# Adder and substrate with signed number

#### Ahahah

Joined Dec 9, 2021
3
I need help in making adder and substrate 3bit signed number
Any ideas ?

#### Papabravo

Joined Feb 24, 2006
19,250
The basic circuits are called a Half Adder and a Full Adder.
The Half-Adder takes two input bits and produces a sum and a carry. So it is a Boolean function with two inputs and two outputs.
The Full-Adder takes three inputs, a carry in and two input bits. It produces a sum and a carry. So it is a Boolean function with three inputs and two outputs.
Once you have those two function blocks, the rest is done with input multiplexers.

#### Ahahah

Joined Dec 9, 2021
3
The Half-Adder takes two input bits and produces a sum and a carry. So it is a Boolean function with two inputs and two outputs.
The Full-Adder takes three inputs, a carry in and two input bits. It produces a sum and a carry. So it is a Boolean function with three inputs and two outputs.
Once you have those the rest is don with input multiplexers.
I tried but it give wrong answer in -3 minus -1 and -1 minus -3

#### Papabravo

Joined Feb 24, 2006
19,250
I tried but it give wrong answer in -3 minus -1 and -1 minus -3
Show me.

#### Ahahah

Joined Dec 9, 2021
3
Sorry ,I don't have laptop now
But I tried to think it with comparator
If it possible to make it on logisim as I have it as assignment and deadline is near
Feel free to help me when you have free time

#### Papabravo

Joined Feb 24, 2006
19,250