Build a Subtractor and Comparator using JLS

Extend your signed adder from the previous project to build

• a circuit than can both add and subtract, and
• a circuit that can compare signed and unsigned integers.

Pre-requisites

You will need your signed added from the previous project.

Task 1: Adder/Subtractor

Build a circuit that can do both addition and subtraction. Specifically, surround your adder with additional gates so that it performs both addition and subtraction. This circuit should contain one adder only. Be sure to ask for clarification if you don't understand this requirement.

Your inputs must be named InputA, InputB, and Op. If not, your circuit won't work with my tests. The Op input specifies whether to perform addition or subtraction. (1 means subtract, 0 means add).

Your outputs must be named Output, CarryOut, and Overflow. CarryOut is the carry out from the last full adder. It is also, in effect, the overflow indicator for unsigned addition. Unsigned subtraction cannot overflow. (Subtraction that generates a negative result is not considered "overflow".) Overflow is the overflow indicator for singed operations. Remember, CarryOut and Overflow will not necessarily have the same value.

You may use this test file (SampleSigned16BitAddSubtractTest.java) as a basis for your own tests. Warning! This test class is incomplete. Don't assume that a circuit that passes this sample test will pass all of my tests.

Task 2: Set if Less Than

Build a circuit that can compare two integers (either signed or unsigned).

Use your adder/subtractor as a starting point and leverage the fact that

• if A < B, then A - B < 0, and
• if A ≥ B, then A - B ≥ 0.

Be aware, however, that this algorithm works only as long as A - B doesn't overflow. You'll need to think about the conditions under which A - B might overflow, and use alternate techniques for comparing A and B in this case.

Your inputs must be named InputA, InputB, and Signed. If not, your circuit won't work with my tests. The Signed input specifies whether to perform a signed or unsigned comparison (1 means signed, 0 means unsigned).

Your output must be named LessThan

You may use this file (SampleSlt16BitTest.java) as a basis for your own tests. Warning! This test script is incomplete. Don't assume that a circuit that passes this sample test will pass all of my tests.

Rules

1. You are strongly encouraged to work in groups of two. You may work alone only with permission of the instructor.
2. The simulator comes with a built-in adder. You may not use the built-in adder.
3. To build the required circuits, you will probably find it useful to design and build several subcircuits. These subcircuits may have any interface you choose; however, you are responsible for testing them.
4. You must write all of your own chips. In other words, you may not share "internal" chips with your classmates, or download them off the Internet.
5. Be sure to test your components thoroughly. Just because it passes the test included here doesn't mean it will pass my test.
6. You may use the built-in mux component.

Hints and shortcuts

• Remember, JLS inserts the code for sub-circuits directly into the circuits into which they are imported. Thus, changes to sub-circuits are not seen by the circuits that contain them. If you discover a bug in a sub-circuit, you must delete and re-import that sub-circuit into the parent. (Again, test your subcircuits thoroughly before importing them!)
• Remember that no software is perfect. This includes JLS and JLSCircuitTester. It is in your best interest to start early in case problems arise. Also, please e-mail me with any bugs you find or feature requests you have. If JLS crashes, please e-mail me the JLSerror file.
• Remember to test your slt circuit thoroughly. My sample test file will not catch several common mistakes.

Grading

This assignment will be scored out of 50 points.

Penalties

• You will lose 6 points for each unnecessary copy of an adder or adder/subtractor.
• You will lose 3 points for each buggy submission (-5 if the submission contains too few tests).

Deliverables

To submit your project:

1. Add a text box to your jls files that includes
   • your name,
   • the file name,
   • the assignment name, and
   • the assignment due date.
2. Either
   • Attach all relevant .jls files, bug reports, and test scripts using Assignment Manager. (Attaching your test scripts allows me to determine how well you tested your code, and assess partial credit if necessary.)
   • Add me (kurmasz) as a collaborator to your GitHub or BitBucket project and send me a pull request.