2.2.2. ARM and Thumb state

ARM architecture versions v4T and above define a 16-bit instruction set called the Thumb instruction set. The functionality of the Thumb instruction set is a subset of the functionality of the 32-bit ARM instruction set. Refer to Thumb instruction set overview for more information.

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 CODE16 and CODE32 directives. Refer to CODE16 and CODE32 for details.

ARM processors always start executing code in ARM state.

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