Inside Macintosh: Memory
In system software version 7.0 and later, suitably equipped Macintosh computers can take advantage of a feature of the Operating System known as virtual memory, by which the machines have a logical address space that extends beyond the limits of the available physical memory. Because of virtual memory, a user can load more programs and data into the logical address space than would fit in the computer's physical RAM.
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 applications and data that are not currently needed in RAM. When some of those portions of memory are needed, the Operating System swaps out unneeded parts of applications or data to the secondary storage, thereby making room for the parts that are needed.
It is important to realize that virtual memory operates transparently to most applications. Unless your application has time-critical needs that might be adversely affected by the operation of virtual memory or installs routines that execute at interrupt time, you do not need to know whether virtual memory is operating. For complete details on virtual memory, see the chapter "Virtual Memory Manager" later in this book.