4-bit Calculator Project in Logisim-evolution

A 4-bit calculator ties together three things you have already met: adders, multiplexers, and displays. The result is a complete mini-project you can demo: two 4-bit inputs, an operation switch, and the answer on hex displays.

Architecture

1. Inputs: two 4-bit input pins, A and B, plus a 1-bit SUB select signal.
2. Add/subtract core: build it from the 4-bit adder technique — XOR each B bit with SUB and feed SUB into the carry-in. When SUB=0 you get A+B; when SUB=1 you get A−B (two’s complement). The carry-out doubles as an overflow/borrow flag.
3. Operation select (optional stretch goal): add a second unit (for example bitwise AND) and use a multiplexer to choose which result reaches the display.
4. Display: split the 4-bit result to a Hex Digit Display, and show the carry/borrow on an LED. Negative results display in two’s complement — note that on your lab report.

Make it a proper project

• Wrap the adder core in a subcircuit so the top level stays readable: inputs on the left, display on the right.
• Add a truth table test: 0+0, 15+1 (watch the carry), 5−5, 0−1. Record each in your report with screenshots.
• Extension ideas: a 7-segment output via the BCD-to-7-segment decoder, or chaining two calculators into an 8-bit version.