Register Preservation

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.

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.