CS · Topic 3

Section 1 · Chapter 3 · IGCSE & O Level Computer Science

Hardware 🖥️

Crack the case open. Spin a motherboard in 3D, watch a program run through the CPU one register at a time, and see how drives, screens, printers and sensors actually work.

🕶️ with Guru Jazzy

🎯 What you'll unlock this chapter

🧠 CPU · von Neumann architecture, registers, buses, fetch–decode–execute
⚡ Performance · clock speed, cache, cores, bus width · instruction sets · embedded systems
⌨️ Input, output and sensors · how each device works · monitoring vs control
💾 Storage · RAM and ROM · magnetic, optical, solid-state · virtual memory · cloud
🌐 Network hardware · NICs, MAC and IP addresses, routers
🧊 4 interactive 3D models · all 8 activities · every worked example · 72 book figures (tap to enlarge)

🧊 3D · Inside the computer

Drag to spin, scroll or pinch to zoom. Tap any part (or use the buttons) to find out what it does. The glowing dots are data travelling along the buses.

Tap a part to learn about it.

🧠 The CPU 3.1.1

📛 Also called the processor or microprocessor
⚙️ Executes every instruction and processes all data — in PCs, tablets and phones
🔲 Usually one integrated circuit on a single chip
🧩 Made of: control unit (CU) · ALU · registers · buses

🏛️ Von Neumann architecture 3.1.2

Stored program computer — described by John von Neumann in the mid-1940s. Every modern computer is based on it. Key ideas:

🧠 A CPU that accesses memory directly
💾 Memory stores programs and data together
📜 Instructions are executed in sequence

Figure 3.1: Von Neumann architecture — **Fig 3.1** Von Neumann architecture

ALU vs control unit

	ALU	Control unit (CU)
Job	Arithmetic (+, −, shifts) and logic (AND, OR)	Reads each instruction; runs the fetch–decode–execute cycle
Key detail	× and ÷ done with additions, subtractions and shifts	Sends signals on the control bus; timed by the system clock
How many	A computer can have more than one	One per core

RAM = immediate access store (IAS) — holds the programs and data the CPU needs right now, copied in from backing store because RAM is far faster.

Special-purpose registers

Register	What it holds
PC · program counter	Address of the next instruction to fetch
MAR · memory address register	Address being read from or written to
MDR · memory data register	Data just read from, or about to be written to, memory
CIR · current instruction register	Instruction being decoded and executed
ACC · accumulator	Temporary data during ALU calculations

Reading vs writing memory

Step	📖 Read	✍️ Write
1	Address → MAR	Data → MDR
2	Read signal sent to memory	Address → MAR
3	Contents of that address → MDR	Write signal sent; data stored at that address

🚌 System buses 3.1.2

🧵 Buses are parallel — one wire per bit
🔗 Three buses link the CPU, memory and I/O devices

Figure 3.2: System buses and memory — **Fig 3.2** System buses and memory

Bus	Carries	Direction
Address bus	Memory addresses	Unidirectional (CPU → memory)
Data bus	Data — an instruction, an address or a value	Bidirectional
Control bus	Control and timing signals from the CU (usually 8 bits)	Bidirectional

Why bus width matters

Bus	Wider means…	Example
Address bus	More memory locations can be addressed	16 bits → 2¹⁶ = 65 536 · 32 bits → 4 294 967 296
Data bus	A longer word moves in one go → better performance	16, 32 or 64-bit words

🔄 The fetch–decode–execute cycle 3.1.2

Fetch · PC → MAR (address bus)
Instruction at that address → MDR (data bus)
MDR → CIR
PC + 1 — now points at the next instruction
Decode · CU decodes the instruction in the CIR
Execute · signals on the control bus to the right components · repeat

Figure 3.3: Fetch–decode–execute cycle flowchart — **Fig 3.3** Fetch–decode–execute cycle flowchart

🧊 3D · Watch a program run

Memory holds a tiny program: LDA 5 (load), ADD 6, STO 7 (store), END. Step through it and watch every value fly between the registers and RAM — yellow on the address bus, green on the data bus, pink on the control bus.

FetchDecodeExecute

Ready

⚡ Cores, cache and the clock 3.1.3

Factor	What it is	Effect on performance
⏱️ Clock speed	Clock cycles per second that sync every operation (3.5 GHz = 3.5 billion/s)	Faster → more instructions/s, but not a guarantee of better overall performance
🚌 Bus width	Number of wires in the address and data buses	Wider → more memory, bigger words per transfer
⚡ Cache	Very fast memory inside the CPU for frequently used data and instructions	Checked before RAM · bigger cache → faster
🧩 Cores	Each core = own ALU, CU and registers (dual = 2, quad = 4)	More cores help, but 2× cores ≠ 2× speed

🔥 Overclocking (changing the clock in the BIOS)

• Instructions can run out of sync → crashes, instability
• The CPU overheats

Why doubling cores doesn't double speed

🔗 Cores must communicate with each other and the CPU
📈 Channels grow fast: 2 cores → 1 channel · 4 cores → 6 channels

Figure 3.4: Dual core — 1 channel between 2 cores — **Fig 3.4** Dual core — 1 channel between 2 cores

Figure 3.5: Quad core — 6 channels between 4 cores — **Fig 3.5** Quad core — 6 channels between 4 cores

📖 Instruction sets 3.1.4

Part of an instruction	Meaning
Opcode	What operation to do (e.g. ADD, JMP, LDA)
Operand	The data to act on — or an address / register

📚 Instruction set = the limited list of opcodes a CPU understands
💻 X86 — common set; Intel and AMD chips differ inside but share almost the same X86 set

⚠️ Don't mix them up — an instruction set isn't program code. High-level programs need a compiler or interpreter to become CPU instructions.

🔌 Embedded systems 3.1.5

🎯 Hardware + software built for a specific set of tasks
📥 Inputs: an operator (keypad, dial) or sensors
📤 Outputs: actuators and other components
🔄 Updates: plug into a computer (e.g. car GPS maps) or automatically over Wi-Fi, satellite or mobile

Figure 3.6: Embedded systems — **Fig 3.6** Embedded systems

What it's built on

Microcontroller	Microprocessor	System on chip (SoC)
CPU + RAM + ROM + peripherals on one chip	Only a CPU — RAM, ROM, peripherals added separately	CPU, memory, I/O ports and storage on one chip (may include a microcontroller)

Benefits vs drawbacks

✅ Benefits	⚠️ Drawbacks
Small — fit easily into devices	Hard to upgrade
Cheap to make; mass production → reliable	Troubleshooting needs a specialist
One task → simple interface, often no OS	"Simple" interfaces can still confuse (cooker clocks!)
Can be controlled remotely (e.g. from a phone)	Internet-connected → open to hackers and viruses
React fast to changing inputs (real time)	Often thrown away, not repaired — "throw-away society"

Examples

🚗 Cars — engine management, ABS, airbags, GPS, security
📺 Set-top boxes · security alarms · smart lighting
🥤 Vending machines — in: keypad, coin, temperature and tilt sensors · out: motors, cooling, LCD
🧺 White goods — washing machines, fridges, cookers
🚫 A general-purpose computer is not embedded — it's multi-functional

Figure 3.7: Embedded systems in a car — **Fig 3.7** Embedded systems in a car

Figure 3.8: Embedded system in a set-top box — **Fig 3.8** Embedded system in a set-top box

Figure 3.9: Embedded system in a security system — **Fig 3.9** Embedded system in a security system

Figure 3.10: Smart LED light bulb — **Fig 3.10** Smart LED light bulb

Figure 3.11: Embedded system in a vending machine — **Fig 3.11** Embedded system in a vending machine

Figure 3.12: Helix in a vending machine — **Fig 3.12** Helix in a vending machine

▮▯▮ Barcodes 3.2.1

▮ A series of dark and light bars
7️⃣ Each digit = 7 modules wide · 2 dark + 2 light bars
🚧 Guard bars mark the start, middle and end

	Left-hand digits	Right-hand digits
Starts with	Light bar	Dark bar
Dark modules	Odd number	Even number
Why	The scanner can tell left from right → barcode reads either way round

Figure 3.13: Guard bars — **Fig 3.13** Guard bars

Figure 3.14: Sample barcode — **Fig 3.14** Sample barcode

How it's read

A red laser or LED shines on the barcode
Dark bars reflect little light, light bars reflect a lot
Photoelectric cells read the reflected light
Pattern → binary · left-hand "3" = L D D D D L D → 0111101 (L = 0, D = 1)

Figure 3.15: Barcode digit patterns — **Fig 3.15** Barcode digit patterns

Figure 3.16: Barcode section for 5 4 3 0 5 2 — **Fig 3.16** Barcode section for 5 4 3 0 5 2

At the supermarket checkout

Barcode number = key field in the stock database
Price sent to the till
Stock count − 1
At the re-order level → new stock ordered automatically
A flag is set so it isn't re-ordered on every scan

🎮 Barcode Builder

Type an even number of digits (up to 12). The first half uses left-hand patterns, the second half right-hand patterns.

🔳 QR codes 3.2.1

🔳 Quick response code — a grid of dark and light squares (pixels)
📐 3 large corner squares = alignment · small square = checks size and angle
📱 Scan with a phone camera + app → opens the website, phone number or boarding pass
🖼️ Frame QR codes add a "canvas" for logos — software usually isn't free

Figure 3.17: Sample QR code — **Fig 3.17** Sample QR code

Figure 3.18: QR code on a boarding pass — **Fig 3.18** QR code on a boarding pass

Figure 3.19: Frame QR code — **Fig 3.19** Frame QR code

QR code vs barcode

	QR code	Barcode
Capacity	Up to 4296 characters (7089 digits), incl. URLs	About 30 digits
Errors	Fewer — built-in error checking	More
Reader	Any phone camera	Laser / LED scanner
Sharing	Easy to send as text or image; can be encrypted	Printed labels
⚠️ Risks	Several formats · can carry malicious code (attagging) → fake sites, malware	—

📷 Digital cameras 3.2.1

🤖 An embedded system controls shutter speed, focus, flash, aperture, image size, red-eye removal

Light through the lens hits a CCD — millions of photodiodes, one per pixel
Each photodiode turns light into electric charge
An ADC converts charge → brightness level (8 bits → 256 levels)
Colour stored too — usually 24-bit RGB

More pixels → more detail → bigger file.

Figure 3.20: Digital camera — **Fig 3.20** Digital camera

Figure 3.21: Pixel brightness and colour values — **Fig 3.21** Pixel brightness and colour values

Figure 3.22: How a digital camera works — **Fig 3.22** How a digital camera works

⌨️ Keyboards, microphones and mice 3.2.1

Keyboard

Key press completes a circuit in the membrane
CPU works out which key, looks it up in an index file
Gets its ASCII value

⚠️ Slow and error-prone · heavy use → RSI · ergonomic keyboards help

Figure 3.23: Keyboard — **Fig 3.23** Keyboard

Figure 3.24: Ergonomic keyboard — **Fig 3.24** Ergonomic keyboard

Figure 3.25: How a keyboard works — **Fig 3.25** How a keyboard works

Microphone

Sound vibrates the diaphragm
Attached copper coil moves in a magnetic field
→ an analogue electric current is induced
Sound card's ADC → digital values

Figure 3.27: Analogue to digital conversion — **Fig 3.27** Analogue to digital conversion

Optical mouse

Red LED bounces light off the surface
CMOS sensor takes ~1500 images a second
Digital signal processor compares images → movement
Coordinates sent to the computer

✅ No moving parts → more reliable · dirt can't get in · no special surface
🔌 Wired mice: no signal loss, no batteries

Figure 3.28: Optical mouse — **Fig 3.28** Optical mouse

2D vs 3D scanners

	2D scanner	3D scanner
Captures	A flat image of a page	x, y and z coordinates of a solid object
How	Bright light → moving scan head → mirrors + lens → CCD	Lasers, light or X-rays; CT scanners stack thin 2D "slices" (tomography)
Used for	OCR (text → editable) · passports at airports + face recognition	CAD · 3D printing · medical imaging

Figure 3.29: How a 2D scanner works — **Fig 3.29** How a 2D scanner works

Figure 3.30: Face recognition key points — **Fig 3.30** Face recognition key points

Figure 3.31: Tomography (CT scanning) — **Fig 3.31** Tomography (CT scanning)

👆 Touch screens 3.2.1

	Capacitive	Infrared	Resistive
How it works	Conductive layer under glass · finger changes the electrostatic field · surface: corner sensors · projective: X–Y grid	Grid of IR beams from edge transmitters · finger breaks beams · microcontroller finds the crossing	Two resistive layers with a gap · press → layers touch → voltage change gives position
✅ Pros	Clear in sunlight · durable, scratch-resistant · projective = multi-touch, thin gloves	Multi-touch · durable · works if scratched or cracked	Dust and water resistant · finger, stylus or glove
⚠️ Cons	Surface type: bare finger / special stylus only · electromagnetic interference	Water / moisture · accidental touches · light interference	Low sensitivity · no multi-touch · poor in sunlight · scratches easily

Figure 3.32: Capacitive touch screen — **Fig 3.32** Capacitive touch screen

Figure 3.33: Infrared transmitters and sensors around the screen — **Fig 3.33** Infrared transmitters and sensors around the screen

Figure 3.34: Touched infrared screen — blocked beams — **Fig 3.34** Touched infrared screen — blocked beams

Figure 3.35: Resistive touch screen — **Fig 3.35** Resistive touch screen

🎮 Infrared Touch Screen

Tap or click anywhere on the "screen". Your finger blocks some of the infrared beams, and the microcontroller finds where the blocked beams cross.

Waiting for a touch…

🧊 3D · Output devices

Watch a 3D printer build an object layer by layer, a DLP chip's micromirrors make a picture, and a loudspeaker turn current into sound.

Speed

⚙️ Actuators 3.2.2

🦾 Used when a computer controls something physical (conveyor belt, valve)
⚙️ Mechanical / electromechanical: relay, solenoid, motor
🧲 Solenoid = coil · current → magnet → pulls a plunger (straight-line motion; rotary solenoids turn)

Figure 3.36: A solenoid — **Fig 3.36** A solenoid

📽️ Projectors 3.2.2

	DLP (digital light projector)	LCD projector
Key part	DMD chip — millions of micromirrors, one per pixel	Three LCD screens (R, G, B)
How	Mirror tilted to light = ON, away = OFF · flipping fast → ~1024 greys · colour filter → 16 million+ colours	Dichroic mirrors split white light → each colour through its LCD → prism recombines → lens
✅ Pros	Higher contrast · longer life · quieter · one chip (no alignment issues) · smaller, lighter · better in dust/smoke	Better colour saturation · more energy-efficient → less heat
⚠️ Cons	"Shadows" on moving images · weaker colour saturation	Lower contrast · shorter life — panels yellow and degrade

Figure 3.37: Digital light projector (DLP) — **Fig 3.37** Digital light projector (DLP)

Figure 3.38: LCD projector — **Fig 3.38** LCD projector

🖨️ Printers 3.2.2

Inkjet vs laser

	Inkjet	Laser
Prints with	Liquid ink droplets	Dry powder toner + static electricity
Prints	Line by line (head moves side to side)	A whole page at a time
Key parts	Print head + nozzles · stepper motor and belt	Charged drum · laser · fuser · discharge lamp
Droplets / image	Thermal bubble (heated resistor) or piezoelectric (vibrating crystal)	Laser removes charge → toner sticks to those areas
Best for	One-off photos · a few pages of good colour	High volume · fast · big toner and paper trays

Figure 3.39: Inkjet printer — **Fig 3.39** Inkjet printer

Figure 3.40: Laser printer — **Fig 3.40** Laser printer

Step	Inkjet	Laser
1–4	Data → printer driver → printer format · checks printer is free (busy / offline / out of ink) · data stored in the printer buffer
5	Sheet fed in; sensor checks for paper (error if empty / jammed)	Drum given a positive charge; laser removes charge → negative image of the page
6	Head moves side to side spraying 4 inks	Positive toner sticks only to negative areas
7	Paper advances after each pass until the page is done	Negatively charged paper rolls over the drum…
8	More data in buffer? → repeat from step 5	…toner transfers to the paper
9	Buffer empty → interrupt to CPU for more data	Paper's charge removed so it doesn't stick
10	—	Fuser (heated rollers) melts toner onto paper
11	—	Discharge lamp clears the drum

🧱 3D printers 3.2.2

🧱 Build solid objects layer by layer (often 0.1 mm)
🧪 Materials: powdered resin, metal, paper, ceramic · newer: liquid polymer hardened by laser / UV

Additive (3D printing)	Subtractive (carving, CNC)
Builds the object up by adding layers	Removes material from a block

Direct 3D printing	Binder 3D printing
Inkjet technology · head also moves up and down	Two passes per layer: dry powder, then a binder (glue)

Design in CAD (or scan with a 3D scanner)
Software converts it to a printer format
Printer set up
Built layer by layer — can take hours
Removed and finished (wash supports, remove powder, cure)

Figure 3.41: Typical 3D printer — **Fig 3.41** Typical 3D printer

Figure 3.42: Alloy wheel made by a 3D printer — **Fig 3.42** Alloy wheel made by a 3D printer

Figure 3.43: Making an object with a 3D printer — **Fig 3.43** Making an object with a 3D printer

Uses

🦿 Prosthetic limbs made to fit exactly · parts for reconstructive surgery
✈️ Lightweight aerospace parts
🎨 Fashion and art · spare parts no longer made

🖥️ Screens and loudspeakers 3.2.2

LED vs LCD vs OLED

	LED screen	LCD screen	OLED screen
Light source	Red, green and blue LEDs	None of its own → needs backlighting (now LEDs)	Organic layers glow on their own — no backlight
Strengths	Very bright — big outdoor displays	Many "LED TVs" are LED-backlit LCDs	Thinner, lighter, bendable · brighter · less power · ~170° viewing angle

LED vs CCFL backlighting

LED backlight	CCFL (old tubes)
Full brightness instantly	Needs warm-up
Whiter, brighter light → vivid colours	Slightly yellow tint
Thinner screens · longer life	Thicker · shorter life
Less power, less heat	More power, more heat

Figure 3.44: How an OLED screen works — **Fig 3.44** How an OLED screen works

Figure 3.45: OLED television (curved screen) — **Fig 3.45** OLED television (curved screen)

Loudspeakers

Digital sound → DAC → analogue current
Amplifier boosts the current
Current through a coil on an iron core → temporary electromagnet beside a permanent magnet
Changing current pushes and pulls the core → cone vibrates → sound waves

Figure 3.46: Digital to analogue conversion — **Fig 3.46** Digital to analogue conversion

Figure 3.47: How a loudspeaker works — **Fig 3.47** How a loudspeaker works

🌡️ Sensors 3.2.3

📏 Input devices that measure physical properties
〰️ Real-world data is analogue → ADC → digital for the microprocessor
🦾 Driving motors / valves → DAC → analogue for the actuator
🔁 Sensors send readings constantly — the microprocessor decides what to do

Figure 3.48: Thermometer — an analogue reading — **Fig 3.48** Thermometer — an analogue reading

Figure 3.49: ADC — analogue to digital — **Fig 3.49** ADC — analogue to digital

Sensor	Example uses
Temperature	Central heating, chemical processes, greenhouses
Moisture	Water in soil (greenhouse), moisture in food processing
Humidity	Water vapour in the air — buildings, microchip factories, greenhouses
Light	Street lights, automatic car headlights, (rain detection for) wipers
Infrared	Active: an intruder breaks a beam. Passive: detects body heat
Pressure	Weighing lorries, gas pressure in a reactor, intruder alarms
Acoustic	Footsteps or breaking glass in alarms, leaks at a pipe joint
Gas	Pollution, O₂ / CO₂ in greenhouses, oxygen in car exhausts
pH	Acidity of soil or a chemical process
Magnetic field	Anti-lock brakes, phones and CD players
Accelerometer	Airbags (sudden deceleration), phone portrait/landscape
Proximity	Turning a phone screen off when held to your ear
Flow	Gas flow in pipes, respiratory devices
Level	Petrol tank level, powder in tablet production, coolant leaks

👀 Monitoring vs 🎛️ control 3.2.3

	👀 Monitoring	🎛️ Control
What happens	Reads sensors · compares with acceptable range · shows readings or warnings	Out of range → signals (via DAC) to actuators that change conditions
Effect on the process	None — it just watches	Output affects the next input → feedback
Examples	Patient's vital signs · burglar alarm · engine temperature · river pollution	Street lights · central heating · chemical plant · ABS brakes · greenhouse

Figure 3.50: Monitoring and control systems — **Fig 3.50** Monitoring and control systems

Systems step by step

System	Sensor(s)	Decision	Action
🚨 Security alarm	Acoustic, infrared, pressure	Intruder detected?	Siren and lights on
💡 Street light	Light (via ADC, sampled e.g. every minute)	Reading below stored value?	Lamp on (else off) · stays 30 min before re-checking — no flicker from passing clouds
🔥 Central heating	Temperature vs user's pre-set value	Below pre-set?	Via DAC: open gas valve + start water pump · else no action
🧪 Chemical process	Temperature, pH	Temp < 70 °C? · pH > 3.5?	Heater on / off · acid valve open / closed
🚗 ABS brakes	Magnetic field (wheel rotation)	One wheel slower (locking)?	Reduce its brake pressure · checked many times a second (the pedal "judder")
🌱 Greenhouse	Humidity, moisture, temp, pH, light	Above / below pre-set?	e.g. humidity high → motors open windows · low → valves spray water

Figure 3.51: Security system — **Fig 3.51** Security system

Figure 3.52: Street lighting — **Fig 3.52** Street lighting

Figure 3.53: Controlling central heating — **Fig 3.53** Controlling central heating

Figure 3.54: Controlling a greenhouse — **Fig 3.54** Controlling a greenhouse

🎮 Central Heating Control Loop

The sensor reading goes through the ADC, the microprocessor compares it with the pre-set value, and the actuators switch the boiler on or off. Watch feedback keep the temperature hovering around the target.

Pre-set temperature:

Outside temperature:

🖼️ Activity 3.5 diagrams

Figure 3.55: Reactor control (Activity 3.5) — **Fig 3.55** Reactor control (Activity 3.5)

Figure 3.56: Traffic-light junction (Activity 3.5) — **Fig 3.56** Traffic-light junction (Activity 3.5)

🗄️ Memory vs storage 3.3

	Primary memory	Secondary & off-line storage
CPU access	Directly addressable	Not directly addressable
Examples	RAM, ROM, cache	HDD, SSD, DVD, memory stick, Blu-ray
Volatile?	RAM yes · ROM no	All non-volatile · internal or external
Size / speed	Smaller, much faster	Much larger, slower — holds OS, apps, files permanently

Figure 3.57: Typical memory and storage devices — **Fig 3.57** Typical memory and storage devices

Figure 3.58: Primary, secondary and off-line storage — **Fig 3.58** Primary, secondary and off-line storage

Figure 3.59: Primary memory — **Fig 3.59** Primary memory

⚡ RAM and ROM 3.3.1

	RAM	ROM
Lasts	Temporary · volatile	Permanent · non-volatile
Access	Read and write · any location directly	Read only — can't be altered
Holds	Data, files, programs and OS parts in use	BIOS — start-up (bootstrap) instructions
Upgrade	More RAM → faster (less swapping to storage)	—

DRAM vs SRAM

	DRAM	SRAM
Stores a bit with	Transistor + capacitor	Flip-flops
Refresh	Constantly (about every 15 µs)	Not needed
Speed	~60 ns	Much faster, ~25 ns
Cost / power / capacity	Cheaper · less power · higher capacity	Dearer · lower capacity
Used for	Main memory	Cache

🧊 3D · Storage up close

Switch between a hard disk, a solid-state drive and an optical disc.

💽 Magnetic storage — hard disk drives 3.3.3

💿 Platters (aluminium, glass, ceramic) spin at 5400–7200 rpm
🧲 Read-write heads = electromagnets on a moving arm · both sides of every platter
🗺️ Data laid out in tracks and sectors
🐢 Latency — wait for the sector to rotate to the head · many head moves → slow ("Please wait…")
🧩 Fragmentation — file split across non-adjacent sectors → slower · fix: defragmentation
🔌 Removable HDDs (USB) for back-ups and moving files

Figure 3.62: Hard disk drive — **Fig 3.62** Hard disk drive

Figure 3.63: Tracks and sectors — **Fig 3.63** Tracks and sectors

🔲 Solid-state storage 3.3.3

🚫 No moving parts — controls electrons in NAND (or NOR) flash chips
🔳 Chips arranged as a matrix · each intersection: floating gate + control gate
⚡ Voltage on control gate → electrons through tunnel oxide → trapped by the dielectric in the floating gate
1️⃣ Charged = 1 · empty = 0 · non-volatile (may leak after ~1 year unused)

Figure 3.64: Flash memory cell — **Fig 3.64** Flash memory cell

SSD vs HDD

	SSD	HDD
Moving parts	None → more reliable	Spinning platters, moving heads
Speed	Much faster access · no spin-up	Latency · must get up to speed
Size / weight	Lighter, thinner	Heavier, bulkier
Power / heat	Lower · runs cooler (great for laptops)	Higher
⚠️ Weak spots	Limited writes (endurance) · must erase before rewriting	Fragmentation · damaged by knocks

Memory sticks — same flash technology · small, light, for moving files and small back-ups · some software uses one as a dongle (won't run without it).

📀 Optical storage 3.3.3

🔦 Data stored as pits and lands on one spiral track (centre → outside), read by a laser
✍️ R = write once · RW = rewrite many times
📏 Shorter wavelength → smaller pits → more data

	CD	DVD	Blu-ray
Laser	Red, 780 nm	Red, 650 nm	Blue, 405 nm
Track pitch	1.6 µm	0.74 µm	0.32 µm
Capacity	~700 MB	4.7 GB per layer (dual-layer possible)	25 GB per layer · 50 GB dual-layer
Notes	Too small for most movies	Two 0.6 mm layers read separately	Built-in anti-piracy encryption · faster transfer · more interactive

Figure 3.65: Optical media — spiral track, pits and lands — **Fig 3.65** Optical media — spiral track, pits and lands

Figure 3.66: Dual-layering on a DVD — **Fig 3.66** Dual-layering on a DVD

Figure 3.67: Blu-ray disc — **Fig 3.67** Blu-ray disc

🧠 Virtual memory 3.3.4

💥 Programs need more RAM than exists → system could crash
➕ Virtual memory = RAM + swap space on the HDD/SSD
📄 Data moved in fixed-size pages (paging) · RAM full → oldest page moved out
🗺️ A memory map tracks where every page is

✅ Benefits	⚠️ Drawback: disk thrashing
Programs bigger than RAM can run	RAM fills → more and more swapping → huge numbers of head movements
No RAM wasted on unused data	More time swapping than processing · thrash point = execution stops
Less need to buy expensive RAM	Can wear out the HDD · fix: more RAM, fewer programs, smaller swap file, SSD

Figure 3.68: Normal memory management — **Fig 3.68** Normal memory management

Figure 3.69: Just before program 4 is given RAM — **Fig 3.69** Just before program 4 is given RAM

Figure 3.70: Program 0 swapped to disk, program 4 gets RAM — **Fig 3.70** Program 0 swapped to disk, program 4 gets RAM

🎮 Virtual Memory Simulator

RAM has room for 4 programs' pages. Run programs and watch the oldest page get swapped out to the disk. Then try "Run them all ×3" to see thrashing.

RAM (4 frames)

Swap space on the HDD/SSD

☁️ Cloud storage 3.3.5

🌐 Data on remote servers
📑 Copied across several servers (data redundancy) → always available

Public cloud	Private cloud	Hybrid cloud
Client and provider are different companies	Dedicated environment behind the company's firewall	Sensitive data private · the rest public

✅ Benefits	⚠️ Drawbacks
Access any time, any device, anywhere online	Slow / unstable internet → can't reach data
No storage device to carry	Costs for big capacity and data transfer
Remote back-up — recover after disk failure	Provider could fail and lose your data
Almost unlimited capacity	You lose control of security — physical, disasters, staff, hackers

🔌 NICs and MAC addresses 3.4.1–3.4.2

🔌 NIC (network interface card) — connects a device to a network · holds its MAC address
📶 WNIC — wireless version, uses an antenna
🔢 MAC address = 48 bits · six pairs of hex: NN-NN-NN-DD-DD-DD
🏭 First half = manufacturer · second half = device serial number
🔁 Replace the NIC → the MAC address changes

	UAA (universally administered)	LAA (locally administered)
Set by	Factory	The user
How common	By far the most common	Rare
Why	—	Fit an addressing format · bypass a MAC filter or network restrictions
Risk	—	Clashes if the new address isn't unique

🏷️ IP addresses 3.4.3

🏠 Private IP — given by the router; unique on that network
🌍 Public IP — the router's, from the ISP; unique on the whole internet

IPv4 vs IPv6

	IPv4	IPv6
Size	32 bits	128 bits
Format	4 groups of 8 bits in denary · 109.108.158.1	8 groups of hex, separated by colons
Extras	—	Far more addresses · no address collisions · built-in authentication · more efficient routing

MAC vs IP

	MAC address	IP address
Identifies	The physical device	Where it's connected on the internet
Assigned by	Manufacturer · part of the NIC · unique	ISP (dynamic ones by DHCP) · not necessarily unique
Size	48 bits	32 bits (IPv4) / 128 bits (IPv6)
Types	UAA or LAA	Static or dynamic
Used for	Identifying sender and receiver	Routing

Static vs dynamic

	Static	Dynamic
Assigned	Permanently by the ISP	By a DHCP server at each connection
Used for	Web servers, online databases, FTP servers	Everyday devices
Privacy	Device fully traceable	Greater — it changes
Performance	Faster up/downloads · consistent location	Can drop VoIP calls if it changes mid-call
Cost / reliability	More expensive to maintain	Cheaper (addresses reused) · less reliable

Figure 3.71: Dynamic vs static IP addressing — **Fig 3.71** Dynamic vs static IP addressing

🎮 Getting a web page — DHCP and DNS

Step through what happens when a device opens a website, with a dynamic or a static IP address.

📡 Routers 3.4.4

🔀 Moves data packets between different networks (e.g. LAN ↔ internet)
🔄 Converts data between network protocols and formats
🔍 Inspects each packet's IP → sends it to the right switch
🎯 The switch delivers it using the MAC destination address
🛡️ Broadband routers sit behind a firewall · wired or wireless

Figure 3.72: Router flow diagram — **Fig 3.72** Router flow diagram

✏️ Practice hub 8 activities · every example

Every activity from the coursebook chapter, as an auto-marked practice set. Same skill as the book's activity with the same number — but fresh questions every time you hit 🔀 New questions.

📘 Worked examples

Every type of worked example in the chapter, solved step by step. Hit 🔀 Another example on any of them for a new one.

🃏 Key-term flashcards Tap to flip

🧠 Mini-quiz

📝 Exam-style questions Try, then peek

✅ The PC holds the address of the next instruction; this is copied into the MAR.
✅ The instruction at the address in the MAR is copied into the MDR (then into the CIR).
✅ The PC is incremented so it points at the next instruction.

✅ The CPU has to communicate with each core, and the cores with each other.
✅ More cores need more channels (4 cores need 6), and that communication reduces the overall gain.

✅ A combination of hardware and software dedicated to a specific task.
✅ Based on a microcontroller, microprocessor or system on chip.
✅ Small, low cost, often no operating system; reacts in real time to sensor inputs.

✅ The drum is given a positive charge; a laser removes the charge where the text/images will be.
✅ Positively charged toner sticks only to those negatively charged areas.
✅ Negatively charged paper is rolled over the drum and the toner transfers to it.
✅ The paper's charge is removed; a fuser (heated rollers) melts the toner onto the paper.
✅ A discharge lamp removes the charge from the drum ready for the next page.

✅ The sensor constantly sends readings; an ADC converts them to digital.
✅ The microprocessor compares each reading with the pre-set value stored in memory.
✅ If it's too low, a signal is sent (via a DAC) to an actuator to switch the heater on / close the windows.
✅ If it's too high, the heater is switched off / motors open the windows.
✅ The output affects the next reading (feedback); this repeats while the system is on.

✅ No moving parts, so more reliable if the laptop is knocked or carried.
✅ Lighter and thinner.
✅ Lower power consumption (longer battery life) and run cooler; faster access too.

✅ Virtual memory uses part of the HDD/SSD (swap space) as if it were RAM when RAM is full.
✅ Pages of data are swapped between RAM and the disk; the oldest page is moved out to make room.
✅ Disk thrashing: so much swapping that there's a very high rate of read/write head movement…
✅ …so more time is spent moving data than processing; at the thrash point execution halts.

✅ MAC is the physical address of the device; IP identifies where it's connected on the internet.
✅ MAC is set by the manufacturer and fixed (UAA); IP is assigned by the ISP and can be static or dynamic.
✅ MAC is 48 bits; IP is 32 bits (IPv4) or 128 bits (IPv6).

🚀 Going further A Level preview

Beyond the syllabus, the coursebook looks at how devices connect:

🔌 USB — serial, the industry standard
🖥️ HDMI — digital replacement for VGA · HD sound + video · high bandwidth
📶 Bluetooth — 79 channels at ~2.45 GHz · each pair picks a free channel and keeps hopping (spread-spectrum frequency hopping)

	Bluetooth	Wi-Fi
Range	Under ~30 m	~100 m
Speed	Lower bandwidth	Faster
Best for	Simple links — headphones, keyboards	Full networks

	Wired (twisted pair, coaxial, fibre)	Wireless
Speed / stability	Faster, more stable	Interference · dead spots
Security	More secure	Easier to intercept
Flexibility	Fixed cables	Easy to expand · devices can move

📒 Revision notes · 1.3.3 Input Devices Answers

Every question from the 1.3.3 revision sheet, answered. Test yourself first: press 🙈 Cover the notes, say your answer, then click a note to reveal it.

Question / key point	Revision notes
🖨️ Scanners
How does a 2D scanner work?	The lid is raised, the document is placed face down on the glass panel and the lid is closed. A bright light (xenon lamp or LED) illuminates the document. A scan head moves across the page. Mirrors send the image of each strip to a lens, which focuses it. The focused image falls on a CCD (charge-coupled device): thousands of light-sensitive elements (pixels) that each produce an electric charge depending on how much light hits them. The charges go through an ADC, and software builds a digital image. OCR software can then turn scanned text into an editable text file; a photo becomes an image file such as a JPEG.
What can a 2D scanner scan?	Flat, hard-copy (paper) items. Examples: flatbed scanners for documents, photos and drawings; sheet-fed / document scanners for many pages; passport scanners at airports; handheld scanners and barcode/QR readers are specialised kinds.
How does a 3D scanner work?	It scans a solid object from many positions, taking readings along the x, y and z coordinates, and builds a 3D digital model. Technologies include lasers, white/structured light, X-rays and magnetic resonance. Medical scanners use tomography: they image the object as a series of very thin 2D "slices" and stack the slices in memory to make the 3D image. The model can be used in CAD or sent to a 3D printer.
What can a 3D scanner scan?	Solid objects and people. Examples: CT scanners (X-rays), MRI (radio frequencies/magnetic fields) and SPECT (gamma rays) for the human body; laser scanners for car parts, buildings and museum artefacts; handheld 3D scanners for making models to 3D print; the face-ID depth camera on a phone.
How does a scanned image compare to a photo?	Both are made of pixels from a light-sensitive sensor (CCD/CMOS) and an ADC. A scan is taken in close, even, controlled light, so it's flat, distortion-free and usually high resolution (good for documents, and OCR can turn text into editable text). A photo is taken from a distance through a lens in natural light, so it can capture 3D scenes, depth and moving subjects, but can suffer from shadows, glare, blur and perspective distortion.
🏷️ Barcodes and QR codes
How does a barcode reader work?	A red laser or red LED shines on the barcode. Light is reflected back; dark bars reflect little or no light, light spaces reflect a lot. Sensors (photoelectric cells) read the reflected light as the beam moves across the code. The pattern of light/dark is converted to binary (e.g. light = 0, dark = 1), which the computer decodes into the digits. Guard bars separate the left and right halves; the left and right digits use different patterns, so the code can be read in either direction.
What info is held in a barcode?	Only a number (up to about 30 digits): typically the country code, the manufacturer, the product number and a check digit. It does not hold the price or the name. The number is the key field used to look up the product's record in the stock database.
What is a QR code and what can it store?	A quick response code: a 2D matrix of dark and light squares (pixels). It holds far more than a barcode — up to 4296 characters or 7089 digits: website addresses, phone numbers, text, contact details, Wi-Fi logins, payment details, boarding passes and tickets.
How does a QR reader work?	The camera on a phone or tablet takes an image of the code. A QR app finds the three large corner squares to locate and straighten the code, reads the grid of dark/light squares as binary data, uses the built-in error-checking data to fix damaged parts, then decodes the text or web address.
How is a QR code used to get information?	Point the camera at the code → the app decodes it → you're taken automatically to the website, the phone app (if a phone number is stored), or the boarding pass/ticket is saved to the phone. Uses: adverts and posters, menus, product information, payments, boarding passes, event tickets. vs barcodes: ✅ more data, built-in error checking, read by any phone camera, can be sent as images/messages, can be encrypted. ❌ several formats exist; can hide malicious links ("attagging") leading to fake websites or malware.
📷 Cameras, keyboards and pointing devices
How does a digital camera work?	Light passes through the lens onto a light-sensitive sensor (a CCD or CMOS) made of millions of tiny photodiodes — the pixels. Each pixel produces an electric charge proportional to the light hitting it. An ADC converts each charge to binary — e.g. 8 bits per colour gives 256 levels; 24-bit RGB gives red, green and blue values of 0–255 for each pixel. The embedded system controls focus, shutter speed, aperture, flash and red-eye removal, then compresses and saves the image (e.g. JPEG). More pixels and a better lens and sensor → better quality and bigger files.
What memory / storage / communication does a digital camera use?	Storage: a removable flash memory card (SD/microSD, solid state) or built-in flash memory in a phone. Memory: RAM as a buffer while processing; ROM/firmware for the embedded system. Communication: USB cable, Bluetooth or Wi-Fi to transfer files to a computer, phone or cloud storage.
How does a keyboard work?	Under the keys are two conductive layers separated by an insulating layer (a membrane or circuit board). Pressing a key pushes the layers together and completes a circuit at that point. The keyboard's controller detects which circuit closed, the CPU looks the key up in an index (key map) and converts it to the character's ASCII/Unicode value.
What different types of keyboard are there?	QWERTY (standard), ergonomic (split/curved to reduce RSI), numeric keypads (ATMs, checkouts, chip-and-PIN), concept/overlay keyboards (fast-food tills), virtual/on-screen keyboards (phones, tablets), wireless/Bluetooth, mechanical and membrane keyboards, and Braille keyboards.
How can data input by a keyboard be verified?	Double entry — type it twice and the computer compares the two copies (e.g. new passwords, email addresses). Visual check — the user reads what's on screen against the original document. (Validation — range, length, type, format checks — then checks the data is reasonable, but not that it's correct.)
How does a mouse / pointing device work?	Optical mouse: a red LED lights the surface; a tiny CMOS camera takes about 1500 images a second of the reflected light; a digital signal processor compares the changing images to work out the direction and distance moved, and sends the new coordinates to the computer, which moves the pointer. Buttons and the scroll wheel send click/scroll signals. ✅ No moving parts, so it's reliable, dirt can't get in, and it works on most surfaces.
What types of mouse / pointing device are there?	Optical mouse, laser mouse, mechanical (ball) mouse, wireless/Bluetooth mouse, trackpad/touchpad (laptops), trackerball (ball on top — good for limited movement/disabilities), joystick, stylus/graphics tablet, touch screen, and the pointing stick on some laptops. Wired vs wireless: wired has no signal loss and needs no batteries; wireless is more flexible.
👆 Touch screens and whiteboards
What are the different types of touch screen? How does each type work?	Capacitive (glass, a transparent conductive layer, glass base). Skin conducts electricity, so a finger changes the electrostatic field; a microcontroller works out where. Surface capacitive has sensors at the corners; projective capacitive uses an X–Y grid, so it allows multi-touch and works through thin gloves. Infrared — infrared transmitters and sensors around the edge make a grid of invisible beams; a finger breaks beams, the sensors detect less infrared, and the microcontroller calculates the position. Resistive — two layers with a resistive coating (a flexible polyethylene top layer and a glass bottom layer) separated by air or an inert gas. Pressing pushes them together, completing a circuit; the change in voltage gives the position.
For each type, a benefit and a drawback (no opposites)	Capacitive ✅ very clear image, even in strong sunlight ❌ affected by electromagnetic radiation (magnetic fields, microwaves) Infrared ✅ still works if the screen is scratched or cracked ❌ can be set off accidentally — anything breaking the beams (an insect, a sleeve) registers as a touch Resistive ✅ good resistance to dust and water ❌ easily scratched (the top layer is plastic)
Advantages and disadvantages of each touch screen type for a mobile phone or tablet?	Capacitive (projective) — ✅ multi-touch for pinch/zoom/swipe, very durable and scratch-resistant glass, sharp display, needs only a light touch ❌ needs a bare finger or special stylus (surface type), affected by electromagnetic interference, more expensive. This is why almost all phones use it. Infrared — ✅ multi-touch, durable, works with any object (gloves, pen) ❌ sensitive to water/moisture and to strong light, accidental touches; the frame of sensors makes the device bulkier — unsuitable for a phone. Resistive — ✅ cheap, works with gloves or any stylus, resists dust and water ❌ no multi-touch, low sensitivity (must press harder), poor visibility in sunlight, plastic surface scratches.
How does an interactive whiteboard work?	A projector shows the computer's screen on a large touch-sensitive board. Touching the board with a finger or special pen is detected — by a resistive or capacitive surface, an infrared frame or cameras in the corners — and the position is sent to the computer as mouse coordinates. So the board is both an input device (touch/pen) and, with the projector, an output surface: you can click, drag, write notes and save them.
What hardware and software do you need with an interactive whiteboard?	Hardware: the board, a computer, a (short-throw) projector (or a large touch-screen panel instead), a connection (USB/wireless and HDMI), special pens/stylus, speakers. Software: device drivers, calibration software so the touch position matches the image, and whiteboard/presentation software for writing, annotating and saving lessons.
How does a microphone work?	Sound makes the air vibrate → the diaphragm vibrates → it moves a coil of wire back and forth around a permanent magnet → the coil cutting through the magnetic field induces an electric current (analogue) → the current is amplified and a sound card / ADC samples it into digital values so the computer can store or process it.
Systems that use a microphone, and how	Voice assistants / smart speakers — voice commands. Phones & video calls — capturing speech. Voice recognition / dictation software — speech to text. Security systems — acoustic sensor detects footsteps or breaking glass. Voice authentication at banks — identifies the speaker. Cars — hands-free calls and voice control. Games consoles — voice chat. Music recording / podcasts. Hearing aids — pick up and amplify sound. Noise-cancelling headphones — detect outside noise to cancel it.
🛫 Input devices at work
Input and output devices when scanning passports at airports	Input: 2D passport scanner (with OCR to read the text into the database), RFID reader for the chip in an e-passport, digital camera to photograph the passenger's face (compared with the passport photo using face recognition), sometimes a fingerprint scanner, and a touch screen. Output: screen/monitor with instructions, gate actuator (motor) opening the barrier, lights (green/red), speaker, and a printer for a slip if needed.
Input and output devices with barcode readers at supermarket checkouts	Input: barcode reader/scanner, keypad (number of identical items, weights, or the barcode number if it won't scan), touch screen (choose loose items such as fruit), weighing scale, card reader / chip-and-PIN / contactless reader. Output: screen (item and price), speaker (a beep for a good read, another sound for an error), printer (receipt).
How is a product barcode used at a supermarket checkout? (the steps)	The barcode is scanned and the number decoded (the check digit is checked). The number is the key field: it's looked up in the stock database. When the record is found, the name and price are sent back to the checkout (POS terminal), shown on screen and added to the bill/receipt. The number in stock in that record is reduced by 1 and written back.
How do barcodes allow automatic stock control?	Every sale reduces the stock level in the database automatically. After each sale the new level is compared with the reorder level; if it's less than or equal to it, an order is automatically sent to the supplier and a flag is set so the item isn't reordered with every later sale. When the delivery arrives, the new items are scanned in and the stock level is updated (and the flag cleared). Management benefits: up-to-date sales information and trends, prices changed in one place, no need to price every item, fewer items run out, and customer buying habits can be analysed (e.g. with loyalty cards).
🌡️ Sensors and control
Name 7 sensors that input data, with two applications each	Temperature — central heating; greenhouse; chemical process · Light — street lights; car headlights · Moisture — greenhouse soil; food factory · Humidity — greenhouse air; microchip factory · Infrared (active / passive) — burglar alarm beam / body heat; automatic doors · Pressure — burglar alarm (weight on a mat); weighing lorries; reactor gas pressure · Acoustic — footsteps/breaking glass in security; detecting leaking pipes · Gas (O₂/CO₂) — greenhouse; pollution monitoring; car exhaust · pH — soil acidity; chemical process · Magnetic field — phones' compass; anti-lock brakes · Accelerometer — airbags; phone portrait/landscape · Proximity — phone screen off at your ear; parking sensors · Flow — gas pipes; inhalers/respirators · Level — petrol tank; river height; tablet powder levels.
What is an ADC?	An analogue-to-digital converter. Real-world data from sensors is analogue (continuously varying, e.g. a voltage). A computer can only process digital data, so the ADC turns the analogue signal into discrete binary values (e.g. 8 bits → 256 levels). A DAC does the reverse so a computer can drive analogue devices such as motors and valves (actuators).
Sensors for street lights	Light sensor (on at dusk, off at dawn); infrared / motion sensor (brighten only when people or cars pass, saving energy); acoustic sensor (traffic noise) in smart street lights.
Sensors for security devices	Infrared (active beam broken; passive body heat), pressure (mats under doors and windows), acoustic (footsteps, breaking glass), magnetic field (door/window contacts), proximity, light (torch at night), plus cameras.
Sensors for pollution control	Gas (CO₂, CO, NO₂, O₂), pH (acid in rivers and rain), temperature (water/thermal pollution), light (water clarity / turbidity), acoustic (noise pollution), humidity, flow and level (rivers, waste outflows).
Sensors for games	Accelerometer and gyroscope (tilting/swinging a controller or phone), infrared (motion cameras such as Kinect, TV-remote style pointing), pressure (dance mats, balance boards, trigger buttons), acoustic/microphone (voice control, singing games), proximity and touch, light/camera (AR games).
Sensors for household applications	Temperature (fridge, oven, central heating, kettle), moisture/humidity (tumble dryer stops when clothes are dry, bathroom extractor fan), pressure (washing machine water level), light (automatic garden lights), infrared (TV remote, motion-sensing lights), gas (carbon-monoxide/smoke alarms), proximity (robot vacuum), acoustic (smart speakers).
Sensors for industrial applications	Temperature and pH (chemical processes), pressure (nuclear reactor gas, hydraulic systems), flow (gas/liquid in pipes), level (tanks, powder in tablet production), gas (leak detection), proximity/infrared (items on a conveyor belt, robot safety), humidity (microchip manufacture), magnetic field, accelerometer (vibration in machinery).
Why use sensors instead of humans to collect data?	Readings are more accurate (no human error); they're taken 24/7 and at regular intervals without breaks or tiredness; the computer responds faster; sensors work in dangerous places (nuclear reactors, chemicals, deep sea, space); data is stored automatically for analysis and graphs; it's cheaper over time (fewer staff); and changes too small or too quick for a person to notice are detected.
How is a sensor used in a control system to control the output?	The sensor constantly sends readings (it never "decides" anything itself). An ADC converts them to digital, if they're analogue. The microprocessor compares each reading with the pre-set (stored) value. If it's outside the range, it sends a signal (through a DAC if needed) to an actuator — e.g. switch the heater on, open a valve or a window. The change affects the next readings: this is feedback. The loop repeats as long as the system is on. (A monitoring system only warns — sounds an alarm or shows a message — and doesn't change the process.)
How is data from a sensor processed?	The microprocessor samples the digital data at regular intervals (e.g. every second), compares each value with stored values/limits, decides on an action (warning, alarm or control signal), may store it to show graphs and trends, and sometimes averages several readings to smooth out "noise" from a faulty or jumpy reading.
How does a control system avoid frequent changes of output (flickering)?	It doesn't switch at exactly one value. It uses a range / two thresholds (hysteresis): e.g. a heater turns on below 19 °C but only off above 21 °C, so a reading wobbling around 20 °C doesn't keep toggling it. A street light might come on below one light level and off only above a higher one. Other methods: wait until the reading has stayed past the limit for a set time (a delay — a passing car's headlights won't switch a street light off), and average several readings before deciding.

📌 Summary for 1.3.3 Input Devices — Input devices turn real-world data into a form a computer can process. Scanners (2D: light, mirrors, lens, CCD, ADC, OCR; 3D: x-y-z points or tomography slices), barcode readers (laser/LED + photoelectric sensors → binary number → key field in the stock database → automatic stock control) and QR codes (camera + app; far more data, error checking, but can hide malicious links). Digital cameras (lens → CCD pixels → ADC → RGB values, flash card storage, USB/Bluetooth/Wi-Fi). Keyboards (a key press completes a circuit → ASCII; verify by double entry/visual check). Optical mice (LED + CMOS images → DSP → coordinates). Touch screens — capacitive (electrostatic field), infrared (beam grid), resistive (two layers touch). Microphones (diaphragm + coil + magnet → current → ADC). Sensors send analogue readings through an ADC to a microprocessor, which compares them with pre-set values and controls actuators through a DAC — feedback, with hysteresis to stop flickering.

📌 Chapter summary — The CPU (CU, ALU, registers, buses) runs the fetch–decode–execute cycle using the PC, MAR, MDR, CIR and ACC; performance depends on clock speed, cache, cores and bus width. Embedded systems are dedicated hardware+software. Input devices (barcode/QR scanners, cameras, keyboards, microphones, mice, scanners, touch screens) and output devices (actuators, projectors, printers, screens, speakers) convert between the real world and data; sensors with ADCs/DACs and actuators allow monitoring and feedback control. RAM is volatile, ROM isn't; secondary storage is magnetic, optical or solid-state; virtual memory swaps pages to disk; cloud storage keeps data on remote servers. NICs hold MAC addresses; IP addresses (static/dynamic, IPv4/IPv6) and routers move packets between networks.

Guru Jazzy 🕶️ · Chapter 3 · Hardware · it's silicon all the way down 🧠