On i386, because of the dearth of general-purpose registers, the calling convention passes all arguments on the stack. This makes the vaarg implementation easy – A valistis simply a pointer into the stack, andvaargjust adds the size of the type to be retrieved to thevalist, and returns the old value. In fact, the i386 ABI reference simply specifiesva_arg in terms of a single line of code:

On amd64, the problem is much more complicated. To start, amd64 specifies that up to 6 integer arguments and up to 8 floating-point arguments are passed to functions in registers, to take advantage of amd64's larger number of registers. So, for a start, va_arg will have to deal with the fact that some arguments may have been passed in registers, and some on the stack.