COSC 201: Computer Organization

Lab 5: Using the Logisim Simulator

Purpose: Lear how to use the Logisim simulator.
Method: Complete the circuit for an 8-bit adder and verify it's operation using the simulator.
Preparation: Read Appendix B in the text.
Files to Use: add.sim
What to Hand In: Completed file add.sim

In this lab, you will learn how to use the Logisim simulator by completing the circuit for an 8-bit adder. Use the logisim command to start the simulator. Take some time to explore the information available in the Help menu. When you begin work, be sure to select "Advanced Tools" in the "Options" menu in order to work with the subcircuits in the add.sim project file.

C_i	A	B	C_o	S
0	0	0	0	0
0	0	1	0	1
0	1	0	0	1
0	1	1	1	0
1	0	0	0	1
1	0	1	1	0
1	1	0	1	0
1	1	1	1	1

Using the truth table for a full adder, we see that the equation for the sum output (S) in sum-of-products form is:

S = (C_i·A·B) + (C_i·A·B) + (C_i·A·B) + (C_i·A·B)

The "sum" subcircuit in the file add.sim is a direct implementation of this equation.

Using the truth table, write the sum-of-products equation for the carry output (C_o) and complete the "carry" subcircuit in the file add.sim. Change the values of the input switches to verify that your circuit implements the truth table accurately.

Observe how the "sum" and "carry" subcircuits are combined in the "add" subcircuit to complete the 1-bit adder.

Complete the "8-bit add" subcircuit by creating 8 instances of the "add" subcircuit and making appropriate connections. Change the values of the input switches in the "main" subcircuit to test your 8-bit adder.

Hand in your file add.sim by placing a copy in your ~/courses/201/assignments directory, and sending e-mail to your instructor. Be sure that your name appears in a the comment on the "main" subcircuit of the file that you hand in.