Question

The contents of register BL and register AL of 8085 microprocessor are 49H and 3AH, respectively. The contents of AL, the status of carry flag (CF), and sign flag (SF) after executing the instruction SUB AL, BL are:

• A. AL = F1H; CF = 1; SF = 1
• B. AL = 0FH; CF = 1; SF = 1
• C. AL = F0H; CF = 0; SF = 0
• D. AL = 1FH; CF = 1; SF = 1

Hint:

In 8085 subtraction, check for negative results by evaluating the carry (borrow) and sign (MSB) flags.

Answer 1

The Correct Option is A

The instruction SUB AL, BL performs the operation AL = AL − BL. In this case: AL = 3AH, BL = 49H. Performing the subtraction: 3AH − 49H = −0F H. In hexadecimal, negative results are represented in two’s complement form: −0F H = F1H. Carry Flag (CF): Since the result of the subtraction is negative, a borrow occurs, setting CF = 1. Sign Flag (SF): The most significant bit (MSB) of the result is 1, indicating a negative number, so SF = 1.