it must be commented

function call protocol of x86-64 says that the register %edi contains the parameter (or argument) x, the register %esi contains the parameter (or argument) y and that the register %eax carries the return value. Make sure the code of all your four functions abides to this protocol.

You may use registers %rax, %rcx, %rdx, %rsi, %rdi, %r8, %r9, %r10 and %r11 in your code.

You must not modify the values of registers %rbx, %rbp, %rsp, %r12, %r13, %r14 and %r15

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