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

In a 32-bit environment, this area is at least 8 words long, regardless of the length
or existence of any argument list. If more than 8 words are being passed, an
extension list is constructed beginning at offset 56 from the current 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, this area is at least 8 doublewords long, regardless of the
length or existence of any argument list. If more than 8 doublewords are being
passed, an extension list is constructed, which begins at offset 112 from the current
stack pointer.
The first 8 doublewords only appear in registers at the call point, never in the
stack. Remaining doublewords are always in the stack, and they can also be in
registers.
Linkage Convention for Argument Passing
The system linkage convention takes advantage of the large number of registers
available. The linkage convention passes arguments in both GPRs and FPRs. Two
fixed lists, R3-R10 and FP1-FP13, specify the GPRs and FPRs available for
argument passing.
When there are more argument words than available argument GPRs and FPRs,
the remaining words are passed in storage on the stack. The values in storage are
the same as if they were in registers.
The size of the parameter area is sufficient to contain all the arguments passed on
any call statement from a procedure that is associated with the stack frame.
Although not all the arguments for a particular call actually appear in storage, it is
convenient to consider them as forming a list in this area, each one occupying one
or more words.
For call by reference (as is the default for Fortran), the address of the argument is
passed in a register. The following information refers to call by value, as in C or as
in Fortran when %VAL is used. For purposes of their appearance in the list,
arguments are classified as floating-point values or non-floating-point values:
Interlanguage Calls 361