2 bits Full adder-subtractor

Discussion in 'Homework Help' started by alexmath, Nov 22, 2014.

  1. alexmath

    Thread Starter New Member

    May 2, 2014
    17
    0
    Hi everyone! I've got 2 full adder-subtractors in front of me, both work, but in different ways. Both add in the same way, but when it comes to substraction they subtract different.

    My 2 bits full adder-subtractor is done exactly like this one

    where it says there is a problem with the borrow bit at the end.
    Here aswell tells us that there's a problem at the end. 12:30
    https://www.academia.edu/4020322/DE...OR_COMPOSITE_UNIT_USING_2_S_COMPLEMENT_METHOD

    And the second one, made by my roomate, works with no problem at all even at subtraction. The question is, why don't we use the second version? Take a look at his circuit below.

    [​IMG]

    when p is 0 ,subtracts, 1 adds
     
  2. alexmath

    Thread Starter New Member

    May 2, 2014
    17
    0
    As you can see here, mine should have a not borrow at the end.
    [​IMG]
    Mine works in C2, and his doesn't? What's wrong here?
     
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