Know and understand differences between ROM and RAM

1 Types and Components of Computer Systems – Internal Memory (RAM & ROM)

Learning Objectives (AO1 / AO2)

• Identify the two main types of primary memory – ROM and RAM.
• Explain the key characteristics of each (volatility, speed, capacity, location, read/write capability).
• Describe how ROM and RAM fit into the overall computer system, including the boot process and normal program execution.
• Distinguish primary memory from secondary (backing) storage and from cache memory.
• Discuss security and e‑safety considerations related to firmware and volatile memory.
• Evaluate the impact of emerging non‑volatile primary‑memory technologies.

Key Concepts

• Primary (main) memory – memory directly addressable by the CPU. Consists of ROM and RAM.
• Secondary (backing) storage – long‑term, non‑volatile devices such as HDDs, SSDs, USB sticks, CDs/DVDs.
• Cache memory – a small, very fast memory situated between the CPU registers and RAM; part of the memory hierarchy.
• Memory hierarchy – Registers → Cache → RAM → Secondary storage. Each level is larger but slower than the one above it.
• Fetch‑decode‑execute cycle – the CPU repeatedly fetches instructions from memory, decodes them, and executes them.

1.1 Hardware – Internal Memory

ROM – Read‑Only Memory

• Non‑volatile – retains its contents when power is removed.
• Stores firmware (e.g., BIOS or UEFI) that initialises the computer and provides basic hardware control.
• Traditionally read‑only for the user; modern ROM is often re‑programmable (EEPROM, Flash) via special firmware‑update procedures.
• Speed: typical access time ≈ 1–10 µs (microseconds), slower than RAM.
• Typical capacity: 2 MB – 16 MB for a BIOS/UEFI chip; larger for specialised embedded systems.
• Physical location: soldered chip on the motherboard or a separate ROM module.
• Examples: BIOS/UEFI firmware, embedded controller code in a digital camera, read‑only lookup tables.

RAM – Random‑Access Memory

• Volatile – all data is lost when power is switched off.
• Provides the fast, temporary workspace for the CPU while programs are running.
• Supports rapid read & write operations; any address can be accessed directly.
• Speed: typical access time ≈ 10–30 ns (nanoseconds) for modern DDR4/DDR5 modules.
• Typical capacity: 4 GB – 64 GB for most desktops and laptops; higher capacities are available for workstations.
• Physical location: installed in DIMM or SO‑DIMM slots on the motherboard.
• Examples: DDR4‑3200, DDR5‑5600 modules, video RAM (VRAM) on graphics cards, dual‑channel kits.

Comparison of ROM and RAM

Why Both Types of Memory Are Needed

1. Boot process – On power‑up the CPU fetches the initial instruction set from ROM (firmware) to perform the Power‑On Self‑Test (POST) and to locate the operating system.
2. Program execution – After the OS is loaded into RAM, the CPU repeatedly reads and writes data in RAM for fast processing.
3. Data preservation – ROM guarantees that essential system software is always available; RAM provides a fast workspace for user data that can later be saved to secondary storage.
4. Interaction with secondary storage – When RAM becomes full, the OS may move infrequently used data to the hard‑disk/SSD (paging or swapping), illustrating the memory hierarchy.
5. Cache support – A small amount of cache memory sits between the CPU and RAM, reducing the average access time for frequently used data.

Security, e‑Safety & Data Protection

• Firmware stored in ROM can be a target for malware (e.g., BIOS rootkits). Secure firmware updates and signed BIOS/UEFI images help protect against such threats.
• Because RAM is volatile, many operating systems erase (zero‑fill) RAM during shutdown or before entering sleep mode to prevent data leakage.
• Good e‑safety practice: always power‑off a computer before performing hardware upgrades to avoid accidental exposure of RAM contents.

Emerging Memory Technologies (Link to Syllabus “Impact of Emerging Technologies”)

New non‑volatile primary‑memory technologies aim to combine the speed of RAM with the permanence of ROM. Examples include:

• NV‑RAM / MRAM – magnetoresistive RAM that retains data without power and offers nanosecond‑level access times.
• 3D‑XPoint (Intel Optane) – a cross‑point memory that is faster than traditional SSDs and can be used as a high‑speed cache or as system memory.

These technologies blur the traditional volatile/ non‑volatile distinction and may appear in future IGCSE case‑studies that ask students to evaluate the advantages and disadvantages of replacing conventional RAM/ROM.

Mobile‑Device Example (Desktop & Mobile Context)

• ROM equivalent: The bootloader and firmware stored in a small eMMC/UFS chip (often called “boot ROM”). It performs the same initialisation role as a PC’s BIOS/UEFI.
• RAM equivalent: LPDDR4X/LPDDR5 memory modules soldered onto the phone’s motherboard. They are volatile, provide the working space for apps, and are typically 4 GB – 12 GB in modern smartphones.
• Both mobile and desktop systems follow the same sequence: firmware (ROM) → load OS from secondary storage (eMMC/SSD) → run applications in RAM.

Case‑Study Prompt (AO3 – Analysis & Evaluation)

“A school’s computer lab has experienced repeated crashes during the start‑up sequence. A technician discovers that the BIOS chip is corrupted. Explain what would happen if the ROM were damaged, how this differs from a RAM failure, and suggest two practical steps the school could take to prevent similar problems in the future.”

Key Points to Remember

• ROM = non‑volatile, primarily read‑only (but can be re‑programmed with EEPROM/Flash), stores firmware and boot instructions.
• RAM = volatile, read‑write, provides the fast workspace for active programs and data.
• Both are **primary memory**, but only RAM is used for the working space of applications.
• During boot the CPU reads from ROM; during normal operation it reads/writes to RAM.
• The operating system is stored on secondary storage, loaded into RAM at start‑up, while a small boot loader remains in ROM.
• Memory hierarchy: Registers → Cache → RAM → Secondary storage; each level is larger and slower than the one above.
• Security of firmware (ROM) and proper clearing of RAM are important e‑safety considerations.
• Emerging non‑volatile memories (NV‑RAM, MRAM, 3D‑XPoint) may eventually replace traditional RAM/ROM distinctions.