XOR can do everything in 1 cycle (which is hopefully far, far less than the clock). SUB-if done the simple way-has to take n cycles where n is the number of bits subtracted.

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anvuong 3 hours ago | parent | next [-]

That's just not true. You think subtracting/adding 64-bit numbers actually take 64 cycles?

There is sequential implementation of ripple carry adder that uses clock and register, this will add 1-bit per cycle, but no body uses this for obvious reason, it's just a toy example for education. A normal ripple carry adder will have some delay in propagation time before the output is valid, but that is much less a clock cycle. You can also design a customized adder circuit for 4-bit 8-bit 16-bit etc separately that would greatly minimizes the propagation delay to only 2 or 3 levels of gates, instead of n gates like in the ripple carry adder.

	▲	valleyer 3 hours ago \| parent [-]
		Right. In other words, the clock cycle is already made to be long enough to allow a word-sized SUB to settle. An XOR-with-self surely settles faster, but it still has to wait for that same clock cycle before proceeding.

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jonathrg 3 hours ago | parent | prev [-]

What do you mean by cycles? A ripple-carry adder needs to wait for the carry bits to ripple through yes, but there's no clock cycle involved.

	▲	toast0 an hour ago \| parent [-]
		Maybe they mean gate delays?