A method, apparatus and system are provided for diagnosing a processor executing a stream of instructions by causing the processor to execute the stream of instructions in a sequence of stages with a diagnostic exception being taken between each stage. The method involves controlling the processor in a current stage, when a point is reached where the diagnostic exception is to be taken, to store in a storage location type indicator information comprising a type indicator for a current instruction in the stream and a type indicator for a next instruction in the stream. The diagnostic exception is then taken, causing a diagnostic operation to be performed which includes accessing the type indicator information from the storage location and, dependent on the type indicator for the current instruction and the type indicator for the next instruction, determining control information to identify at least one trigger condition for a next diagnostic exception. Thereafter, return from the diagnostic exception causes the processor to operate in a next stage in accordance with the determined control information. By capturing information not only about the current instruction being processed at the point that the diagnostic exception is to be taken, but also information about the next instruction, this can provide a significant improvement in the efficiency of the handling of the diagnostic process.