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Inside Macintosh: Mac OS Runtime Architectures /

Chapter 4 - PowerPC Runtime Conventions / Routine Calling Conventions

Register Preservation

Table 4-3 lists registers used in the PowerPC runtime environment and their volatility in routine calls. Registers that retain their value after a routine call are called nonvolatile. All registers are 4 bytes long.

Table 4-3 Volatile and nonvolatile registers

Type	Register	Preserved by a routine call?	Notes
General- purpose register	GPR0	No
	GPR1	See next column	Used as the stack pointer to store parameters and other temporary data items.
General- purpose register	GPR2	See next column	Used as the base register to access the direct data area. GPR2 is preserved by direct calls; for indirect calls the caller must restore the value after the call.
	GPR3	See next column	Holds the return value or the address of the return value in function calls. For routine calls that do not return a value, GPR3 is used to pass parameter values.
	GPR4-GPR10	No	Used to pass parameter values in routine calls.
	GPR11-GPR12	No
	GPR13-GPR31	Yes
Floating- point register	FPR0	No
	FPR1-FPR13	No	Used to pass floating- point parameters in routine calls.
	FPR14-FPR31	Yes
Link Register	LR	No	Stores the return address of the calling routine during a routine call.
Count Register	CTR	No
Fixed-point exception register	XER	No
Condition Registers	CR0-CR1	No
	CR2-CR4	Yes
	CR5-CR7	No