2.2.1. Unpredictable encodings

The ARM and Thumb instruction sets include many instruction encodings that the ARM architecture defines as unpredictable. In particular, in the Thumb instruction set there are many instructions where the use of the PC as a destination register results in unpredictable behavior. The PFT architecture does not specify unpredictable instruction encodings as waypoint instructions. However the unpredictable behavior of such an instruction might require the PTM to trace it as a waypoint instruction.

Because unpredictable encodings are never executed in valid program code, trace decompression tools can safely ignore any unpredictable encodings. However, ARM recommends that, if a decompression tool encounters an unpredictable encoding, it indicates this to the user to ensure they are aware that some unpredictable behavior might have occurred.


Trace decompression might not accurately represent the real execution of the instruction stream, particularly if an unpredictable encoding causes a branch that is not explicitly traced.

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