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.
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.
(A modified Mac II with a Motorola MC68851 PMMU can, however, run Virtual Memory.)
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
Under Mac OS X, however, virtual memory is on permanently.
- Performance is largely degraded if the Mac's total memory is set to more than twice the amount of real RAM the Mac has (e.g. over 32 MB on a Mac with 16 MB of built-in, real RAM, will cause serious performance bottlenecks.)
- Activated, Virtual Memory will, on Power Macs, make application programmes use less memory.
- 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).