update

calling-convention/Output-explanation.md

@@ -21,4 +21,10 @@ Clean stack: []
 RS: 0
 Collected result: {'result': {'type': 'result', 'value': 42}}
 
-
+**Explanation:**
+Since there is only one argument and it fits in a single register, it is
+placed directly in `RA` (the first available argument register).
+Before the call, the 4 caller-saved registers (R0-R3) are pushed onto the
+stack to preserve their values. `RS` points to position 4, reflecting those
+4 entries. After `undo_function_call`, the stack is empty, `RS` returns to 0,
+and the result value `42` is successfully collected from `RA`.