GATE CS 1996 Q2.16 — Lexical Analysis

To determine which macro can cause an infinite loop during macro expansion, let us trace the expansion of both macros with an initial argument of $X = 0$.

1. Analysis of Macro (i)

Let us call M1 0 (i.e., $X = 0$):

• The condition .IF EQ, X is evaluated. Since $X = 0$, this condition is true.
• The macro recursively calls M1 X + 1, which evaluates to M1 1.
• In the expansion of M1 1 (where $X = 1$):
  • .IF EQ, X is false (since $1 \neq 0$).
  • .IF NE, X is true (since $1 \neq 0$), which generates .WORD 1.
  • The expansion of M1 1 terminates.
• Returning to M1 0, the next block .IF NE, X is false (since $X = 0$).
• The expansion of M1 0 terminates successfully.

Since the recursion has a well-defined base case that increments $X$ to break the condition, Macro (i) does not cause an infinite loop.

2. Analysis of Macro (ii)

Let us call M2 0 (i.e., $X = 0$):

• The condition .IF EQ, X is evaluated. Since $X = 0$, this condition is true.
• The macro recursively calls M2 X, which is M2 0.
• This recursive call M2 0 again finds $X = 0$, evaluates the EQ condition to true, and calls M2 0 again.

This creates an infinite recursion of M2 0 → M2 0 → M2 0 → ..., which puts the macro assembler into an infinite loop.

Thus, only macro (ii) can put the macro assembler into an infinite loop.