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At what point in time is the boot security map fixed into the chip?

Ed Player

This question was raised in the 'How to implement a secure IoT system on ARMv8-M' webinar, view all the questions in the round up blog post. 

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    If the ARMv8-M implementation decides that the SAU configures all memory as Secure at boot time, which is the case for Cortex-M33 and Cortex-M23, then on reset the SAU_CTRL.ALLNS register is always reset to zero, setting all memory, apart from some specific regions in the PPB space, to Secure state. Setting SAU_CTRL.ALLNS bit to zero prevents an external SAU (Implementation Defined Attribution Unit) overriding any security level. If the ARMv8-M implementation decides that on reset that the SAU_CTRL.ALLNS register is always reset to 1, then it is up to the IDAU to determine the default security settings at reset. This will typically be fixed at the time of manufacturer.
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  • +1
    If the ARMv8-M implementation decides that the SAU configures all memory as Secure at boot time, which is the case for Cortex-M33 and Cortex-M23, then on reset the SAU_CTRL.ALLNS register is always reset to zero, setting all memory, apart from some specific regions in the PPB space, to Secure state. Setting SAU_CTRL.ALLNS bit to zero prevents an external SAU (Implementation Defined Attribution Unit) overriding any security level. If the ARMv8-M implementation decides that on reset that the SAU_CTRL.ALLNS register is always reset to 1, then it is up to the IDAU to determine the default security settings at reset. This will typically be fixed at the time of manufacturer.
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