Question

Comment and show the result and how the flag register (CF, PF, SF, ZF) is affected after following.

MOV AL, 05H

MOV BL, 0FH

MUL BL

Answer 1

IN FLAG REGISTER,

SIGN FLAG ( SF ) = 1 ( IF MOST SIGNIFICANT BIT  OF ALU RESULT IS 1 ),

ZERO FLAG ( ZF ) = 1 ( IF ALU RESULT IS EQUAL TO 00H),

PARITY FLAG ( PF ) = 1 ( IF RESULT HAS EVEN NUMBER OF 1'S),

CARRY FLAG ( CF ) = 1 ( IF ADDITION OF TWO NUMBER CAUSE A CARRY OUT OF MOST SIGNIFICANT (LEFTMOST) BITS ADDED )

( MOV AL, 05H ) WILL STORE O5H IN AL REGISTER

( MOV BL, 0FH ) WILL STORE OFH IN BL REGISTER

(MUL BL ) WILL PERFORM MULTIPLICATION OF AL AND BL REGISTER AND STORE THE RESULT BACK IN AL REGISTER.

AFTER PERFORMING THE CODE ( MUL BL ) , AL WILL CONTAIN 4BH.

4BH CAN BE WRITTEN AS 01001011 IN HEXADECIMAL.

SINCE THE MOST SIGNIFICANT BIT IS 0, SO SF = 0.

SINCE THE RESULT IS NOT ZERO , SO ZF = 0.

SINCE RESULT CONTAIN ODD NUMBER OF 1'S, SO PF = 0.

SINCE NO CARRY OR BORROW IS DONE SO, CF = 0