Debugging code stored in EEPROM by inserting a breakpoint into code using a read/modify/write cycle on block containing individual instruction to be modified

The invention relates to a method for operating a data processor, especially a single-chip microcontroller, in a test environment (debugger) in which the program code to be tested is modified before, during and/or after the execution of the program to be tested by exchanging individual program instructions, wherein the program instructions are held in a non-volatile and rewritable memory (flash). According to the invention, it is proposed among other things that the temporally or sequentially last command for modifying a program instruction is replaced by a command sequence which causes the data processor to enter the debug mode. The number of rewrite cycles of the non-volatile rewritable memory is hereby minimised in the interactive analysis of the program code to be tested.