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

Addressing modes specify how the operand of an instruction is located. Let's analyze each instruction option, typically for a microprocessor like the 8085 (which uses similar mnemonics). 1. LDA 2700H: \textbullet \ Description: Load Accumulator Direct. This instruction loads the content of the memory location whose 16-bit address is 2700H into the Accumulator (A). \textbullet \ Addressing Mode: This is an Direct Addressing Mode. The effective address of the operand is directly specified in the instruction itself. 2. ADI 36H: \textbullet \ Description: Add Immediate to Accumulator. This instruction adds the 8-bit immediate data 36H to the content of the Accumulator (A). The result is stored in A. \textbullet \ Addressing Mode: This is an Immediate Addressing Mode. The operand data itself is part of the instruction. 3. DAA: \textbullet \ Description: Decimal Adjust Accumulator. This instruction adjusts the content of the Accumulator after a binary addition to make it a valid BCD (Binary Coded Decimal) number. It does not involve any external memory access or register content as an address. \textbullet \ Addressing Mode: This is an Implied (or Implicit) Addressing Mode. The operand (Accumulator) is implicitly specified by the instruction itself; no explicit address or register is provided. 4. LDAX B: \textbullet \ Description: Load Accumulator Indirect (using B-C pair). This instruction loads the content of the memory location pointed to by the 16-bit contents of the B-C register pair into the Accumulator (A). \textbullet \ Addressing Mode: This is a Register-Indirect Addressing Mode. The effective address of the operand is contained in a register pair (here, BC). The instruction does not directly specify the memory address, but rather the register that holds the address. Therefore, \( \text{LDAX B} \) is an example of a register-indirect addressing mode.