74F157A: Quad 2-input data selector/multiplexer, non-inverting
74F158A: Quad 2-input data selector/multiplexer, inverting

DESCRIPTION
The 74F157A is a high speed Quad 2-Input Multiplexer which selects 4 bits of data from one of two sources under the control of a common Select input (S). The Enable input (E) is active when Low. When E is High, all of the outputs (Yn) are forced Low regardless of all other input conditions. Moving data from two registers to a common output bus is a common use of the 74F157A. The state of the Select input determines the particular register from which the data comes.
The device is the logic implementation of a 4-pole, 2-position switch where the position of the switch is determined by the logic levels supplied to the Select input.