Hello, I have an application consisting of a atmel 89c51RD µC, 32k external SRAM and a memory mapped i/o device at adress 8000h (spc3 profibus controller). As the controller has integrated 1k xdata ram I tried to spare the cost for the external ram. I have done the following to get it running: 1. modifiying startup.asm to enable the xram. i have tested it with a simple testprogram. this works. 2. in the target option i specify 1k xdata from 0000h and length of 0400h. i didn't specify the spc3 which is explicit specified with the _at_ command at adress 8000h. memory model : large. 3. it didn't work. 4. now i put some printf statements into the program (main()) to locate the point of error over the serial port which i connedted to a pc with a terminal program running. And it worked (!). 5. the problem is, whenever i remove these "debug" commands, it's not working. not working means that the controller is reset, but i did turn the watchdog off. the strange thing is the following: i have one working version with some printfs. i know the point where the program is reset. if i remove a printf after this point it will also not work(!). i think that it is some point of memory overlap or misleading pointer, or simple misconfiguration of the compiler or linker, but i really have no idea at the moment. if someone has expierience with using the internal xdata of the 89c51rd or an idea i would be thankful for a little help. thanks.