Once you map the 256kB RAM into 16kB, the upmost two address bits (A14 and A15) are fixed for the RAM acces, aren't they? Say, if the 16kB window is 0x8000-0xBFFF, these bits are _always_ 10(binary) when you access this window, so there is no reason to add them up to the "page".

So, I would simply map A0-A13 from the '51 bus to the memory's address bus, and tie A14-A17 of the memory to a simple latch (or, if available, spare pins in P0 or P3); which is a fairly standard "paging" scheme, (even to some extent supported by C-compilers).

Do I miss something?

JW