Transcribed from the May 1977 National Semiconductor Datasheet by Paul Robson (email@example.com)
The I/O Chip contains a 128 x 8 bits of RAM and 2 8 bit I/O ports. These are addressed via the address lines AD0-AD6 and the Memory I/O Select line.
On the MK14 the memory is mapped to bytes 00-7F of the page, and the I/O to bytes 80-FF of the page.
The I/O area is unused on the MK14 and will be described later.