You can access very easily the external memory by using the XBYTE , as some one has already suggested, but here i have given the detail,

when using XBYTE, there must be the declaration of header file as following on the top of C program.
#include <ABSACC.H>
after that you can use XBYTE as below.

XBYTE [0x8001] = 0xFF ; to write at external address
P1 = XBYTE [0x5001] ; to read from external address

the above statements will produce the following code by using keil compiler,
------------------------------------------------------

; XBYTE [0x8001] = 0xFF ;

MOV DPTR,#08001H
MOV A,#0FFH
MOVX @DPTR,A

; P1 = XBYTE [0x5001] ;

MOV DPTR,#05001H
MOVX A,@DPTR
MOV P1,A
-----------------------------------------------------

thanks