Description
Overview
All computer processors have a very important component known as the Arithmetic Logic Unit (ALU).
This component allows the computer to do, as the name suggests, arithmetic and logical operations. For
this assignment, you’re going to build an ALU of your own.
DO NOT USE TRANSISTORS!
1. Create a 1-bit full adder
2. Use your 1-bit full adder to create a 4-bit full adder
3. Use your 4-bit full adder and other components to construct a 4-bit ALU
4. Use your 4-bit ALU’s to make a 16-bit ALU
Requirements
You may use anything from the Base and Wiring sections, basic gates (AND, OR, XOR, NOT, NAND,
NOR, XNOR), multiplexers, and decoders. Use of anything not listed above will result in heavy
deductions. Your designs for the first three problems must each be a sub-circuit.
More information on sub-circuits is given below
Use tunnels where necessary to make your designs more readable
Sub-circuit tutorial
As you build circuits that are more and more sophisticated, you will want to build smaller circuits that
you can use multiple times within larger circuits. In Logisim, this is called a sub-circuit. Sub-circuits
behave like classes in Object-Oriented languages. Any changes made in the design of a sub-circuit are
automatically reflected wherever it is used. The direction of the IO pins in the sub-circuit correspond to
their locations on the representation of the sub-circuit.
To create a sub-circuit:
1. Go to the “Project” menu and choose “Add Circuit…”
2. Name your sub-circuit
To use a sub-circuit:
1. Click the sub-circuit you want to use from the sidebar.
2. Place it in your design.
To set a sub-circuit as the main circuit:
1. Right-click the sub-circuit and choose “Set As Main Circuit”.
Part 1: 1-bit Full Adder
The full adder has three 1-bit inputs (A, B, and CarryIn), and two 1-bit outputs (Answer and CarryOut).
The full adder adds A+B+CarryIn and places the answer in Answer and the carry-out in CarryOut.
For example:
A = 0, B = 1, CarryIn = 0 then Answer = 1, CarryOut = 0
A = 1, B = 0, CarryIn = 1 then Answer = 0, CarryOut = 1
A = 1, B = 1, CarryIn = 1 then Answer = 1, CarryOut = 1
Hint: making a truth table of the inputs will help you
Make your 1-bit adder a sub-circuit. You will use it in part 3.
Part 2: 4-bit Full Adder
For this part of the assignment, you will daisy-chain together 4 of your 1-bit full adders together in order
to make a 4-bit full adder.
This circuit should have two 4-bit inputs (A and B) for the numbers you’re adding, and one 1-bit input
for CarryIn. Note that the CarryIn can be used for more than just CarryIn, which will come in handy in
the next two parts.
There should be one 4-bit output for the answer and one 1-bit output for CarryOut.
Make your 4-bit full adder a sub-circuit; you will use it in Part 4.
Part 3: 4-bit ALU
Now you will use your 4-bit adder to make a 1-bit ALU capable of the following operations:
1. Addition [A + B]
2. Subtraction [A – B]
3. Increment [A + 1]
4. Negation [-A]
5. AND [A & B]
6. OR [A | B]
7. NOT [~A]
8. XOR [A ^ B]
Notice that Increment, Negate, and NOT only operate on the A input
This ALU has two 4-bit inputs for A and B, one 1-bit CarryIn, and three 1-bit inputs for S0, S1, and S2
(the selectors for the op-code of your ALU’s functions)
This ALU should have one 4-bit output for the answer, and one 1-bit CarryOut.
You may assign the op-codes to the operations any way that you want as long as you implement every
operation and each op-code only corresponds to one operation.
Add a label to your circuit that lists which operation each op-code
corresponds to.
Part 4: 16-bit ALU
Daisy-chain 4 of your 4-bit full ALUs with some other circuitry to create a 16-bit ALU with the
following operations (same as before):
1. Addition [A + B]
2. Subtraction [A – B]
3. Increment [A + 1]
4. Negation [-A]
5. AND [A & B]
6. OR [A | B]
7. NOT [~A]
8. XOR [A ^ B]
Notice that Increment, Negate, and NOT only operate on the A input
This ALU has two 16-bit inputs for A and B and three 1-bit inputs for S0, S1, and S2 (the selectors for
the op-code of your ALU’s functions)
This ALU should have one 16-bit output for the answer.
You may assign the op-codes to the operations any way that you want as long as you implement every
operation and each op-code only corresponds to one operation.
Add a label to your circuit that lists which operation each op-code corresponds to.
Also, set this sub-circuit as the main circuit.
Deliverables
Save the file as hw3.circ and turn it in through T-Square
Once again, your designs for the four problems must be contained in the same .circ file as subcircuits
You may also include a README file if there is anything you wish your grading TA to know about your
designs. This would be a good place to discuss your choice of op-codes or other concerns.
