ARM Cortex-M3 Technical Reference Manual page 353

Embedded Trace Macrocell
The final address byte uses bits [7:6] set to 0b01 to indicate the end of the address field.
Exception data follows this field. Exception byte 0 sets bit [7] to 1 if a second exception
byte follows. If there is no exception present, and only address bits [6:1] change, then a
single byte is used. If an exception is present, then at least two bytes are used to signal
the address.
When turning off trace immediately before entry to an exception handler, the ETM
remains enabled until the exception is taken. This enables it to trace the branch address,
exception type and resume information.
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