Inside Macintosh: Memory
In the Macintosh multitasking environment, your application is limited to a particular memory partition (whose size is determined by information in the 'SIZE' resource of your application). The size of your application's partition places certain limits on the size of your application heap and hence on the sizes of the buffers and other data structures that your application can use.
If for some reason you need more memory than is currently available in your application heap, you can ask the Operating System to let you use any available memory that is not yet allocated to any other application. This memory, called temporary memory, is allocated from the available unused RAM; in general, that memory is not contiguous with the memory in your application's zone
Your application should use temporary memory only for occasional short-term purposes that could be accomplished in less space, though perhaps less efficiently. For example, if you want to copy a large file, you might try to allocate a fairly large buffer of temporary memory. If you receive the temporary memory, you can use the large buffer to copy data from the source file into the destination file. If, however, the request for temporary memory fails, you can instead use a smaller buffer within your application heap. Although the use of a smaller buffer might prolong the copy operation, the file is nonetheless copied.
One good reason for using temporary memory only occasionally is that you cannot assume that you will always receive the temporary memory you request. For example, if two or more applications use all available memory outside the system partition, then a request by any of them for some temporary memory would fail.
Another strategy for using temporary memory is to use it, when possible, for all nonessential memory requests. For example, you could allocate window records and any associated window data using temporary memory. This scheme allows you to keep your application partition relatively small (because you don't need space for nonessential tasks) but assumes that users will not fill up the available memory with other applications.