Mardi,
try these..

o vcc
|
|

/ R = 10k

diode /
psen_pin>------|<|----+----<RAMread_pin
|
rd_pin>--------|<|----+
diode

now u got your self a physical RAM area that logically act as Program Memory (psen) and External Data Memory (RD)

contact me for any question..
have fun...