Question

Which of the following is a register-indirect addressing mode in instruction set?

• A. \( \text{LDA 2700H} \)
• B. \( \text{ADI 36H} \)
• C. \( \text{DAA} \)
• D. \( \text{LDAX B} \)

Hint:

Understanding addressing modes is crucial for assembly language programming. \textbullet \ Direct Address is specified directly in the instruction. \textbullet \ Immediate The data itself is part of the instruction. \textbullet \ Register Operand is directly in a CPU register. \textbullet \ Register-Indirect A register or register pair holds the memory address of the operand. \textbullet \ Implied/Implicit The operand is implicitly defined by the instruction (e.g., Accumulator for DAA).

Answer 1

The Correct Option is D

To solve this problem, let's dive into the different types of addressing modes in instruction sets and how they work.

1. What is Addressing Mode?

In computer architecture, an addressing mode is a method used to specify where the operand (the data to be operated on) of an instruction is located. The operand can be in memory, a register, or an immediate value. Different types of addressing modes allow different ways of accessing operands during instruction execution.

2. Types of Addressing Modes:

Some common addressing modes are:

• Immediate Addressing: The operand is specified directly in the instruction itself.
• Register Addressing: The operand is located in a register, and the instruction specifies the register.
• Direct Addressing: The operand is located at a specific memory address, and the address is given directly in the instruction.
• Indirect Addressing: The operand is located at an address stored in a register or memory location, so the instruction specifies a register or memory location that contains the address of the operand.

3. Explanation of the Options:

- Option 1: "LDA 2700H" – This is incorrect. The "LDA" instruction uses direct addressing, where the operand (the data) is located at the specified memory address (2700H). This is not register-indirect addressing.

- Option 2: "ADI 36H" – This is incorrect. "ADI" uses immediate addressing mode. The operand 36H is provided directly in the instruction, and it is not a register-indirect addressing mode.

- Option 3: "DAA" – This is incorrect. The "DAA" (Decimal Adjust After Addition) instruction does not use any addressing mode since it operates on the accumulator and performs an internal adjustment based on the current value. This is not a register-indirect addressing mode.

- Option 4: "LDAX B" – This is correct. "LDAX" is a register-indirect instruction. It uses the contents of the specified register (in this case, register B) to find the address of the operand in memory. The operand is loaded into the accumulator from the address pointed to by the register, making it an example of register-indirect addressing.

4. Why This Matters:

Understanding addressing modes is crucial for working with machine-level instructions in assembly language. Different addressing modes provide flexibility in how operands are accessed, which allows for more efficient memory usage and instruction execution. Register-indirect addressing is particularly useful when accessing variables or data structures that are stored at dynamically determined memory addresses.

5. Final Answer:

The correct answer is "LDAX B."