//main.cpp
#include <iostream>

using namespace std;

extern "C" int GetValueFromASM(unsigned long long num);

int main()
{
    unsigned long long x = 18446744073709551615; //max for an unsigned long long number (64bit)
    cout << GetValueFromASM(x) << endl; // prints out -1, why?
    cout << x << endl; // prints out 18446744073709551615 as expected
    cin.get();
    return 0;
}

// GetValueFromASM.asm

.code
GetValueFromASM proc
    mov rax, 0
    mov rax, rcx
    ret
GetValueFromASM endp
end