GATE CS 1988 Q2xv — Runtime Environments

To find the output of the program under the call-by-reference parameter passing method, we trace the execution step-by-step:

1. Initial State

Before calling procedure P, the variables in the main program are initialized as:

• a = 2
• b = 3

2. Parameter Binding in Call-by-Reference

The procedure call is p(a+b, a, a) with formal parameters (x, y, z):

• For expression a+b: Since an expression does not have an l-value (memory address), the compiler evaluates the expression at the time of the call, stores the result in a temporary location, and passes the reference of this temporary location to x.
  $$\text{temp} = a + b = 2 + 3 = 5$$ Thus, x references temp (value is $5$).
• For variable a: Both y and z receive the reference (address) of the variable a. This creates an alias, meaning any modification to y or z directly affects a.

3. Execution of Procedure P

• Statement 1: y := y + 1

  Since y references a, this is equivalent to:

  $$a := a + 1 \implies a := 2 + 1 = 3$$
• Statement 2: z := x + x

  Since z references a and x references temp (which holds $5$):

  $$a := \text{temp} + \text{temp} \implies a := 5 + 5 = 10$$

4. Output

After returning from the procedure, the main program executes Write(a). The value of a is now 10.