I present to you my Kogge-Stone adder! Now, I love pistons and instant ripple-carry as much as anyone. In fact, the vast majority of my creations have relied on pistons for speed - and I've made some very fast circuits. The thing is, I thought it would be a lot of fun to start playing with some more complex logic and build a real life fast adder while taking a break from pistons.



You might have heard the terms "Kogge-Stone" or "carry-lookahead" before, and while the circuitry sure is convoluted, the concept isn't too hard to understand. In a ripple carry adder the carry is calculated as the sum is calculated, so you can't calculate the carry or sum of a bit until the carry of the previous bit has been calculated. A carry-lookahead adder produces the carry of each bit before the sum, so you have much less waiting to do. The Kogge-Stone Adder is a type of carry-lookahead adder with reduced fan-out (fan-out refers to the number of connections between a logic gate output and other logic gate inputs), making it quite fast. The one I built has a slowest speed of 9 ticks - not to shabby for a torch-based adder!

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