CY74FCT157CT,pdf(Quad 2-Input

The \x92FCT157T devices are quad two-input multiplexers that select four bits of data from two sources under the control of a common data-select (S) input. The output-enable (E\) input is active low. When E\ is high, all of the outputs (Y) are forced low, regardless of all other input conditions.

Moving data from two groups of registers to four common output buses is a common use of the \x92FCT157T devices. The state of S determines the particular register from which the data comes. It also can be used as a function generator. These devices are useful for implementing highly irregular logic by generating any 4 of the 16 different functions of 2 variables, with 1 variable common.

The \x92FCT157T devices are logic implementations of a four-pole, two-position switch, where the position of the switch is determined by the logic levels at S.