Virtual Memory

The MIPS R3000 has an address space of 2³² bytes. A Gigabyte = 2³⁰, so it has 4 gigabytes of address space. Ideally, all of these memory locations would be implemented using memory chips (usually called RAM). RAM costs about $2 per megabyte. Installing the maximum amount of memory as RAM would cost 2¹² × $2 = $8192. This might be more than you want to spend. Hard disk storage costs much less per megabyte. A 20 gigabyte hard disk costs about $90 (summer, 2001).

On modern computers, the full address space is present no matter how much RAM has been installed. This is done by keeping some parts of the full address space on disk and some parts in RAM. The RAM, the hard disk, some special electronics, and the operating system work together to provide the full 32 bit address space. To a user or an applications programmer it looks as if there are all 2³² bytes of main memory.

This method of providing the full address space by using a combination of RAM memory and the hard disk is called virtual memory. The word virtual means "appearing to exist, but not really there." Some computer geeks have a virtual social life.