in the example in the folder Keil\C51\EXAMPLES\FarMemory\16MB RAM on ADuC812.
FVAR (char, 0x015678) = 0x5A; i = FVAR (char, 0x015678);
FVAR (char, 0x016000) = 0x5A; i = FVAR (char, 0x016000);
Well, did you, or didn't you set up a memory mapping that covers address 0x16000 in XRAM?
A search in the Knowledgebase helps in such situations: http://www.keil.com/support/docs/814.htm