The MUX is a common circuit, and has been encapsulated into a single chip, the 74153 dual 4-to-1 data selector/multiplexer. This chip implements two multiplexers which share the two select lines. This section will use the 74153 chip to implement a circuit to mirror the input on/off switches. This circuit could easily be implemented by simply connecting the switches directly to the LED, as in Figure 3-11 for one switch and LED. The output is the same as in exercise 3-2. The reason this circuit is more interesting than the ones in Chapter 3 is that it shows how a MUX can be used to store and retrieve data values, and how those data values can represent a program.