Hopefully someone can Help, I'm working on a project with the Atmel AT89C51ED2 processor. I have selected the AT89C51RD2 controller from the device database (If I'm not mistaken the only difference is the EEPROM which I'm not using..... Yet). I have a project which works correctly (in the simulator) except when Xdata is used. I have gone through tens of threads and can not find the answer. I have edited the Startup.A51 code which seems to work (clears 0x6FF bytes of data) cleared bit 1 of AUXR. All seems to be OK. I suspect the simulator, I have checked the memory mapping, and that also seems to be OK. Everything is working except Xdata. Any thoughts of anything else I can try..?? Any suggestions appreciated. Regards John Garrelts