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2.4. Fault handling

Faults are a subset of exceptions, see Exception model. All faults result in the HardFault exception being taken or cause Lockup if they occur in the NMI or HardFault handler. The faults are:

  • Execution of an SVC instruction at a priority equal or higher than SVCall.

  • Execution of a BKPT instruction without a debugger attached.

  • A system-generated bus error on a load or store.

  • Execution of an instruction from an XN memory address.

  • Execution of an instruction from a location for which the system generates a bus fault.

  • A system-generated bus error on a vector fetch.

  • Execution of an Undefined instruction.

  • Execution of an instruction when not in Thumb state as a result of the T-bit being previously cleared to 0.

  • An attempted load or store to an unaligned address.

  • If the device implements the MPU, an MPU fault because of a privilege violation or an attempt to access an unmanaged region.


Only Reset and NMI can preempt the fixed priority HardFault handler. A HardFault can preempt any exception other than Reset, NMI, or another HardFault.

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