For functions not fully prototyped, the caller will pass integer values as integers and floating-point values as double precision. For floating-point values only, both the integer register and the floating-point register will contain the float value in case the callee expects the value in the integer registers.

func1();  
func2() {   // RCX = 2, RDX = XMM1 = 1.0, and R8 = 7  
   func1(2, 1.0, 7);  
}  

Calling Convention