The ’HC157, ’HCT157, ’HC158, and ’HCT158 are quad 2-input multiplexers which select four bits of data from two sources under the control of a common Select input (S). The Enable input (E\) is active Low. When (E\) is High, all of the outputs in the 158, the inverting type, (1Y\-4Y\) are forced High and in the 157, the non-inverting type, all of the outputs (1Y\-4Y\) are forced Low, regardless of all other input conditions.
Moving data from two groups of registers to four common output busses is a common use of these devices. The state of the Select input determines the particular register from which the data comes. They can also be used as function generators.