A lot of the old parts used a design technique known as dynamic logic. A down side of dynamic logic was that the clock had to be running all of the time. If it did not run, then the charge on the internal dynamic nodes would bleed off, and the power consumption would greatly increase. Sometimes, the power increase would be enough to actually damage the part.

You can tell if you are dealing with dynamic logic when the specification has a minimum clock frequency.

Even though there were a lot of problems with dynamic logic, it did have a number of advantages. It provided twice the speed, half the operating power, and at half the cost of static logic.