CS201 Lab: Design Adders & Subtractors

Purpose

	Arithmetic circuits play a central role in the operation of a microprocessor.
	This week we will look at how basic arithmetic circuits are built.

Half-Adder

    A Half-adder is a combinational circuit that performs the addition of 2 bits.

	Inputs: 	2 input bits to be added 

	Outputs: 	sum bit (least significant) 
			carry bit (most significant)  

    For example: 

	1 + 1 = 1 0 		c = 1;  s = 0

	1 + 0 = 0 1		c = 0;  s = 1
      

Full-Adder

    A Full-adder is a combinational circuit that forms the arithmetic sum of 3 input bits.

	Inputs: 	2 input bits to be added (x, y) 
			1 carry bit from previous lower significant position (z)  

	Outputs: 	sum bit (least significant) 
			carry bit (most significant) 

    For example: 

	1 + 1 + 1 = 1 1 	c = 1;  s = 1

	1 + 0 + 1 = 1 0		c = 1;  s = 0
	      

    Note: it takes 2 half-adders to make a full adder.

Assignments