Sirinivas:

If you don't use external ROM and it is a single RAM chip the you ca unse my first schmatic. You don't loose any adress.

In my last post I refer to inverting inputs as CS-, and non inverting as CS.

To select A16 one free port (P1.0)
To select between memory and I/O another free port (p1.6)

Then if you use a RAM's CS you must put the same line to 138's CS-. If you do this you select RAM by high and I/O by low in p1.6.
OR or RAM CS- and 138 CS to select RAM by low and I/O by high in P1.6

Another cuestion is the size of the decoding spaces. If you use 8255 with 4 adresses it is enougt, but may be not for another device, so if yo want to expand in future with other devices put A,B,C of 138 to i.e. A5,6,7. then you have blocks of 32bytes in decoder´s outputs.

For completing a few the decoding you can put the unused CS pins to higher lines adress. Then you will got less repeats of adressing and you will can expand in future with another decoder or devices.

Félix.