Virtual memory

Virtual memory makes more memory available than physically installed in a computer by making use of unused free space on its hard drive. Because the extra memory comes from a slower storage medium, performance is not as fast as RAM chips alone.

System requirements
Apple Computer first officially added support for virtual memory with the release of System 7 in 1991. However, Connectix had previously added its own implementation of virtual memory in System 6 through its Virtual software. Virtual memory became permanently enabled in Mac OS X (now macOS); before this, it was recommended for all Power Macintosh computers.

Only Macintosh computers with a Motorola 68030 or later processor can use Virtual Memory. This excludes the Macintosh Plus, Macintosh SE, Macintosh Classic, Macintosh II and Macintosh LC.

(A Macintosh II modified with the addition of a Motorola 68851 PMMU can, however, run virtual memory.)

Operations
A large chunk of hard drive space (called a swap file) is set aside for the use of virtual memory. Often, this will be amount of RAM installed plus the size of the requested additional memory.


 *  Example: If you requested another 4 MB of RAM (via Virtual Memory) on a Mac that already has 12 MB of RAM installed, the total size of the swap file, that is, the total RAM available will be 16 MB.

Note that in the example above, 16 MB of hard drive space will go to virtual memory -- not just 4 MB!

Virtual memory operates by quickly shuffling data from the disk onto RAM, and vice-versa. This can cause slowdowns, but is most noticeable only if switching between different programmes.

Activating and deactivating virtual memory
On a System 7 Mac, go to the Memory control panel, and activate or de-activate Virtual Memory. The computer must be restarted for the changes to kick into effect.

Under Mac OS X, however, virtual memory is on permanently.

Memory compression

 * RAM Doubler from Connectix doubled, and in more recent versions, tripled, the amount of available RAM in classic Mac OS through the compression of less-often used memory contents. This improved performance by reducing the amount of memory data that was swapped to disk.
 * OS X 10.9 "Mavericks" added memory compression as a standard feature. This can be disabled through Terminal commands.