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 don't understand what you mean by 'sub directory config'.

Try entering 'gcc -v' to print out the specs. The first thing displayed is the location of the specs file.

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.