Intel used an asynchronous technique in their Pentium-4 processors. You may recall that the internal core ALUs ran at 2x the frequency of the rest of the chip.

Intel used an asynchronous technique in their Pentium-4 processors. You may recall that the internal core ALUs ran at 2x the frequency of the rest of the chip. This was done with self timed domino circuits. These are notoriously difficult to get working.