hi,

I am unable to write data to program memory(4k internal)

...because it is not possible for AT89C51.

if you work with Atmel and need to save some data into non-volatile memory then I may suggest you to look at AT89S8252 which has 2kb of internal EEPROM. Another way is the usage of Atmel`s X2 chips, for example, AT89C51RD2, AT89C51ED2, etc. They allow user to write into internal flash program memory via intergated API.

Regards,
Oleg