# 4-bit Parallel Adder -> Subtractor

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

1. ### ishanz Thread Starter New Member

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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

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Feb 4, 2008
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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

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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

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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

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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

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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

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