74ALS257 Quad 2-input data selector, non-inverting (3-State)
74ALS258 Quad 2-input data selector, inverting (3-State)

DESCRIPTION
The 74ALS257 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 output enable input (OE) is active when Low. When OE is High, all of the outputs (Yn) are forced to a High impedance state (3-State) regardless of all other input conditions.
Moving data from two registers to a common output bus is a typical use of the 74ALS257. The state of the select input determines the particular register from which data comes.