Hi Steve,

thank you for your reply.
Some times its hard to find the questioners problem.
Nevertheless I try to answer:

Most problems on paging CODE. So there are many ways
to handle interrupts independend from the currrent page.

But normally no problems on data space expansion.
Since any XDATA using can be preceded by setting the right page.

E.g. for using 512kByte you need 19 address lines.
The easiest way was to use P1.0 - P1.2 to generate
the upper 3 address lines. The lower 16 lines must be connected,
like described in the data sheet. So to using XDATA, you must
only load DPTR with the 2 lower address bytes and P1
with the 3. address byte. Then with MOVX you can access
any address pointed by: P1, DPH, DPL
E.g.:
load address 0789ABh with 0CDh:
mov p1, #07h
mov dptr, #089ABh
mov a, #0CDh
movx @dptr, a


Hope, I hit your problem now,

Peter