74ALS157 Quad 2-input data selector/multiplexer, non-inverting
74ALS158 Quad 2-input data selector/multiplexer, inverting

DESCRIPTION
The 74ALS157 is a 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 typical use of the 74ALS157. The state of the select input determines the particular register from which data comes.