ABI, according to http://www.x86-64.org/documentation/abi-0.99.pdf:

Up to 6 integer and pointer arguments are passed in registers. Nine
registers, %rax, %rcx, %rdx, %rsi, %rdi and %r8-%r12 can be used
freely. Integers and pointers are returned in %rax.

Registers	May be		Argument
		clobbered	number

%rax 		Y
%rbx		
%rcx		Y		4
%rdx		Y		3
%rbp		
%rsi		Y		2
%rdi		Y		1 
%rsp		(SP)
%r8		Y		5
%r9		Y		6
%r10		Y
%r11		Y
%r12		
%r13		
%r14		
%r15