Question regarding function cvu64 in minix3.1

[LeoHaveFun123]

I am not sure if this is the correct place to ask this question, I am reading minix3 source code these days but I cannot understand the asm code of this function. The original code is as below, this function is used to covert the input 32bit unsigned value to 64 bit.

_cvu64: ! u64_t cvu64(unsigned i);
_cvul64: ! u64_t cvul64(unsigned long i);
mov eax, 4(esp) !this line is saving input paramter to eax, which is i
mov edx, 8(esp) !this line saves something above i which is something unknown on caller stackframe? Why???
mov (eax), edx !move unknown value to address pointed by i??
mov 4(eax), 0
ret

I have added my understanding in above comments, I may have made mistakes but I dont know how. Thanks for any help in advance.