This 2-bit adder was a lot of work to build. It uses a total of thirty-six 555 timers and it does have the option of adding or subtracting numbers. It’s a rather unorthodox use of the part, d… ...

The yield on eight-bit registers, for example, was only 71 percent, and that dropped to only 7 percent for a 64-bit register (which required 1,152 transistors).