4.1.2 Normal Argument List Structure

The argument list in an Alpha call is an ordered set of zero or more argument items, which together comprise a logically contiguous structure known as the argument item sequence. An argument item is represented in 64 bits.

An argument item may be used to pass immediate arguments, arguments by reference, and arguments by descriptor. The standard permits any combination of these mechanisms in an argument list.

Although the argument items form a logically contiguous sequence, they are in practice mapped to integer and floating-point registers and to memory in a fashion that may produce a physically discontiguous argument list. Registers R16..R21 and F16..F21 are used to pass the first six items of the argument item sequence. Additional argument items must be passed in a memory argument list that must be located at 0 (SP) at the time of the call.

The following table specifies the standard locations in which argument items can be passed.

Table 4-1 Argument Item Locations

Item	Integer registers	Floating-point registers	Stack
1	R16	F16
2	R17	F17
3	R18	F18
4	R19	F19
5	R20	F20
6	R21	F21
7...n			0(SP)...(n-7)*8(SP)

The general rules that determine the location of any specific argument can be summarized as follows:

All argument items are passed in the integer registers or on the stack, except argument items that are floating-point data passed by immediate value.
Floating-point data passed by immediate value is passed in the floating-point registers or on the stack.
Only one location in any row in the above table may be used by any given argument list. So, for example, if argument item 3 is an integer passed by value, and argument item 4 is a single-precision floating-point number passed by value, then argument item 3 is assigned to R18 and argument item 4 is assigned to F19.
A single- or double-precision complex value passed by immediate value is treated as two arguments for the purpose of these rules, with the real part coming first. In particular, the real part of a complex value may happen to be passed as the sixth argument item in register F21, in which case the imaginary part will then be passed as the seventh argument item in memory.

The argument list, including both the in-memory portion as well as the portion that is passed in registers, may be read and written by the called procedure. The calling procedure must therefore not make any assumptions about the validity of any part of the argument list after the completion of a call.