In general terms I can add that not all 8051 derivatives have on-board flash programming code. And in some cases even if the part does have on-board flash programming code the data transfer protocol used may not be appropriate to the IAP needed/desired. In those cases it may be necessary to devise your own flash proramming procedures and include it as a part of the application code. Of course it needs to be designed in in a way to deal with the application as a two part animal (the bootstrap/re-programming part versus the run time application part). And then there are issues which become completely processor derivative dependant on just how that is done in the best manner possible.

Michael Karas