5.2.2. ARM and Thumb state

Versions 4T and 4TxM of the ARM architecture 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.

The Thumb instruction set:

Refer to Thumb instruction set overview for more information.

A processor that is executing Thumb instructions is said to be operating in Thumb state. A Thumb-capable processor that is executing ARM instructions is said to be operating in ARM state.

ARM processors always start in ARM state. You must explicitly change to Thumb state using a BX (Branch and exchange instruction set) instruction.

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