RAM (Random Access Memory)
RAM is the computer's main working memory. It temporarily stores:
- The operating system (loaded at startup)
- Currently running programs
- Data being actively processed by the CPU
RAM is volatile — contents are lost when the computer is powered off. It is much faster than secondary storage but slower than cache. Larger RAM allows more programs to run simultaneously without performance slowdown.
ROM (Read-Only Memory)
ROM is non-volatile — it retains its contents without power. It stores the BIOS (Basic Input/Output System) or firmware — the instructions the computer needs to start up (boot) before the operating system is loaded from secondary storage.
- Cannot normally be written to or modified by the user
- Contents are set by the manufacturer
- Very small compared to RAM
Virtual Memory
When RAM is full, the operating system uses a section of secondary storage (e.g. hard drive or SSD) as virtual memory. Data that is currently not being used is temporarily moved (paged out) to this space to free up RAM for active processes.
- Allows the computer to run programs that require more RAM than is physically available
- Significantly slower than real RAM (secondary storage is much slower)
- Heavy use of virtual memory causes thrashing — the system spends more time swapping data than executing programs
- More physical RAM reduces the need for virtual memory
Memory Comparison
| Type | Volatile? | Speed | Typical size | Used for |
|---|
| Cache | Yes | Fastest | MB | CPU's current working data |
| RAM | Yes | Fast | GB | Running OS, programs, data |
| ROM | No | Moderate | MB | BIOS/firmware |
| Virtual memory | No | Slow | Configurable | Overflow when RAM is full |
📝 Exam Tip: If asked why a computer with 4 GB RAM is running slowly when many programs are open, a good answer mentions that RAM is full, the OS is using virtual memory (secondary storage), and this is much slower — causing performance issues.
⚠️ Common Mistakes
- Confusing RAM with storage — RAM is temporary and volatile; hard drives/SSDs are permanent
- Saying ROM is "random access" — RAM stands for Random Access Memory; ROM is Read-Only Memory
- Thinking virtual memory is fast — it uses secondary storage and is significantly slower than physical RAM