pointer in gap memory

Hello everybody,

First of all, sorry for my english langugae because it is not my origin language.

So, i would like to know, how to define a pointer to point in gap memory in xdata.
Look this example :
unsigned char xdata * Ptr;

The adresse of Ptr is in internal data, and it point to xdata memory but anywhere (out of physical memory)!! I want to define this pointer to point in available gap memory xdata.

I hope it's clear.
Thanks.