8085 Microprocessor Instructions: Accumulator Modification

The question asks us to identify which instruction among the given options in an 8085 microprocessor will change the content of the accumulator. The accumulator is a very important 8-bit register in the 8085 that is used for most arithmetic and logical operations.

Let's analyze each instruction option to determine its effect on the accumulator.

MOV Instruction (MOV B, M)

• The MOV B, M instruction is a data transfer instruction.
• It moves the content of a memory location (M) into register B.
• The memory location (M) is pointed to by the HL register pair. That is, the address in the H and L registers is used to fetch data from memory.
• This instruction only affects register B and the memory location from which data is read.
• It does not change the content of the accumulator.

PCHL Instruction

• The PCHL instruction is a control transfer instruction.
• It transfers the content of the H and L registers into the Program Counter (PC). Specifically, L's content goes to the lower 8 bits of PC, and H's content goes to the upper 8 bits of PC.
• This effectively changes the next instruction to be executed by the microprocessor, as the PC holds the address of the next instruction.
• It does not change the content of the accumulator.

RNZ Instruction

• The RNZ instruction is a conditional return instruction.
• It means "Return if Not Zero".
• This instruction is executed after a subroutine call. If the Zero flag (Z flag) in the Flag Register is 0 (meaning the previous operation resulted in a non-zero value), then the program control returns to the main program by popping the return address from the stack into the Program Counter.
• If the Zero flag is 1, the return operation is skipped, and the program continues with the next instruction in the subroutine.
• This instruction only affects the Program Counter and the stack pointer (during the pop operation).
• It does not change the content of the accumulator.

SBI Instruction (SBI BEH)

• The SBI instruction stands for "Subtract Immediate with Borrow".
• It is an arithmetic instruction that performs subtraction.
• The instruction SBI BEH means to subtract the immediate 8-bit data BEH (where H denotes hexadecimal) from the content of the accumulator, and also subtract the content of the Carry flag (CY) from the result.
• The operation can be represented as: $\text{Accumulator} \leftarrow \text{Accumulator} - \text{Immediate Data} - \text{Carry Flag}$
• After the operation, the result is stored back into the accumulator.
• Therefore, this instruction directly modifies and changes the content of the accumulator.

Based on the analysis, only the SBI BEH instruction directly performs an operation that updates the content of the accumulator.