a logic expression from a problem statement or from either of the other two forms

Key definitions

Term	Definition
Primary memory	Memory the processor accesses directly (RAM and ROM)
Secondary storage	Non-volatile storage for programs and data not currently in use; includes removable media
Volatile	Contents are lost when the power is switched off
Embedded system	A computer system built into a larger device to carry out a dedicated function
Buffer	A temporary memory area holding data while it is transferred between devices or processes that work at different speeds
Sensor	An input device that measures a physical quantity such as temperature or pressure
Actuator	A device that turns an electrical signal from a computer into physical movement, e.g. a motor or valve
Feedback	Output of a control system affecting its next input, so readings taken after an actuator acts decide the next action
Truth table	A table giving the output of a gate or circuit for every combination of inputs

Why does a computer need input, output, primary memory and secondary storage?

• Input devices capture data from the user or environment; output devices present results.
• Primary memory holds the instructions and data the processor is working on, because the processor can only execute what is in main memory.
• Secondary storage keeps programs and files permanently, because RAM is volatile and too small to hold everything. Removable storage (USB flash drive, optical disc, external drive) lets data be moved between computers or kept off-site as a backup.

What are the benefits and drawbacks of embedded systems?

Benefits	Drawbacks
Small and cheap to mass-produce	Hard or impossible for the user to upgrade
Low power consumption	Limited to the one task they were designed for
Dedicated to one task, so fast and reliable at it	Faults usually need a specialist or a replacement unit
Little or no user setting-up needed	If connected to a network, they can be a security weak point

Examples: washing machine controller, microwave oven, car engine management, central heating controller. An answer must be about the device in the question; June 2024 examiners noted that generic embedded-system answers scored poorly.

How do the named hardware devices work?

Device	Principal operation
Laser printer	A rotating drum is charged; the laser changes the charge on parts of the drum to form an electrostatic image of the page; charged toner sticks to those parts; the paper is charged so the toner transfers to it; a heated fuser melts the toner onto the paper; the drum is discharged and cleaned
3D printer	The design is sliced into thin layers by software; the printer builds the object one layer at a time, e.g. by extruding melted plastic through a moving nozzle, or by curing liquid resin with a laser
Microphone	Sound waves vibrate a diaphragm; the movement (e.g. a coil moving in a magnetic field) produces a varying analogue electrical signal; an ADC converts it to digital
Speaker	Digital data goes through a DAC and is amplified; the varying current passes through a coil attached to a cone, inside the field of a permanent magnet; the coil and cone move in and out, creating sound waves
Magnetic hard disk	Platters coated in magnetic material spin at high speed; read/write heads on an actuator arm move across the surface; data is stored as magnetised regions in tracks and sectors
Solid state (flash) memory	Data is stored as electric charge trapped in transistor cells (floating gates); no moving parts; data is erased and rewritten in blocks, and cells wear out after many writes
Optical disc reader/writer	A laser is shone onto the spinning disc; the reflected light differs between pits and lands, detected by a photodiode and read as bits. A writer uses a stronger laser to change the reflectivity of a dye or phase-change layer. Blu-ray uses a shorter-wavelength laser, so data is packed more densely
Touchscreen	Resistive: pressing pushes two conductive layers together and the voltage at the contact point gives its position. Capacitive: the screen carries an electrostatic field; a finger changes the capacitance at that point, and the controller calculates the position (supports multi-touch)
Virtual reality headset	A screen (or two) shows a slightly different image to each eye through lenses, giving a 3D effect; sensors such as gyroscopes and accelerometers track head movement so the image changes to match

What is a buffer used for?

A buffer lets a fast device (the processor) hand data to a slower one (a printer) and get on with other work while the slower device catches up. When streaming video, data is downloaded into a buffer ahead of playback so short drops in network speed do not stop the picture. The buffer holds the data; the processor or device does the sending. Examiners in June 2024 and November 2024 penalised answers saying the buffer “sends” data.

How do RAM, ROM, SRAM and DRAM differ?

	RAM	ROM
Volatile?	Yes	No
Can be written in normal use?	Yes	No
Typical contents	Operating system in use, running programs and their data	Bootstrap loader / BIOS, firmware of embedded devices

	SRAM (static)	DRAM (dynamic)
Stores each bit in	A flip-flop	A capacitor (with a transistor)
Needs refreshing?	No	Yes, constantly, as the charge leaks
Speed	Faster access	Slower
Cost and density	More expensive per bit, fewer bits per chip	Cheaper per bit, more bits per chip
Typical use	Cache memory	Main memory

What is the difference between PROM, EPROM and EEPROM?

• PROM: blank when made; programmed once by the user or manufacturer, and then cannot be changed.
• EPROM: can be erased by shining ultraviolet light through a window on the chip (removed from the device), then reprogrammed. It is not write-once; November 2024 candidates lost marks for saying it was.
• EEPROM: erased and rewritten electrically while still in the circuit, so firmware can be updated without removing the chip. Flash memory is a form of EEPROM.

What is the difference between monitoring and control?

• Monitoring: sensors take readings; the computer compares them with stored values and displays, records or warns (e.g. an alarm), but does not change the conditions. Example: monitoring a patient’s heart rate.
• Control: the computer also sends signals to actuators to change the conditions. Example: a greenhouse that opens windows with a motor when it is too hot.
• Feedback makes control continuous: after the actuator acts, new sensor readings are taken and compared again, so the system keeps the value within the required range.
• Sensors in the syllabus: temperature, pressure, infra-red and sound. Sensor readings are analogue, so an ADC converts them for the computer; actuators may need a DAC.

Logic gates in one table

Gate	Expression	Output 1 when	00	01	10	11
AND	A AND B	both inputs are 1	0	0	0	1
OR	A OR B	at least one input is 1	0	1	1	1
NAND	NOT (A AND B)	not both inputs are 1	1	1	1	0
NOR	NOT (A OR B)	both inputs are 0	1	0	0	0
XOR	A XOR B	the inputs differ	0	1	1	0

NOT has one input and inverts it. The NAND and NOR symbols are the AND and OR shapes with a small circle on the output; XOR is the OR shape with an extra curved line on the input side.

Worked example: truth table to logic expression

A circuit has inputs A, B, C and output X = 1 only in the rows A=0, B=1, C=1 and A=1, B=1, C=0.

1. Write one AND term per row where X = 1, using NOT for each input that is 0: (NOT A AND B AND C) and (A AND B AND NOT C).
2. Join the terms with OR: X = (NOT A AND B AND C) OR (A AND B AND NOT C).
3. Check: both terms need B = 1, and A and C must differ, so X = B AND (A XOR C) is an equivalent, simpler answer. Either form earns the marks unless a particular gate set is required.

Common mistakes and exam traps

• Saying SRAM is cheaper than DRAM, or that DRAM is used for cache. It is the other way round.
• Describing a device by what it is used for instead of how it works when the question says “describe the principal operation”.
• Giving a monitoring example when the question asks for control (control must change conditions through an actuator).
• Swapping the NOR, XOR and NAND symbols, or describing a gate’s output for only some input combinations (June 2024, June 2025 examiner reports).
• Leaving rows out of a truth table. Three inputs give 2^3 = 8 rows; list them in binary order.
• Writing XOR as “A or B”; it is 0 when both inputs are 1.

Frequently asked questions

Is virtual memory part of Topic 3? No. In 9618 the details of virtual memory, paging and segmentation are in Topic 16 (A Level). In Topic 3 you need RAM, ROM, SRAM, DRAM, PROM, EPROM and EEPROM.

How much detail do I need on how a laser printer works? Enough to give the stages in order: charge the drum, form the image with the laser, attract toner, transfer it to charged paper, fuse it with heat, clean the drum. Each correct stage is typically one mark.

Do I need three-input gates? Not for single gates: the syllabus says all gates except NOT have two inputs. Circuits can still have three or more inputs overall, built from two-input gates.

What is the difference between a sensor and an actuator? A sensor is an input device that measures a physical quantity. An actuator is an output device that creates movement, such as opening a valve, in response to a signal from the computer.

Official syllabus

Cambridge International AS & A Level Computer Science 9618, syllabus for 2026 and syllabus for 2027, 2028 and 2029, Topic 3 Hardware (sections 3.1 and 3.2). Examiner points are from Cambridge’s Principal Examiner Reports for June 2024, November 2024 and June 2025, as summarised on our 9618 past papers page.