Classic Mac OS memory management

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Historically, the classic Mac OS used a form of memory management that has fallen out of favor in modern systems. Criticism of this approach was one of the key areas addressed by the change to Mac OS X.

"About This Computer" Mac OS 9.1 window showing the memory consumption of each open application and the system software itself

The original problem for the engineers of the Macintosh was how to make optimum use of the 128 KB of RAM with which the machine was equipped, on Motorola 68000-based computer hardware that did not support virtual memory.[1] Since at that time the machine could only run one application program at a time, and there was no fixed secondary storage, the engineers implemented a simple scheme which worked well with those particular constraints. That design choice did not scale well with the development of the machine, creating various difficulties for both programmers and users.