RAM and virtual memory

A desk and a cupboard

Memory stores the data and instructions a program needs, so the CPU can reach them quickly while the program runs.

Primary memory, or Random Access Memory (RAM), is the computer's main working area. It is fast and temporary: its data is erased when the computer is turned off. When you open a Word document, the program and the document are placed in RAM so the CPU can work on them at once.

When RAM is full, the OS uses part of the storage drive, such as a hard drive, SSD or NVMe drive, as virtual memory. It acts like temporary RAM, so more programs can run at the same time.

Storage drives transfer data more slowly and take longer to reach than RAM. So using virtual memory can slow the system. The OS moves data of less active programs there, which frees RAM for the programs you are using but makes the others slower to respond.

Your book's Do You Know box: the first personal computer, the IBM 5150 (1981), had only 16 KB of RAM. Modern DDR5 RAM can move more than 50 GB per second.

RAM: fast, temporary, the main working area. Virtual memory: storage drive used as extra RAM when RAM is full; more programs fit, but it is slower.

Key terms

RAM

Holding the program and data the CPU is working on now.

Virtual memory

Letting more programs run when RAM is full.

Temporary

Describing data that is erased when power is turned off.

Access time

How long it takes to reach stored data.

Short questions with model answers

1. Q1. Short Question 7 (p. 17): why is virtual memory slower than RAM?

   • Say where it lives.
   • Compare the speeds.

   It is stored on a hard drive or SSD, which moves data more slowly and takes longer to reach than RAM.

2. Q2. You have a game, a browser and a music app open, and RAM is full. What does the OS do?

   • Find the less active program.
   • Move its data.

   The game stays fast in RAM; the less active app may now respond more slowly.

Common mistakes

• ✗ Saying RAM keeps your work after the computer is switched off.

  ✓ RAM is temporary: its data is erased when the computer is turned off.

• ✗ Thinking virtual memory makes the computer faster.

  ✓ It lets more programs run, but it is slower than RAM and can reduce performance.

• ✗ Describing virtual memory as an extra RAM chip.

  ✓ It is part of the storage drive (hard drive, SSD or NVMe) that the OS uses like RAM.

MCQs

1. 1. Which memory is erased when the computer is turned off?

   1. (a) The hard drive
   2. (b) An SSD
   3. (c) RAM
   4. (d) A USB drive
   Show answer

   (c) 1.4.1, pp. 9–10.

2. 2. When does the OS start using virtual memory?

   1. (a) When RAM is full
   2. (b) When the computer starts
   3. (c) Only when a file is deleted
   4. (d) When the internet is slow
   Show answer

   (a) 1.4.2, p. 10.

3. 3. Why can virtual memory reduce system performance?

   1. (a) It deletes programs
   2. (b) It uses the CPU's ALU
   3. (c) It turns the computer off
   4. (d) Storage drives are slower and have higher access times than RAM
   Show answer

   (d) 1.4.2, p. 10; Short Q7, p. 17.

4. 4. How much RAM did the IBM 5150 (1981) have?

   1. (a) 50 GB
   2. (b) 16 KB
   3. (c) 16 GB
   4. (d) 1 MB
   Show answer

   (b) Do You Know, p. 10.

5. 5. The OS moves a less active program's data to virtual memory. What happens to that program?

   1. (a) It closes at once
   2. (b) It becomes faster
   3. (c) It may respond more slowly
   4. (d) It moves to the shell
   Show answer

   (c) 1.4.2, p. 10.

Quick revision

• RAM is the fast, temporary main working area; it is erased when the computer is turned off.
• When RAM is full, the OS uses part of the storage drive as virtual memory so more programs can run.
• Virtual memory is slower than RAM, so less active programs moved there respond more slowly.