i m not able to find the top level gcc directory which contains the sub directory config in which i must add the particular code.
i have searched for such a directory in /usr/lib and /usr/bin without any success.

I do not understand the question. What do you mean by top level gcc directory or sub directory config?

What are you trying to accomplish?

I wanted to add a backend for 8051 microcontroller. For this purpose the documents (online documents on gcc) said that u need to add the related files in the directory gcc/config/
Once in this directory l had to create a new directory by the name of 8051 which will store its associated files but i could'nt even start as i could not figure out the location of this particular sub-directory of gcc i.e. gcc/config/

PS:The documents also mentioned the existence of other sub-directories under gcc directory like gcc/languages/ etc but i couldn't locate any of these also.

I didn't see anything in the documentation on 8051s. Can you post the link?

This is what I use to compile 8051 code.
http://sdcc.sourceforge.net/

i was referring to the documentation in gcc on how to add a backend

http://gcc.gnu.org/onlinedocs/gccint/Subdi...#Subdirectories

shows the subdirectories which should be there while the top level directory structure is at
http://gcc.gnu.org/onlinedocs/gccint/Top-L....html#Top-Level