4.1.4 Argument Lists and High-Level Languages

High-level language functional notations for procedure call arguments are mapped into argument item sequences according to the following rules:

• Arguments are mapped from left to right to increasing offsets in the argument item sequence. R16 or F16 is allocated to the first argument, and the last quadword of the memory argument list (if any) is allocated to the last argument.
  
  
• Each source language argument corresponds to one or more contiguous Alpha calling standard argument items.
  
  
• Each argument item consists of 64 bits.
  
  
• A null or omitted argument — for example, CALL SUB(A,,B), is represented by an argument item containing 0.

  No arguments passed by the immediate mechanism may be omitted unless a default value is supplied by the language. (This is to enable called procedures to distinguish an omitted immediate argument from an immediate value argument with the value 0.)

  Trailing null or omitted arguments — for example, CALL SUB(A,,), are passed by the same rules as embedded null or omitted arguments.