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-%r11 can be used
freely. Integers and pointers are returned in %rax.

At entry, it is required that %rsp == 8 (mod 16).

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		

W64 ABI, according to
http://msdn.microsoft.com/en-us/library/9z1stfyw.aspx:

Registers	May be		Argument
		clobbered	number

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

Additional arguments are passed on the stack. "backing store" on the
stack for the four register arguments is also required. %xmm6 to
%xmm15 are callee-saved. The "long" type is just 32 bits.

If we have six arguments, and push the additional callee-save
registers %rdi and %rsi on the stack, we get a stack frame like

64(%rsp): Sixth argument
56(%rsp): Fifth argument
48(%rsp): Space for fourth argument
40(%rsp): Space for third argument
32(%rsp): Space for second argument
24(%rsp): Space for first argument
16(%rsp): Return address
8(%rsp) : Saved %rdi
(%rsp): Saved %rsi

If, in addition, we use more than 6 %xmm registers, we push them 
*after* %rdi (but before %rsi), so that they are stored at 16-byte
aligned addresses.