8085 Instruction Set Questions and Answers

Answer: D

RAL rotates the accumulator one bit to the left through the carry flag: the most significant bit goes into CY, and the old CY goes into bit 0.

\[ \text{AAH} = 1010\ 1010,\quad CY = 0 \]

After the first RAL: the accumulator is \(0101\ 0100 = 54\text{H}\) and CY = 1.

After the second RAL: the accumulator is \(1010\ 1001 = \text{A9H}\) and CY = 0.

The value AAH would result from rotating by 8 positions through the carry, 9AH and 99H come from wrong rotations. The answer is A9H, option D.

Answer: B

In the 8085, the instruction LDA 2600H loads the accumulator with the contents of the memory location whose 16 bit address, 2600H, is given in the instruction itself. An instruction that carries the address of the operand in this way uses direct addressing.

In immediate addressing the operand itself is in the instruction (as in MVI A, 26H), in register addressing the operand is in a register (MOV A, B), and in register indirect addressing a register pair points to the memory location (as in MOV A, M, where HL holds the address). The answer is direct addressing, option B.

Answer: A

In the 8085, LXI (load register pair immediate) is the instruction that loads a 16 bit number into a register pair. The instruction LXI H, 2500H puts 25H in the H register and 00H in the L register, so the H-L pair holds 2500H.

MOV copies one 8 bit register into another (or to memory), so it cannot load the 16 bit value 2500H, and MOV H, 2500H is not a valid instruction. LXI L, 2500H is not valid either, because the register pair is named by its first register, H for the H-L pair. The answer is option A.