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AQA 7517 · Paper 2 · 4.7.5

Storage Devices
& I/O

HDD, SSD, optical, solid-state · Input/output devices · Section 4.7

WHAT YOU'LL LEARN
Primary vs secondary storage · HDD vs SSD · Optical media · RAM vs ROM · I/O devices
AQA SPEC LINK
4.7.5 — Storage devices, media, and I/O
Memory Hierarchy

Storage Hierarchy

Storage is organised in a hierarchy: faster = smaller = more expensive, slower = larger = cheaper.
Registers (~0.1ns, bytes)
L1/L2/L3 Cache (~1–15ns, KB–MB)
RAM (~60ns, GB)
SSD (~100μs, TB)
HDD (~10ms, TB)
Optical/Tape (slowest, TB+)
RAM & ROM

Primary Storage: RAM & ROM

RAM (Random Access Memory)
Volatile — loses contents when power removed. Holds currently running programs and data. Fast read/write. DRAM (main RAM) and SRAM (cache).
ROM (Read-Only Memory)
Non-volatile — retains data without power. Stores firmware/BIOS. Cannot be easily written to. Modern systems use flash-based ROM (EEPROM) that can be updated.
HDD vs SSD

Hard Disk Drive vs Solid State Drive

FeatureHDDSSD
TechnologyMagnetic platters + moving headsNAND flash chips, no moving parts
Speed~100 MB/s, ~10ms seek~500–7000 MB/s, ~0.1ms
DurabilityFragile (moving parts)Shock-resistant
Cost/GBCheaperMore expensive
NoiseAudibleSilent
Optical Storage

Optical & Solid-State Media

CD (~700 MB)
Pits and lands read by laser; CD-ROM (read-only), CD-R (write once), CD-RW (rewritable)
DVD (~4.7–17 GB)
Higher density than CD; dual layer. DVD-ROM, DVD-R, DVD-RW
Blu-ray (~25–50 GB)
Blue-violet laser (405nm); used for HD/4K movies and large data backup
Flash/USB/SD Card
Solid-state, no moving parts; portable; used in cameras, phones, USB drives
I/O Devices

Input & Output Devices

INPUT DEVICES
Keyboard · Mouse · Touchscreen · Microphone · Webcam · Scanner · Barcode reader · RFID reader · Sensors (temperature, motion, proximity)
OUTPUT DEVICES
Monitor · Printer (inkjet, laser, 3D) · Speakers · Projector · Actuators · Braille reader · LED/LCD displays
Choosing Storage

Choosing the Right Storage

Capacity — how much data needs to be stored? (TB for video; GB for OS)
Speed — SSD for OS/applications; HDD for bulk storage/backups
Portability — USB/SD for portable; cloud for remote access
Durability — SSD/flash for laptops (no moving parts); optical for archival (long lifespan if stored well)
Cost — HDD cheapest per GB; SSD falling in price; Blu-ray cheap for single-write large archival
Magnetic Storage

How HDD Works

HDD uses rotating magnetic platters. A read/write head moves radially across the platter while it spins (5400–7200 RPM). Data is stored as magnetic polarity (N=1, S=0 or vice versa).
Track: concentric circle on platter · Sector: subdivision of a track · Cluster: group of sectors allocated to a file
Seek time: time to move head to correct track (main bottleneck)
Fragmentation: file stored in non-contiguous sectors → slower access
AQA Exam Style

Practice Question

AQA 7517 — Paper 2 Style
(a) Give TWO differences between RAM and ROM. [2]
(b) A laptop manufacturer is deciding between an HDD and SSD. Give TWO advantages of using an SSD. [2]
(c) Explain why optical storage (Blu-ray) might be chosen for archiving data despite being slower than SSD. [2]
(d) Describe the difference between volatile and non-volatile storage. [2]
[8 marks]
2 marks
(a) RAM is volatile (loses data when power removed) vs ROM is non-volatile [1]; RAM can be read and written freely vs ROM is primarily read-only [1]
2 marks
(b) Any 2: faster access speeds / no moving parts so more durable / lighter / silent / lower power consumption
2 marks
(c) Blu-ray discs are physically durable for long-term archiving [1]; cheaper per GB for write-once archival, and don't degrade if stored properly unlike flash memory [1]
2 marks
(d) Volatile storage loses its contents when power is removed [1]; non-volatile storage retains data without a power supply [1]
Summary

Key Points to Remember

RAM — volatile, fast, primary storage; ROM — non-volatile, read-only, firmware
HDD — magnetic, moving parts, slow, cheap/large; SSD — flash, no moving parts, fast, expensive
Optical — CD/DVD/Blu-ray; read by laser; good for archiving
Choose storage by: capacity, speed, portability, durability, cost
Storage hierarchy: registers → cache → RAM → SSD → HDD → optical (speed decreases, capacity increases)
🎉 Lesson complete — move to the quiz!