Calling sequence and stack frame

Stack pointer always kept 16-byte aligned (8-byte aligned for the old unsupported -32 ABI), and it grows downward:

	Caller's frame
	remaining overflow args
16($sp)	1$^{\rm st}$ overflow arg
	16 bytes reserved
$sp	for args passed in regs

Note that integers are promoted to 64 bits, and all optional parameters are allocated at least 64 bits, even if they are short.