# 4-bit Parallel Adder -> Subtractor

#### ishanz

Joined May 22, 2008
5 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?

#### mik3

Joined Feb 4, 2008
4,843 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?
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.

#### ishanz

Joined May 22, 2008
5
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?

#### Caveman

Joined Apr 15, 2008
471
Hint:
A - B = A + 2's complement of B
2's complement of B = B' + 1

#### mik3

Joined Feb 4, 2008
4,843
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?
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.

#### ishanz

Joined May 22, 2008
5
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.

#### mik3

Joined Feb 4, 2008
4,843
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.
yes the carry in is the 1 we add n the 2's complement.