A hardware trace unit is integrated into a computer system that has a main memory. The trace unit includes registers that contain information defining a location in main memory, and has an input connection. This is used to eavesdrop on communications in the computer system, thereby obtaining information about the state and/or operation of the system. A trigger mechanism then compares the information received against one or more trigger conditions, such as whether a particular event has occurred. Responsive to the trigger conditions being satisfied, the trace unit generates an output record containing diagnostic information. This is sent for storage in the main memory of the computer at the location defined in the registers.