Patent ID: 7962664

Claim:
Trace circuitry provided on a chip for generating a trace stream of packets to be output from the chip, the trace stream indicating activities of monitored circuitry within the chip, the monitored circuitry producing data elements indicative of said activities, and the trace circuitry comprising: trace element generation circuitry responsive to at least some of the data elements produced by the monitored circuitry to generate trace elements representative of those at least some of the data elements, the trace elements generated being dependent on a selected trace mode of operation of the trace circuitry; and compression circuitry for applying an encoding operation to a sequence of trace elements to produce a packet whose bit pattern represents the sequence of trace elements, and to cause that packet to be output in the trace stream, the encoding operation applied being dependent on a current compression scheme associated with the compression circuitry; the compression circuitry, whilst in the selected trace mode of operation, is responsive to a compression change stimulus to change the current compression scheme from a first compression scheme to a second compression scheme whose bit pattern encoding space overlaps the bit pattern encoding space of the first compression scheme, such that following the change of compression scheme any further packets produced whilst in said selected trace mode of operation are produced in accordance with the second compression scheme; wherein when the current compression scheme is changed, the compression circuitry is arranged to output a change signal in the trace stream to indicate that the change has been made, so that a trace analysing apparatus used to analyse the trace stream can determine when the compression scheme has changed so as to correctly interpret the packets in the trace stream despite the bit pattern encoding space of the first compression scheme and the second compression scheme overlapping.