While only one of these upper half adders will be correct, the computation time has been cut in half. Such an adder is called a carry select adder . Three of these operations are found in the upper left hand corner. It also contains the carries so that several of these circuits can be wired together to do full-word operations. This type of circuit is called a bit slice.
Many designers use clocks to provide synchronization of events.
A clock is a circuit that emits a series of pulses with a determined width and interval. The clock cycle time is the time interval between two edges of a pulse. For purposes of precision, a crystal oscillator often controls clock frequency. Many events can occur within one clock cycle. For this reason, a single clock cycle must be broken down into subcycles.