Primary storage (memory): Virtual memory

The idea

Virtual memory is a technique that lets a computer run programs larger than the available physical RAM. When RAM becomes full, the operating system moves pages of data that are not currently being used from RAM onto a reserved area of secondary storage, often called the page file or swap file. This frees space in RAM for the pages the CPU needs now. When a program needs data currently held on secondary storage, the operating system swaps it back into RAM and may move another page out. Because secondary storage is much slower than RAM, heavy use of virtual memory slows the computer down.

For example: A laptop with 8 GB of RAM running several large applications may use virtual memory to keep working, but switching between applications becomes noticeably slower.

What earns the marks

• Virtual memory extends the apparent size of RAM by using secondary storage as an overflow area.
• The operating system moves pages or blocks of data between RAM and a page file or swap file on secondary storage.
• When required data is not currently in RAM, the operating system retrieves it from secondary storage and may move another page out.
• Frequent swapping increases disk access and slows performance because secondary storage has a much higher access time than RAM.
• Virtual memory allows programs larger than physical RAM to run, and allows more programs to be open at once.
• The technique is managed by the operating system, not directly by the application program.

Common mistakes and how to put them right

• Thinking virtual memory increases the amount of physical RAM.

  Put it right: It does not; it uses secondary storage to supplement RAM.

• Believing virtual memory makes a computer faster.

  Put it right: It usually slows performance because secondary storage is slower than RAM.

• Confusing virtual memory with cache or with cloud storage.

  Put it right: Cache is fast memory close to the CPU, and cloud storage is remote secondary storage, not a local extension of RAM.

Exam tips

• Link virtual memory to the fetch-execute cycle by explaining that the CPU can only execute instructions and data held in RAM.
• Describe the transfer between RAM and secondary storage in both directions, using GCSE-level terminology.
• When asked about performance, compare access times of RAM and secondary storage rather than saying virtual memory is simply good or bad.