Question

1) State the steps that the SUBB instruction will go through when subtracting 23H – 12H

2) Show the code required to perform 77 x 34 and then show the code required to perform 77 divided by 3

Answer 1

1)CLR C    ;make CY=0 as here we are not using borrow.

MOV A,#23H ;load 23H into accumulator.

MOV R3,#12H ;load 12H into R3.

SUBB A,R3 ;subtract 23H from 12H.

Steps followed in subtraction:

in microprocessor like 8051 subb is performed by 2's compliment method instead of having a separate subtracter circuit it will use the adder circuit so 2's compliment method is inculcated prior to simple subtraction CY flag is made 0 and so we can summarize as take 2s compliment of subtrahend here 12 and add it to A here 23 and then invert the carry provided that address mode is must and should supported.

2)Below is the code for multiplication

MOV A,#4DH ;hexadecimal of 77 is 4D and load this value into A.

MOV B,#03H ;Load 3 into B.

MUL AB ;multiply 77 and 3 and lower significant bits are stored in A and if exceeds upper significant bits are stored in B

Below is the code for division

MOV A,#4DH ;hexadecimal of 77 is 4D and load this value into A.

MOV B,#03H ;Load 3 into B.

DIV AB ;divide 77 and 3 quotient is in A and remainder is in B.