Hi, in school I have written many programs for 8051 compatible CPU. But I never actually knew how our "debug" sets worked. I mean, we test our programs in special sets, which actually allow you to very simply load program to CPU via PC serial port. But I thing you know this musch more better than I. But how it works? I mean, I know there is chip which adjusts signal level from PC serial port to TTL logic, and than connected to serial line of 8051. But thats all I know. Actually even my teacher doesen´t know how it works, since school bought it all. So, I suspect there is some program already running in the 8051 which handles communication and stores your program into memory, am I right? But, how can you make 8051 to process instructions from different location than ROM? Becouse if I am right, you cannot write into ROM memory by any instruction, as well as 8051 can only read instructions from ROM?