Virtual Memory (CIE IGCSE Computer Science)

• The purpose of RAM is to store current data and program instructions in use
• The amount of physical RAM available is limited. The average modern desktop computer has between 4GB  and 16GB of RAM. Some programs are larger than this
• Virtual memory is part of secondary storage which acts as RAM when RAM is full
• It prevents the computer system crashing when RAM is full
• Data that needs to be accessed by the CPU directly is switched by the operating system from virtual memory into RAM and an inactive page from RAM replaces it in virtual memory. This is called page switching

A diagram showing page switching between RAM and virtual memory

• The table describes the steps that are taken when a program is opened on a computer

Step	Description
1	The memory manager software built into the Operating System checks to see if there is sufficient space in RAM to move the new program’s instructions into RAM, so that they can be directly accessed by the CPU
2	If RAM is full, the memory manager will move out the most inactive pages of data and put them in virtual memory until needed
3	The new program instructions are moved into RAM