Legacy Document

Inside Macintosh: Mac OS Runtime Architectures /

Chapter 5 - CFM-68K Runtime Conventions / Routine Calling Conventions

Register Preservation

Table 5-2 lists registers used in the CFM-68K runtime environment and their volatility in function calls. Registers that retain their value after a routine call are called nonvolatile. All registers are 4 bytes long.

Table 5-2 Volatile and nonvolatile registers

Type	Register	Preserved by a routiine call?	Notes
Data register	D0 through D2	No
	D3 through D7	Yes
Address register	A0	No
	A1	No	Used to pass transition vector addresses (+4) when making indirect or cross-fragment calls.
	A2 through A4	Yes
Address register	A5	See next column	Used to access global data objects and the jump table. A5 is preserved by direct (in-fragment) calls, but not by cross-fragment or indirect calls.
	A6	Yes	Used as the back link and frame pointer when making cross-fragment calls.
	A7	See next column	A7 is the stack pointer used to push and pop parameters and other temporary data items
Floating- point register	F0 through F3	No	When present.
	F4 through F7	Yes	When present.
Condition Register	CR	No	Bits are set by compare instructions and used for conditional branching.