2.2.3. ARM and Thumb state

A processor that is executing Thumb instructions is operating in Thumb state. A processor that is executing ARM instructions is operating in ARM state.

A processor in ARM state cannot execute Thumb instructions, and a processor in Thumb state cannot execute ARM instructions. You must ensure that the processor never receives instructions of the wrong instruction set for the current state.

Each instruction set includes instructions to change processor state.

You must also switch the assembler mode to produce the correct opcodes using ARM (CODE32), THUMB, or CODE16 directives. See Instruction set and syntax selection directives for details.

ARM processors always start executing code in ARM state.

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