"I am using a custom built C-compiler for 8051"

Some love to live dangerously. I think you are in that league :-)

OK other members have given you the required inputs - but I have just one query :

How frequent is your writes going to be ? If it is too frequent ( say as an example once every second ) then this may not be the preferred method.

Normally I use this area to write in calibration data that is done at specific periods- say once a week or month. And even then check if the stored data is different from the data to be written - if same I dont write.

Raghu