Thanks for the information.

One more doubt I have in this subject is, how can I use the lower part of external rom.

Actually I am using a s/w Reads51 for writing assembly language program. I wrote the program in a single file and converted into a HEX file, which contains program for both internal and external ROM. Using a notepad i cut the hex file into two .
pgm1.hex ( address < 4k , 0-0FFFh)
pgm2.hex ( adress > 4K , 1000h and above)


Then I place pgm1.hex in internal rom and pgm2.hex in external rom.

It is fine now, but i didn't use the 0- 4k region of external rom.

expecting sugessions, Prasad