Latches
Combinational logic does not directly permit the storage of data as is needed for digital functions such as processor registers. Logic gates can be used to create data storage elements by using feedback from a gate output to the input of a gate preceding that point in the signal chain.
A latch is a single-bit memory device constructed from logic gates. Figure 2.10 shows a simple type of latch called the Set-Reset, or SR, latch. The feature that provides memory in this circuit is the feedback from the output of the AND gate to the input of the OR gate:
Figure 2.10: SR latch circuit
Based on the inputs S and R, the circuit can either set the output Q to high, reset Q to low, or cause the output Q to be held at its last value. In the hold state, both S and R are low, and the state of the output Q is retained. Pulsing S high (going from low to high then back to low) causes the output Q to go high and remain at that level. Pulsing R high causes Q to go low and...
