Assembler-level subroutine linkage conventions

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The subroutine linkage convention specifies the machine state at subroutine entry and exit, allowing routines that are compiled separately in the same or different languages to be linked.

The information on subroutine linkage and system calls in the System V Application Binary Interface: PowerPC Processor Supplement and 64–bit PowerPC ELF Application Binary Interface Supplement are the base references on this topic. You should consult these for full details. This section summarizes the information needed to write mixed-language Fortran and assembler programs or to debug at the assembler level, where you need to be concerned with these kinds of low-level details.

The system linkage convention passes arguments in registers, taking full advantage of the large number of floating-point registers (FPRs), general-purpose registers (GPRs), vector registers (VRs and VSRs) and minimizing the saving and restoring of registers on subroutine entry and exit. The linkage convention allows for argument passing and return values to be in registers (FPRs, GPRs, VRs or multiple register types), memory, or both.

The following table lists floating-point registers and their functions. The floating-point registers are double precision (64 bits).

Table 1. Floating-point register usage across calls
Register Preserved Across Calls Use
0 no  
1 no FP parameter 1, function return 1.
2 no FP parameter 2, function return 2.
3 no FP parameter 3, function return 3, function return complex *32.
4 no FP parameter 4, function return 4 , function return complex *32.
5 no FP parameter 5, function return 5.
⋮ ⋮ ⋮
8 no FP parameter 8, function return 8.
9 no FP parameter 9
10 no FP parameter 10
11 no FP parameter 11
12 no FP parameter 12
13 no FP parameter 13
14-31 yes local variables

The following table lists general-purpose registers and their functions.

Table 2. General-purpose register usage across calls
Register Preserved Across Calls Use
0 no  
1 yes Stack pointer.
2 yes System-reserved TOC/GOT pointer.
3 no 1st word of arg list; return value 1.
4 no 2nd word of arg list; return value 2.
5 no 3rd word of arg list
⋮ ⋮ ⋮
10 no 8th word of arg list.
11-12 no  
13 yes SDA (thread local storage) pointer.
14-30 no Local variables.
31 yes Local variables or environment pointers.
If a register is not designated as preserved, its contents may be changed during the call, and the caller is responsible for saving any registers whose values are needed later. Conversely, if a register is supposed to be preserved, the callee is responsible for preserving its contents across the call, and the caller does not need any special action.
The following table lists vector register conventions.
Table 3. VMX Vector register usage across calls
Register Preserved Across Calls Use
0 no Local variables
1 no Local variables
2 no Vector parameter 1, vector result 1
3 no Vector parameter 2, vector result 2
4 no Vector parameter 3, vector result 3
5 no Vector parameter 4, vector result 4
6 no Vector parameter 5, vector result 5
7 no Vector parameter 6, vector result 6
8 no Vector parameter 7, vector result 7
9 no Vector parameter 8, vector result 8
10 no Vector parameter 9
11 no Vector parameter 10
12 no Vector parameter 11
13 no Vector parameter 12
14-19 no Local variables
20-31 yes Local variables
VMX vector registers VR0-31 correspond to VSX registers 32-63. FPR floating-point registers FP0-31 correspond to the upper halves of VSX registers 0-31.

The following table lists special-purpose register conventions.

Table 4. Special-purpose register usage across calls
Register Preserved Across Calls
Condition register
   Bits 0-7   (CR0,CR1)
   Bits 8-22  (CR2,CR3,CR4)
   Bits 23-31 (CR5,CR6,CR7)
 
no
yes
no
Link register no
Count register no
XER register no
FPSCR register The control flags are preserved except for calls to functions whose purposes are to change them. The status flags are not preserved.