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.