"promero" wrote on Friday, March 09, 2001 6:46 PM > The problem may be located in the handling of PCLATH register and/or page crossing boundaries. > There is a lot of information about it in the archives. You can check Microchip Application note > AN556 about table reads. There is also a chance that W register is being changed because of a > bad/non saving environment interrupt. I don't think so. I ran it in MPLAB SIM and It did'nt work. all the Interrupts were disabled. I even tried to move the code to location 0x200 which must clear PCL and increment PCLATH. But it still does'nt work. And I Don't think Its a problem with the Simulator, Because, when I burned the code on a real 16f877 , the same problem persisted. It began Jumping mysteriously inbetween my ISR code . That means that It does'nt jump to 0x04 but it jumps somewhere to 0x21 Man, this is a really persistant bug. If all else fails, I'll post the whole code on piclist. Thanking you, Jeethu Rao -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.