A control line M holds a binary value of either 0 or 1 which determines the operation is carried out whether addition or subtraction.

The RESULT will be A-B for signed number, for unsigned number if A>=B.

For unsigned A<B the result will be 2's complement of B-A i.e. , first calculate B+A`+1 and then take 2's complement of the result of B+A`+1.

        suppose A=6, B=7  Now , A=110 and B=111 as it is clear that A<B so 

                                                      B+A`+1=111+001+1

                                                                    =1000+1

                                                                     =1001

                                                      2's complement of 1001 is (1001)`+1  i.e. , 0110+1=0111