2.15.10. Undefined instruction

When the processor encounters an instruction that neither it nor any coprocessor in the system can handle, it takes the Undefined Instruction exception. Software can use this mechanism to extend the ARM instruction set by emulating Undefined coprocessor instructions.

After emulating the failed instruction, the exception handler executes the following instruction, irrespective of the processor operating state:

MOVS PC,R14_und

This action restores the CPSR and returns to the next instruction after the Undefined Instruction exception.

IRQs are disabled when an Undefined Instruction exception occurs. See the ARM Architecture Reference Manual for more information about Undefined instructions.

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