4-bit Parallel Adder -> Subtractor

Discussion in 'Homework Help' started by ishanz, May 26, 2008.

  1. ishanz

    Thread Starter New Member

    May 22, 2008
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    [​IMG]

    I just drew this up real quickly in Photoshop. So the objective is to use the IC and four inverters to create a subtractor circuit that'll do the following operation:

    A4 A3 A2 A1 - B4 B3 B2 B1

    So would I just invert all the B's on the circuit?
     
  2. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    Connect the outputs of 4 XOR gates to B1-B4. Connect one of each XOR's gate input to C1. The other four inputs on the XOR gates (one input per gate) are your new inputs for B1-B4. When you apply high logic to Cin the circuit will subtract A-B , when you apply low logic to Cin the circuit will add A+B.
     
  3. ishanz

    Thread Starter New Member

    May 22, 2008
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    Is there a way to just do it with inverters? The worksheet we were given only says "Use the IC below and 4 inverters to subtract..."

    I was trying to figure out how on earth to do the 2's complement method with this, are using the XOR gates the only way?
     
  4. Caveman

    Active Member

    Apr 15, 2008
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    Hint:
    A - B = A + 2's complement of B
    2's complement of B = B' + 1
     
  5. mik3

    Senior Member

    Feb 4, 2008
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    Ok then. Connect 4 NOT gates on the B inputs and apply a high logic to Cin input.

    With this modification you invert all the bits and add 1 as the 2's complements requires. The circuit will do the function A-B.
     
  6. ishanz

    Thread Starter New Member

    May 22, 2008
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    So the carry-in high is basically the 1 that is added when a 2's complement is performed?

    I believe I understand now, thanks a bunch to the both of you! Hopefully I'll be ready for the exam come tomorrow.
     
  7. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    yes the carry in is the 1 we add n the 2's complement.
     
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