Extending the Operating System's Available Memory

In system 7.0, suitably equipped Macintosh computers can take advantage of

an operating-system feature known as virtual memory, by which the

machines have a logical address space that extends beyond the limits of the

available physical memory. This means that a user can load more programs and

data into the logical address space than if limited to the actual physical RAM

installed in the machine. The Operating System extends the address space by

using part of the available secondary storage (that is, part of a hard disk) to

hold portions of programs and data that are not currently needed in RAM. When

some of those portions of memory are needed, the Operating System swaps

other parts of programs or data that are not needed out to the secondary

storage, thereby making room for the parts that are needed.

It is important to realize that virtual memory operates trans parently to most

applications. You allocate and release memory in your application heap exactly

as you always have, by calling Memory Manager routines. You can also

request temporary memory by using special Operating System routines. But

unless your application has time-critical needs that might be adversely

affected by the operation of virtual memory, you do not need to know whether

virtual memory is operating.