Question 11

The struct student is stored in memory starting at a base address held in register R1:

struct data {
    int marks[100];
    char grade;
    int cnumber;
};

The field student.grade is located at a fixed byte offset from the start of the struct. For example, if an integer is 4 bytes, marks[100] occupies $100 \times 4 = 400$ bytes, so grade is located at offset $X = 400$ relative to R1.

To access memory at an address computed by adding a constant displacement $X$ to a base register R1 without modifying R1, the processor uses index addressing mode (or base-displacement addressing mode), denoted $X(\text{R1})$.

Post-increment and pre-decrement modes modify the register R1 itself, which is not desirable for accessing a fixed struct field. Register direct mode accesses only the register contents without referencing memory. Therefore, index addressing mode $X(\text{R1})$ is the appropriate and efficient mode.