Do you want to read/write to any external data location or do you want to put a variable at a specific location?
One way to address any location is to declare a pointer as "xdata".
Ex:
// Declare pointer to external data
// character
unsigned char xdata *your_pointer_name;

// Declare another variable for this
// example
unsigned char data;


// Assign address to pointer
your_pointer_name = what_ever_address;

// Write data to address
data = some_number;
*your_pointer_name = data;

// Or read data from address
data = *your_pointer_name;

Works great for me!
Allen