Four "74HC257" quad 2-to-1 multiplexers with a common select line and separate output enables.

Makes 16 switches in one chip (32 inputs, 16 outputs).

Used in dual slot MVS systems to switch between carts.

Select signal comes from one of the "SLOT" outputs of NEO-F0 ?

Internal logic

This is NOT verified.

Pinouts

Information provided by Channelmaniac. Some pins marked as GND could be "enable" pins.

A*, B*: Inputs
Y*: Outputs
17 SELECT: A if low, B if high
18 is an active low output enable for Y0~Y7
33 and 34 are probably ORed together to provide an active low output enable for Y8~Y15
35 is an active high enable for entire chip

Used to switch the upper 16 bits of the sprite graphics bus.

OpenOffice Draw file: File:Neo-257 L4.odg

Used to switch the lower 16 bits of the sprite graphics bus.

OpenOffice Draw file: File:Neo-257 K4.odg

SYSTEM and SYSTEMB might be reversed ?

OpenOffice Draw file: File:Neo-257 J11.odg