It looks to me as though this is an old masked-ROM version, the program store of which is programmed during manufacture and cannot be erased.

Is this a mental-masturbation project that doesn't matter? If not, you're in for a rough time. The code space is a hard-coded read-only memory that will only contain what it was built to contain. If you want to use external memory, you can use it that way, with an external non-volatile memory or flash, or EPROM memory. The real question would be, WHY?

If you do want to do that, I'd recommend a thorough study of the old datasheet. IIRC, it's still floating around out there on the WWW.

Considering that you can buy some serially reprogrammable 805x devices for under $2, I have to wonder what your goal is.

RE