how does the formater of a TPIU works


I'm learning the trace of ARM cortex_m3 

after run the trace-test, i check the inside signal of TPIU and i found the format data is different from the ITM data

in the picture the fifo1_data_i is from ITM and its a serials of packets  the fromat_data is the final valid data output

for example the formater add a b3 in front of 17 and change the 3 to 2  (..c0 15 3 dd...  to ....c0 15 2 dd ...)

i have read the ARMv7 reference manual   and the manual just tell me the packet protocol. but therer is nothing about the format of a packets stream 

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