GATE CS 2007 Q71 — I/O Interface: Interrupt and DMA Mode

To find the number of memory references for accessing the data, we only count the memory read and write operations performed on data operands (excluding instruction fetches).

1. Instruction Outside the Loop

• MOV R1, (3000): This instruction reads the data from memory location 3000 and loads it into register R1.
  Data memory references = 1 (Read operation).

After this instruction, the value in register R1 becomes 10.

2. Loop Execution

The loop starts at the label LOOP and ends with BNZ LOOP. Since R1 is initialized to 10 and decremented by 1 in each iteration (DEC R1), the loop will execute exactly 10 times.

Let's analyze the data memory references inside one iteration of the loop:

1. MOV R2, (R3): Reads data from the memory location pointed to by R3.
   Data memory references = 1 (Read operation).
2. ADD R2, R1: Register-to-register operation.
   Data memory references = 0.
3. MOV (R3), R2: Writes data to the memory location pointed to by R3.
   Data memory references = 1 (Write operation).
4. INC R3: Register operation.
   Data memory references = 0.
5. DEC R1: Register operation.
   Data memory references = 0.
6. BNZ LOOP: Branch instruction (control transfer).
   Data memory references = 0.

Thus, each iteration of the loop performs exactly $1 + 1 = 2$ data memory references.

3. Total Data Memory References

$$\begin{align*} \text{Total References} &= \text{References outside loop} + (\text{References per iteration} \times \text{Number of iterations}) \\ &= 1 + (2 \times 10) \\ &= 21 \end{align*}$$

Therefore, the total number of memory references for accessing the data is 21.