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The assembler can issue warnings about instructions that cannot assemble to both ARM and Thumb code.
There are a few UAL instructions that can assemble as either ARM code or Thumb code, but not both. You can identify these instructions in the source code using the following command-line option when invoking the assembler:
armasm --diag_warning 1812
It warns for any instruction that cannot be assembled in the other instruction set. This is only a hint, and other factors, like relocation availability or target distance might affect the accuracy of the message.