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I've started a logic design class where there is a chapter on binary codes and their computing (addition and subtraction). While I grasp easily the representation of negative values using sign-magnitude, one's complement, and two's complement, I'm confused about the excess-$N$ one. I've been on Wikipedia and all but I seem not to get it.

Can anyone please explain to me using examples for, let's say, excess-3 and excess-8?

There is also the value of the magic number; in my book it's $2^{(N-1)}$, while on the net I can find $2^{(N+1)}$.

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