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Inside Macintosh: Mac OS Runtime Architectures /

Chapter 11 - Classic 68K Runtime Conventions / Classic 68K Stack Structure and Calling Conventions

Register Preservation

Table 11-2 lists registers used in the classic 68K runtime environment and their volatility in routine calls. Registers that retain their value after a routine call are called nonvolatile. Note that these register conventions are for C and Pascal-style calls. Certain system calls may use different conventions, so you should check their definitions in the appropriate Inside Macintosh book before using them. All registers are 4 bytes long.

Table 11-2 Volatile and nonvolatile registers

Type	Register	Preserved by a function call?	Notes
Data register	D0 through D2	No
	D3 through D7	Yes
Address register	A0	No
	A1	No
	A2 through A4	Yes
	A5	See note	Used to access global data objects and the jump table.
	A6	Yes	Used as the frame pointer, which points to the base of the stack frame.
	A7	See note	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.

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