Virtual memory

Virtual memory adds more memory to your Macintosh by making use of unused free space on the hard drive. Because the extra memory comes from the hard drive, though, it is much slower than the RAM chips.

System requirements
Apple officially supports virtual memory beginning with System 7, but Connectix Corporation works well under System 6, thanks to its Virtual software. Virtual Memory is permanently on beginning with Mac OS X; before this, it was recommended for all Power Macintosh computers.

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

(A modified Mac II with a Motorola MC68851 PMMU can, however, run Virtual Memory.)

Operations
A large chunk of hard drive space is set aside just for the use of virtual memory. (This is called the swap file.) 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.

Utilities

 * RAM Doubler from Connectix Corporation doubles, and in more recent versions for the Classic Mac OS, triples your total, physical RAM (8 MB expanded to 16 MB and 24 MB, respectively).