GATE CS 2016 Set 1 Q36 — Runtime Environments

To find the output, we trace the execution step-by-step under pass-by-reference and dynamic scoping:

1. Initial State: Global variable $a = 3$.
2. Execution of main():
   • Calls m(a). Since parameters are passed by reference, the formal parameter y in m becomes an alias for the global variable a (value $3$).
3. Execution of m(y):
   • a = 1;: Under dynamic scoping, this assignment implicitly creates/binds a local variable a in the scope of m with an initial value of $1$.
   • a = y - a;: Updates m's local a to $\text{global } a - \text{local } a = 3 - 1 = 2$.
   • n(a);: Calls n passing m's local a by reference. Thus, the formal parameter x in n becomes an alias for m's local a.
4. Execution of n(x):
   • x = x * a;: Under dynamic scoping, the variable a is resolved by searching up the active call stack (n $\rightarrow$ m). It resolves to m's local variable a (value $2$).
   • Since x is a reference to m's local a, this statement evaluates to: $$\text{local } a = \text{local } a \times \text{local } a = 2 \times 2 = 4$$
   • print(x); prints $4$.
5. Return to m(y):
   • print(a); prints the current value of m's local a, which is now $4$.

Thus, the printed output is $4, 4$.