Legacy Document

Important: The information in this document is obsolete and should not be used for new development.

Inside Macintosh: Mac OS Runtime Architectures /: Chapter 5 - CFM-68K Runtime Conventions / Routine Calling Conventions

Stack Frames, A6, and Reserved Frame Slots
The CFM-68K runtime architecture requires two long words in the stack frame to be reserved (that is, unused) for future use. The word locations for these reserved slots are -4(A6) and -8(A6).
Routines making calls through procedure pointers (that is, indirect or cross-fragment calls) must have an A6 frame and must reserve the two long words. Leaf routines or routines that make only direct (in-fragment) calls do not need to use A6 as a link, and they do not require reserved stack frame slots. However, some debugging options may require you to set up a stack frame.
In general, you should not use the A6 register except as a frame pointer, and if you do set up an A6 stack frame, you must also reserve the two long frame slots.

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