CS61C Homework 6

ALU in Logisim

Due:	21.03.2006 (little-endian!) @ 2359
HW TA:	David Eitan Poll <cs61c-tb at imail dot eecs dot berkeley dot edu>

Introduction

So, now you’ve learned about logic gates and combinational logic; it’s time to put that newfound knowledge to work!  I’m sure you’ll find it quite ALUminating!  This will add to your experience and shift your perspective on what we’ve been learning!

Assignment

In this assignment, you will implement an 8 bit ALU in logisim (the image above is for a 4 bit ALU).

Getting Started

Copy the homework 6 directory to your home folder with the following command:

	  % cp -r ~cs61c/hw/06 ~/hw

This will copy in alu.circ, the file you will submit your assignment in. You must use this file or points may be deducted for not following directions.

Functions

The eight functions that you will implement are: add, subtract, and, or, shift left logical, less than, shift right logical, and shift right arithmetic. The ALU will perform a desired function on two 8-bit inputs (X and Y, where x0 is the lowest order bit for x, etc...) and output the result (RESULT). The function will be determined by the value of a control signal (S), as listed below. In addition to the 8 bits of output provided in RESULT, two additional outputs will be provided: unsigned overflow, and signed overflow. Unsigned overflow will have a high value iff the command was an add and unsigned overflow occured. Signed overflow will have a high value iff the command was an add or a subtract, and signed overflow occured. (You need not worry about unsigned overflow for subtractions.)

Here's what occurs for each operation:

S	Op	Result
000	add	X + Y (signal if either type of overflow occurs)
001	sub	X - Y (signal if signed overflow occurs)
010	bitwise OR	X | Y
011	bitwise AND	X & Y
100	sll	X << Y (but logically, don't sign extend!)
101	signed less than	X < Y (return 1 if X < Y, otherwise 0)
110	srl	X >> Y (but logically, don't sign extend!)
111	sra	X >> Y (but arithmetically, do sign extend!)

But wait, there's more!

Think about intuitive ways to test your ALU. If the grader can look at your logisim file and immediately see that everything is in working order, it will be to your advantage. You might consider some method for automatically testing your ALU, such as a counter. Perhaps you should add testing logic that signals an error if an incorrect value is seen. Think about this, we want to see some form of testing suite in your outter-most circuit design.

Not Allowed On This Assignment

You may only use tools from the "Base" and "Gates" libraries in Logisim. Do not use any items from the Memory, Plexers, Arithmetic, or Legacy libraries. Any ALU pieces built using any of these illegal components will be given no credit.

Details & Hints

• Logisim can be run on instructional machines using the following unix command: logisim &

  If you are having difficulty using Logisim on nova (i.e. the GUI interface is unusable), ssh into cory and run logisim from there. You can ssh using the following command:

    % ssh cory

  Type in "yes" when it asks you something about a key, then enter in your password.

• You can download logisim for yourself as well as view more documentation at the logisim website: http://ozark.hendrix.edu/~burch/logisim/. It is recommended that you use the JAR version. The Windows and Mac OSX versions have been reported to be notoriously slower than the JAR file.
• LABEL EVERYTHING!
• Please use subcircuits! They make it easier for you, they make it easier for us. Give your subcircuits appropriate labels as well. Keep in mind that this won't be autograded, and humans will have to look at your schematics (and grade them!).
• The key to your sanity on this project is modularity. Don't be afraid to build subcircuits that are multiple levels deep.
• PLEASE use multi-bit buses in logisim.  We do not expect you to connect 8 separate wires everywhere.  You will have to use splitters to get individual bits from the wire, but I guarantee this will make you life SO much easier. If you are unsure how splitters work, refer back to Lab 9.

Submission Details

Create a directory named 'hw6' containing a README, alu.circ, and any additional files needed for your submission. While in that directory, run 'submit hw6'. If you included an image, make sure to say "yes" when the submit script prompts you about it.