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Another way to do signed comparisons (I think) is to simply invert the top bit and do an unsigned comparison. For the 3-bit example this is equivalent to adding 4 to both numbers which will shift the range from (-4,3) to (0,7).

So in hardware this amounts to routing one bit of the instruction to be XORed with the sign bits of the operands. You then get signed and unsigned comparison. It's these trivial hardware implementations that made 2's complement the standard way of doing things. using 2 gates to implement a variation on an instruction is awesome.

edit: it would not surprise me if there is an even simpler way to do it.

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