Hi , I am using the technique described in : http://www.keil.com/support/docs/132.htm This document is about checking in the ISR where your program was - internal area or external. my ROM boot code -(0000 - 1FFF) and flash area (2000H - FFFF) First thing in the ISR I would like to check if I need to run the internal ISR or the external one. But the first c code I locate in the beginning of the ISR is placed after a lot of PUSH commands - which the assembler adds How can I place code in the very beginning of the ISR, or maybe I can change the assembler code myself ?