Abstract: A data processor is provided with an embedded debugger. The debugging function is provided by the execution of a debugging program which is stored in reserved, non-volatile memory which is internal to the data processor. During the debug mode, the data processor allows the internal registers used during execution of a user program to be examined. Debug operation can be initiated via debug instruction which replaces an existing instruction in the user code, the replaced instruction being held in a special purpose register such that it can be executed on return from the debug mode. Single step operation of the data processor can be performed in debug mode and data and instructions can be exchanged with the data processor in debug mode, optionally via a single pin so as not to sacrifice any user resources.