Mac OS Memory Management - Switcher

Switcher

The situation worsened with the advent of Switcher, which was a way for the Mac to run multiple applications at once. This was a necessary step forward for users, who found the one-app-at-a-time approach very limiting. However, because Apple was now committed to its memory management model, as well as compatibility with existing applications, it was forced to adopt a scheme where each application was allocated its own heap from the available RAM. The amount of actual RAM allocated to each heap was set by a value coded into each application, set by the programmer. Invariably this value wasn't enough for particular kinds of work, so the value setting had to be exposed to the user to allow them to tweak the heap size to suit their own requirements. This exposure of a technical implementation detail was very much against the grain of the Mac user philosophy. Apart from exposing users to esoteric technicalities, it was inefficient, since an application would grab (unwillingly) all of its allotted RAM, even if it left most of it subsequently unused. Another application might be memory starved, but was unable to utilize the free memory "owned" by another application.

While an application could not beneficially utilize a sister application's heap, it could most certainly destroy it, typically by inadvertently writing to a nonsense address. An application treating a fragment of text or image, or an unassigned location as an address could easily overwrite the code or data of other applications or even the OS, leaving "lurkers" even after the program was exited. Such problems could be extremely difficult to analyze and correct.

Switcher became MultiFinder in System 4.2, which became the Process Manager in System 7, and by then the scheme was utterly entrenched. Apple made some attempts to work around the obvious limitations – temporary memory was one, where an application could "borrow" free RAM that lay outside of its heap for short periods, but this was unpopular with programmers so it largely failed to solve the problem. There was also an early virtual memory scheme, which attempted to solve the issue by making more memory available by paging unused portions to disk, but for most users with 68K Macintoshes, this did nothing but slow everything down without solving the memory problems themselves. By this time all machines had permanent hard disks and MMU chips, and so were equipped to adopt a far better approach. For some reason, Apple never made this a priority until OS X, even though several schemes were suggested by outside developers that would retain compatibility while solving the overall memory management problem. Third party replacements for the Mac OS memory manager, such as Optimem, showed it could be done.