Hardware: primary memory (RAM and ROM) and secondary storage (types, characteristics and uses)

What you'll learn

This revision guide covers everything you need to know about primary memory and secondary storage for your CIE IGCSE Computer Science exam. You'll learn the differences between RAM and ROM, understand why computers need both primary and secondary storage, and master the characteristics of magnetic, optical and solid-state storage devices. This content directly maps to the hardware component of the CIE syllabus.

Key terms and definitions

Random Access Memory (RAM) — volatile primary storage that temporarily holds data and instructions currently in use by the CPU; contents are lost when power is switched off

Read Only Memory (ROM) — non-volatile primary storage containing permanent instructions (bootstrap loader and BIOS) that cannot be altered during normal computer operation

Volatile memory — storage that loses its contents when power is removed; RAM is volatile

Non-volatile memory — storage that retains its contents even when power is removed; ROM and all secondary storage are non-volatile

Primary storage — memory directly accessible by the CPU (RAM and ROM); fast but limited in capacity

Secondary storage — permanent storage external to the CPU; larger capacity but slower access than primary storage; used for long-term data retention

Solid-state storage — storage with no moving parts that uses electronic circuits to store data persistently; examples include SSD, USB flash drive and memory cards

Access time — the time taken to locate and retrieve data from storage; measured in nanoseconds for RAM, milliseconds for hard disk drives

Core concepts

Primary memory: RAM

RAM is the computer's working memory. When you open an application or document, it is copied from secondary storage into RAM so the CPU can access it quickly. RAM is essential because the CPU can only process data held in primary storage.

Key characteristics of RAM:

• Volatile — all contents are lost when the computer is switched off
• Fast access times (measured in nanoseconds)
• Limited capacity compared to secondary storage (typically 4GB-16GB in consumer devices)
• Directly accessible by the CPU
• Read and write operations possible
• More expensive per gigabyte than secondary storage

Why RAM is needed:

The CPU processes instructions at extremely high speeds. Secondary storage devices like hard disk drives are far too slow to keep up with the CPU's processing speed. RAM acts as a buffer, holding currently executing programs and active data. The more RAM available, the more applications can run simultaneously without performance degradation.

When RAM becomes full, the operating system uses virtual memory — a portion of secondary storage used as an extension of RAM. However, this significantly slows performance because secondary storage has much slower access times than RAM.

Types of RAM:

• DRAM (Dynamic RAM) — must be constantly refreshed; used for main system memory
• SRAM (Static RAM) — faster, doesn't need refreshing; used for cache memory

Primary memory: ROM

ROM contains permanent instructions that the computer needs to start up. When you press the power button, the CPU first executes instructions stored in ROM because RAM is empty at startup.

Key characteristics of ROM:

• Non-volatile — contents retained when power is removed
• Contains the bootstrap loader and BIOS (Basic Input/Output System)
• Cannot be altered during normal computer operation
• Read-only during standard use
• Smaller capacity than RAM (typically measured in megabytes)

What ROM contains:

The bootstrap loader is a small program stored in ROM that runs automatically when the computer starts. It performs the following sequence:

1. Runs power-on self-test (POST) to check hardware is functioning
2. Searches for the operating system on secondary storage
3. Loads the operating system into RAM
4. Transfers control to the operating system

The BIOS (Basic Input/Output System) contains low-level instructions for communicating with hardware devices like keyboards, disk drives and display screens.

Types of ROM:

• ROM — truly permanent; contents set during manufacture
• PROM (Programmable ROM) — can be programmed once after manufacture
• EPROM (Erasable PROM) — can be erased with UV light and reprogrammed
• EEPROM (Electrically Erasable PROM) — can be erased and reprogrammed electronically; used for BIOS updates

Secondary storage: magnetic storage

Hard Disk Drives (HDD) are the traditional form of high-capacity storage in computers.

How HDDs work:

• Data stored magnetically on rotating metal platters coated with magnetic material
• Read/write heads move across the spinning platters to access data
• Platters spin at 5400-7200 RPM (rotations per minute) in consumer drives
• Data organized into tracks (concentric circles) and sectors (segments of tracks)

Characteristics of HDDs:

• Non-volatile
• Large capacity (500GB-10TB common)
• Relatively slow access times (5-10 milliseconds)
• Moving parts make them susceptible to damage from physical shock
• Lower cost per gigabyte than solid-state storage
• Uses more power than solid-state alternatives
• Generates noise and heat from moving components

Uses of HDDs:

• Desktop computer internal storage
• Backup and archival storage where speed is less critical
• Network-attached storage (NAS) devices
• Data centres requiring large, cost-effective storage

Secondary storage: solid-state storage

Solid-state storage has no moving parts and stores data in electronic circuits using flash memory technology.

Types of solid-state storage:

Solid State Drives (SSD):

• Installed internally in computers as a direct HDD replacement
• Connect via SATA or faster NVMe interfaces
• Capacities from 120GB to 4TB commonly available
• Significantly faster than HDDs

USB flash drives (memory sticks):

• Portable storage connecting via USB ports
• Capacities from 8GB to 512GB
• Used for transferring files between devices
• Small, lightweight and convenient

Memory cards:

• Used in cameras, smartphones, tablets and gaming devices
• Various formats: SD, microSD, CompactFlash
• Capacities from 4GB to 1TB

Characteristics of solid-state storage:

• Non-volatile
• No moving parts — more durable and shock-resistant than HDDs
• Fast access times (0.1 milliseconds or less for SSDs)
• Silent operation
• Lower power consumption than HDDs
• More expensive per gigabyte than magnetic storage
• Limited write cycles (typically 100,000+ before degradation)
• Lighter and more compact than HDDs

Uses of solid-state storage:

• Laptop and ultrabook primary storage (prioritizing speed and battery life)
• Smartphones and tablets
• Digital cameras and video recording devices
• Operating system drives for faster boot times
• USB drives for portable file transfer
• Embedded systems requiring reliability

Secondary storage: optical storage

Optical storage uses lasers to read and write data on reflective discs.

Types of optical storage:

CD (Compact Disc):

• Capacity: 700MB
• Available as CD-ROM (read-only), CD-R (recordable once), CD-RW (rewritable)

DVD (Digital Versatile Disc):

• Capacity: 4.7GB (single layer) or 8.5GB (dual layer)
• Available as DVD-ROM, DVD-R, DVD-RW

Blu-ray:

• Capacity: 25GB (single layer) or 50GB (dual layer)
• Uses blue-violet laser (shorter wavelength than DVD's red laser)
• Used for high-definition video distribution

How optical storage works:

• A laser burns pits into the disc surface (for R formats) or alters the reflectivity of a special layer (for RW formats)
• To read data, a lower-power laser shines on the disc surface
• Pitted and unpitted areas reflect light differently
• The pattern of reflections represents binary data (1s and 0s)
• The disc spins while the laser head moves radially to access different areas

Characteristics of optical storage:

• Non-volatile
• Slow access times compared to HDD and SSD
• Relatively low capacity by modern standards
• Susceptible to scratches affecting readability
• Inexpensive per disc
• Portable and lightweight
• Declining in popularity

Uses of optical storage:

• Software distribution (though digital downloads now dominant)
• Music album distribution
• Film and video distribution (particularly Blu-ray for high definition)
• Archival storage and backups
• Data transfer when internet speeds are limited

Comparing storage types

Speed (fastest to slowest):

1. RAM (nanoseconds)
2. SSD (microseconds)
3. HDD (milliseconds)
4. Optical (milliseconds, but slower than HDD)

Capacity (typical consumer devices):

• RAM: 4-16GB
• SSD: 120GB-2TB
• HDD: 500GB-10TB
• Optical: 700MB-50GB per disc

Cost per gigabyte (cheapest to most expensive):

1. HDD
2. Optical discs
3. SSD
4. RAM

Durability:

• Most durable: SSD (no moving parts)
• Moderate: Optical (unless scratched)
• Least durable: HDD (vulnerable to shock due to moving parts)

Choosing appropriate storage

The choice of storage depends on several factors:

For high-speed access to active data: Use RAM (volatile, fastest)

For operating system and frequently used programs: Use SSD (fast boot times, quick program loading)

For large media libraries and backups: Use HDD (large capacity, cost-effective)

For portable file transfer: Use USB flash drive (convenient, durable, adequate speed)

For archival storage with physical media: Use optical discs (inexpensive, relatively stable long-term)

For digital cameras: Use SD/microSD memory cards (compact, adequate speed for photo/video capture)

Worked examples

Example 1: Distinguishing RAM and ROM

Question: Explain two differences between RAM and ROM. [4 marks]

Model answer:

RAM is volatile, meaning it loses its contents when power is switched off, whereas ROM is non-volatile and retains its contents when power is removed. [2 marks]

RAM can be read from and written to during normal computer operation, whereas ROM can only be read from during normal use and contains permanent instructions such as the bootstrap loader. [2 marks]

Mark scheme guidance: Award 1 mark for each correct difference, and 1 mark for appropriate expansion/context. Alternative valid differences include: RAM has larger capacity than ROM; RAM holds currently executing programs and data whereas ROM holds startup instructions; RAM contents change constantly whereas ROM contents are fixed.

Example 2: Choosing appropriate storage

Question: A photographer needs to store 50GB of high-resolution photographs taken at a wedding. She needs to give the photographs to the client and also keep a backup copy herself.

Recommend appropriate storage devices for: (a) Giving the photographs to the client [2 marks] (b) Keeping a backup copy [2 marks]

Model answer:

(a) USB flash drive [1 mark] because it is portable, has sufficient capacity for 50GB, and the client can easily connect it to their computer via a USB port. [1 mark]

Alternative: External SSD/External HDD would also be acceptable with appropriate justification.

(b) External hard disk drive [1 mark] because it provides large, cost-effective storage suitable for long-term backup, and HDDs are reliable for archival purposes. [1 mark]

Alternative: Cloud storage would be acceptable with justification about accessibility and safety from physical damage.

Mark scheme guidance: Award 1 mark for a suitable device, 1 mark for appropriate justification linked to the scenario. The justification must be specific to the use case.

Example 3: Understanding storage characteristics

Question: State three advantages of using a solid-state drive (SSD) instead of a hard disk drive (HDD) for a laptop computer. [3 marks]

Model answer:

• SSDs have no moving parts, making them more shock-resistant and durable for portable devices. [1 mark]
• SSDs have faster access times, resulting in quicker boot times and faster program loading. [1 mark]
• SSDs consume less power than HDDs, extending battery life in the laptop. [1 mark]

Mark scheme guidance: Award 1 mark per valid advantage. Other acceptable answers include: lighter weight, silent operation, generates less heat. General statements like "faster" or "better" without context would not receive marks.

Common mistakes and how to avoid them

• Confusing volatile and non-volatile: Remember that volatile means "easily evaporated" — RAM's contents vanish when power is removed, just like a volatile liquid evaporates. ROM and all secondary storage are non-volatile (permanent).

• Saying ROM cannot be changed at all: ROM contents are permanent during normal operation, but some types (EEPROM) can be updated through special procedures like BIOS updates. Don't use absolutes unless appropriate.

• Mixing up capacity units: Be precise with units. RAM is typically measured in gigabytes (GB), ROM in megabytes (MB), and secondary storage in gigabytes or terabytes (TB). Optical discs have much smaller capacities than hard drives.

• Vague comparisons without context: Saying "SSD is better than HDD" is too simplistic. SSDs are faster and more durable, but HDDs offer larger capacity at lower cost. Choose storage based on requirements, not blanket statements.

• Forgetting that the CPU only accesses primary storage: The CPU cannot directly execute programs stored on a hard drive or SSD. Data must first be copied into RAM. This is why "loading" takes time when opening applications.

• Assuming all solid-state storage is the same: USB drives, SSDs, and memory cards all use flash memory but differ in speed, interface, and capacity. Use the correct specific term in exam answers.

Exam technique for Hardware: primary memory and secondary storage

• Command word awareness: "State" requires a brief answer without explanation (1 mark). "Describe" needs more detail about characteristics or processes (2 marks). "Explain" requires reasons or justification (2+ marks). "Compare" needs similarities and differences between two items.

• Use correct technical terminology: The mark scheme rewards precise vocabulary. Use terms like "volatile," "non-volatile," "access time," and "bootstrap loader" accurately. Avoid colloquialisms like "memory stick" unless it's clearly a USB flash drive.

• Structure comparison questions carefully: When comparing RAM and ROM or different storage types, make direct contrasts. Use "whereas" or "while" to link opposing characteristics: "RAM is volatile whereas ROM is non-volatile."

• Link answers to context: When questions provide a scenario (like the photographer example), your answer must reference that specific context. Generic answers receive fewer marks than those justified by the situation described.

Quick revision summary

Primary storage includes RAM (volatile, fast, limited capacity, holds active programs) and ROM (non-volatile, contains bootstrap loader and BIOS for startup). Secondary storage provides permanent, large-capacity storage. Magnetic storage (HDD) offers high capacity at low cost but has moving parts. Solid-state storage (SSD, USB drives, memory cards) is faster, more durable, but more expensive per gigabyte. Optical storage (CD, DVD, Blu-ray) provides portable, inexpensive media but with limited capacity and slower speeds. Choose storage based on requirements for speed, capacity, portability, durability and cost.