System V ABI

Callee saved

rbx, rbp, rsp, r12, r13, r14, r15

need to be saved and restored inside function

Caller saved

rax, rcx, rdx, rdi, rsi, r8, r9, r10, r11

can be used freely inside function

Function Parameters

rdi, rsi, rdx, rcx, r8, r9

further values on stack in reverse order

Return Value

rax (128-bit: higher go in rdx)