Authored by Matt Mumm, mummm2@unlv.nevada.edu

November 22, 2013

Lab description

For this project I will be creating a 8 input ALU that can AND, OR, ADD and SUBTRACT. 

The first part of this is to figure out how I can do all 4 functions at the same time and only select one of the results. 

So I need 3 different gates.  AND, OR and Full Adder. 

Subtraction is just addition of the negative value. A-B = A+(-B) This is where 2's compliment comes into use.  In order to subtract a value in binary, we invert the second number and then add 1.  So using the full adder, we can invert B and then set the carry in value as 1. 

With the 4 functions figured out, I now need to be able to select anyone of them.  With 4 options, I will need 2 control bits (F1 and F2).  

Using 2 muxs in series I can select my inputs/outputs.  F2 will control my first selection AND or OR.   F2 will also be the control bit if I want to invert the input for B into my full adder and if 1 gets added or not. 

F1 will control the final output between AND/OR and full adder.    

Below is my schematic for a 2 input ALU.    

Now that I have that, I can just cascade the 2 input ALUs eight times to get an 8 input ALU. 

Notice on the Carry ins, the first is from F2 (for the subtract feature) and the rest are from the previous carry outs.

A[7:0]=0000 1100 (12)

B[7:0]=0000 1010 (10)

Bi[7:0]=1111  0110  (+246,-10)

This is my layout for a 1 bit ALU.

 

Now I can cascade my 1 bit ALUs together to get an 8bit ALU.

My library file can be found here

More information can be found at cmosedu.com