John entered his four digit RPN calculator into the Open 7400 Logic Competition. This is the fist of two entries he made while attending “Digital Hardware Lab” courses at the University of Utah. It is a 4 digit RPN calculator that can add and subtract, designed entirely out of logic ICs.

The first few weeks of the first class were spent doing simple labs, mostly to teach students how logic worked, how to use the tools, how to debug, and how to write up your designs. Then you launched into the meat of the class: Design a four digit, stack-based (a la HP RPN) calculator that could add and subtract. Sounds simple, right? Well, remember you’re doing everything with gates and latches. Scanning and debouncing the keyboard matrix, multiplexing the LED digits, managing the stack and computations. By the end of the quarter you had filled three boards with chips.

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If this 4-digit RPN calc cannot multiply or subtract, then it is not a calculator but just a adder-subtractor. Can it perform floating point arithmetic?