The calling convention of the System V AMD64 ABI is followed on Solaris, Linux, FreeBSD, Mac OS X, and other UNIX-like or POSIX-compliant operating systems. The first six integer or pointer arguments are passed in registers RDI, RSI, RDX, RCX, R8, and R9, while XMM0, XMM1, XMM2, XMM3, XMM4, XMM5, XMM6 and XMM7 are used for floating point arguments. For system calls, R10 is used instead of RCX. As in the Microsoft x64 calling convention, additional arguments are passed on the stack and the return value is stored in RAX.
Registers RBP, RBX, and R12-R15 are callee-save registers; all others must be saved by the caller if they wish to preserve their values.
Unlike the Microsoft calling convention, a shadow space is not provided; on function entry, the return address is adjacent to the seventh integer argument on the stack.