Register usage

x86-64 has a much less claustrophobic 16 integer registers, as well as 16 SSE/SSE2 registers. It has the usual complement of 8 x87 registers, aliased with 8 MMX registers. The available x86-64 registers is summarized in Table 4.

Table 4: x86-64 register summary

		CALLEE
REGISTER	USAGE	SAVE
%rsp	Stack pointer	YES
%rbx	optional base pointer	YES
%rbp	optional frame pointer	YES
%rax	integer return val	NO
%rdi	1st int arg	NO
%rsi	2nd int arg	NO
%rdx	3rd int arg	NO
%rcx	4th int arg	NO
%r8	5th int arg	NO
%r9	6th int arg	NO
%r10	used to pass static chain pointer	NO
%r11	scratch reg	NO
%r12-15	callee-saved regs	YES
%xmm0-1	pass & return fp args	NO
%xmm2-7	pass fp args	NO
%xmm8-15	scratch regs	NO
%mmx0-7	scratch regs, aliased to fp stack	NO
%st	returns long double args	NO
	aliased with mmx regs
%st1-7	scratch regs, aliased with mmx	NO