As long as only one application at a time was run, the system worked well. The machine itself implemented two areas in memory available for this scheme – the system heap (used for the OS), and the application heap. If a memory request required compaction of memory, this was done and the table, called the master pointer block, was updated. Apple's scheme was simple – a handle was simply a pointer into a (non-relocatable) table of further pointers, which in turn pointed to the data.
To solve this, Apple engineers used the concept of a relocatable handle, a reference to memory which allowed the actual data referred to be moved without invalidating the handle. The primary concern of the original engineers appears to have been fragmentation – that is, the repeated allocation and deallocation of memory through pointers leading to many small isolated areas of memory which cannot be used because they are too small, even though the total free memory may be sufficient to satisfy a particular request for memory. That design choice did not scale well with the development of the machine, creating various difficulties for both programmers and users. 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.
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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. Criticism of this approach was one of the key areas addressed by the change to Mac OS X. Historically, the classic Mac OS used a form of memory management that has fallen out of favor in modern systems. "About This Computer" Mac OS 9.1 window showing the memory consumption of each open application and the system software itself
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