XL Fortran Enterprise Edition for AIX : User's Guide - IBM

processing, and the fifth doubleword is reserved for future use. The last
doubleword (doubleword 6) is reserved for use by the global linkage routines that
are used when calling routines in other object modules (for example, in shared
libraries).
The Input Parameter Area
In a 32-bit environment, the input parameter area is a contiguous piece of storage
reserved by the calling program to represent the register image of the input
parameters of the callee. The input parameter area is double-word aligned and is
located on the stack directly following the caller’s link area. This area is at least 8
words in size. If more than 8 words of parameters are expected, they are stored as
register images that start at positive offset 56 from the incoming stack pointer.
The first 8 words only appear in registers at the call point, never in the stack.
Remaining words are always in the stack, and they can also be in registers.
In a 64-bit environment, the input parameter area is a contiguous piece of storage
reserved by the calling program to represent the register image of the input
parameters of the callee. The input parameter area is double-word aligned and is
located on the stack directly following the caller’s link area. This area is at least 8
doublewords in size. If more than 8 doublewords of parameters are expected, they
are stored as register images that start at positive offset 112 from the incoming
stack pointer.
The first 8 doublewords only appear in registers at the call point, never in the
stack. Remaining words are always in the stack, and they can also be in registers.
The Register Save Area
The register save area is double-word aligned. It provides the space that is needed
to save all nonvolatile FPRs and GPRs used by the callee program. The FPRs are
saved next to the link area. The GPRs are saved above the FPRs (in lower
addresses). The called function may save the registers here even if it does not need
to allocate a new stack frame. The system-defined stack floor includes the
maximum possible save area:
32-bit platforms:
64-bit platforms:
18*8 for FPRs + 19*4 for GPRs
18*8 for FPRs + 19*8 for GPRs
Locations at a numerically lower address than the stack floor should not be
accessed.
A callee needs only to save the nonvolatile registers that it actually uses. It always
saves register 31 in the highest
v addressed word (in a 32-bit environment)
v addressed doubleword (in a 64-bit environment)
The Local Stack Area
The local stack area is the space that is allocated by the callee procedure for local
variables and temporaries.
The Output Parameter Area
The output parameter area (P1...Pn) must be large enough to hold the largest
parameter list of all procedures that the procedure that owns this stack frame calls.
360 XL Fortran Enterprise Edition for AIX : User’s Guide