Instead of going with silicon-based switching (ie: transistors) this 4-bit adder uses mechanical relays. We like it for its mess of wires (don’t miss the “assembly” page which is arguably ...

[Phillip] wanted to play with the C preprocessor. He decided to do that by creating a 4 bit full adder. We know this is pretty useless in everyday life, but it was a great learning experience.

To provide context, it shows where VHDL is used in the FPGA design flow. Then a simple example, a 4-bit comparator, is used as a first phrase in the language. VHDL rules and syntax are explained, ...