All, We are designing a "loader" for field firmware updates. Unfortunately we do not have the code space (P89C668) to have two sets of code in on-chip flash. Therefore we will use a scheme where we have the loader code located in sector one and all the ISRs in sector zero as the loader will use interrupts. The rest of the application will be in sectors 2 and up. The loader will erase the upper sectors and program the new code there. It will then erase sector zero and reprogram the vector table and the ISRs. Block one, the loader, will remain unchanged. Questions: I think I can locate the code in sector one and the ISRs in sector zero using user classes. I have done this with some ARM code in uVision. How can I be sure that the loader will have all the run-time modules it needs in sector zero? In other words, I cannot have the loader calling code in sectors that have been erased. It will have to be completely self contained except for the ISRs. Is this a reasonable plan? Anyone have a better way to do this? Rich