Dennis, that is incorrect...

according the the data sheet, you also need to commit the write by send eeCON 0x50 and 0xa0 .

From my experience with the T89c51RD2, I would suggest that you do not try to read from the eeprom unless you have done the commit... I would get all 00's if the commit were not completed. I am interested if this is the case with another part.

As for the API calls, it's a simple matter of making assembly routines to be called from C. Each is only a few lines long. Someone (maybe ESCacademy) has some files to do this, but they exaggerate the amount of work involved in creating them.