XPLINK register conventions

XPLINK introduces a register scheme which is different from standard OS linkage, in order to optimize the performance of saving and restoring registers in function prologs and epilogs. The registers which are saved in the register save area of the XPLINK stack frame are described in Save area (48 bytes).

Table 1 shows the layout of the XPLINK interface.

Table 1. Comparing non-XPLINK and XPLINK register conventions
Interface part	Non-XPLINK	XPLINK
Stack pointer	Reg 13	Reg 4 (biased; see The XPLINK stack frame layout)
Return address	Reg 14	Reg 7
Entry point on entry	Reg 15	Reg 6 (not guaranteed; a routine may be called via branch relative)
Environment	Reg 0 (writable static)	Reg 5
CAA Address	Reg 12	Reg 12
Input parameter list	address in R1	Located at offset 2112 ('840'x) off R4 (fixed location in caller's stack frame). First three words are passed in R1-R3, floating point values in FPR0, 2, 4, 6.
Return code	Reg 15	R3 (extended return value in R1,R2)
Start address of callee's stack frame	Caller's NAB value	Caller's Reg 4 - DSA size
End address of callee's stack frame	Caller's NAB value + DSA size	Caller's Reg 4
Where caller's registers are saved	R0-R12 saved in caller's stack frame R13 saved in callee's stack frame R14-R15 saved in caller's stack frame	R0 not saved, not preserved R1-R3 not saved, not preserved R4 not saved, recalculated (or saved, restored) R5 not saved, not preserved R6 saved in callee's stack frame, not restored R7-R15 saved in callee's stack frame (R7 is the return register and is not guaranteed to be restored)

See z/OS Language Environment Vendor Interfaces for additional information about register usage and conventions.