GATE CS 2005 Q66 — Machine Instructions and Addressing Modes

To find the most natural addressing mode for each high-level language statement, we analyze how each operation is translated into machine instructions:

1. Statement (1): $A[I] = B[J]$
   This statement accesses elements of arrays $A$ and $B$ using indices $I$ and $J$. In assembly, the effective address of an array element is computed as $\text{Base Address} + \text{Index}$. This is the definition of Indexed addressing.
   $\therefore (1) \rightarrow (b)$


2. Statement (2): while (*A++);
   This statement dereferences the pointer $A$ and then increments the pointer to point to the next memory location. This sequential access and update operation is directly supported by the Auto-increment addressing mode, where the register holding the address is automatically incremented after the memory access.
   $\therefore (2) \rightarrow (c)$


3. Statement (3): int temp = *x;
   Here, $x$ is a pointer variable that holds the address of the actual data. Accessing the value pointed to by $x$ (dereferencing) requires fetching the address from $x$ first and then accessing that address. This is Indirect addressing (specifically, register indirect if $x$ is in a register, or memory indirect if $x$ is in memory).
   $\therefore (3) \rightarrow (a)$

Thus, the correct matching is $(1, b), (2, c), (3, a)$, which corresponds to Option C.