Question

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

• A. LDA 2700H
• B. ADI 36H
• C. DAA
• D. LDAX B

Hint:

In 8085 assembly language, instructions that use the character 'X' (like ‘LDAX‘, ‘STAX‘) or explicitly reference the memory symbol 'M' (like ‘MOV A, M‘, ‘ADD M‘) typically utilize Register-Indirect Addressing Mode. The symbol 'M' tells the processor to look at the address stored in the HL register pair.

Answer 1

The Correct Option is D

Concept: In microprocessor architectures (such as the 8085), addressing modes define how an instruction specifies the location of the operand it needs to process. In Register-Indirect Addressing Mode, the instruction does not specify the memory address directly. Instead, it names a register pair that holds the target 16-bit memory address where the operand is stored.

Step 1: Detailed analysis of the correct option “LDAX B”
The mnemonic ‘LDAX B‘ stands for "Load Accumulator Indirect from Register Pair BC".
• The operand's location is stored inside the $BC$ register pair.
• The processor reads the 16-bit address held in the $BC$ register pair, accesses that memory location, reads the byte stored there, and copies it into the Accumulator ($A$).
• Because the address is retrieved indirectly via a register pair, this is a classic example of register-indirect addressing.

Step 2: Disproving alternative choices

• LDA 2700H: ("Load Accumulator Direct"). The explicit 16-bit memory address ($2700\text{H}$) is written directly in the instruction code. This is Direct Addressing Mode.
• ADI 36H: ("Add Immediate to Accumulator"). The actual data byte ($36\text{H}$) is provided directly inside the instruction. This is Immediate Addressing Mode.
• DAA: ("Decimal Adjust Accumulator"). This instruction operates directly on a fixed register (the Accumulator) implied by the command itself, without needing external memory addresses. This is Implied/Implicit Addressing Mode.