I am a 2nd year Computer Science and Engineering student from India ..
My Computer Organization sir gave this statement while teaching implementing parameter passing using stacks

mov Ro,20(SP)

To me it looks to be incorrect ! How can you move an item to any location other than the top of the stack?

SP is just a pointer to somewhere in memory; nothing enforces stack discipline.

If there's more code in the example than the one line you provided here, the rest of it might prove enlightening. Could you post anything else that came with the MOV instruction?