3.1.1. Switching state

Entering THUMB state

Entry into THUMB state can be achieved by executing aBXinstruction with the state bit (bit 0) set in the operand register.

Transition to THUMB state also occurs automatically on return from an exception (IRQ, FIQ, UNDEF, ABORT, SWI etc.), if the exception was entered with the processor in THUMB state.

Entering ARM state

Entry into ARM state happens:

  • On execution of the BX instruction with the state bit clear in the operand register.

  • On the processor taking an exception (IRQ, FIQ, RESET, UNDEF, ABORT, SWI etc.). In this case, the PC is placed in the exception mode’s link register, and execution starts at the exception’s vector address.

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