A 74C138 is a hardware component - you cannot use it in your 'C' source code! ;-)
(unless you're using 'C' as a Hardware Description Language, HDL)


Look at the schematic in Michael's post:
See how the address lines A13-15 are connected to the decoder's A,B,C input lines, and the Y0-7 outputs are used to select the different peripherals (in Michael's case, different memory chips)


There are several possibilities:

* Use a pointer - see your compiler manual for details of its pointer implementation to cope with the 8051's different memory spaces.
(Note that this is effectively what Keil's XBYTE, etc, macros do - see the Keil manual)

* Use the Linker to locate a variable at the required address - see your Linker manual.
(this has the advantage of keeping hardware- and compiler-dependencies out of your source code)

* If you are using Keil, try the _at_ keyword extension - see the Keil C51 manual.
(this has the disadvantage of making your source code both hardware- and compiler-dependant!)