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4.5.2. MPU Control Register

The MPU_CTRL register:

  • Enables the MPU.

  • Enables the default memory map background region.

  • Enables use of the MPU when in the HardFault or Non-Maskable Interrupt (NMI) handler.

See the register summary in Table 4.25 for the MPU_CTRL attributes. The bit assignments are:

Table 4.27. MPU_CTRL register bit assignments
BitsNameFunction
[31:3]- Reserved.
[2]PRIVDEFENA

Enables privileged software access to the default memory map:

0 = If the MPU is enabled, disables use of the default memory map. Any memory access to a location not covered by any enabled region causes a fault.

1 = If the MPU is enabled, enables use of the default memory map as a background region for privileged software accesses.

When enabled, the background region acts as if it is region number -1. Any region that is defined and enabled has priority over this default map.

If the MPU is disabled, the processor ignores this bit.

[1]HFNMIENA

Enables the operation of MPU during HardFault and NMI handlers.

When the MPU is enabled:

0 = MPU is disabled during HardFault and NMI handlers, regardless of the value of the ENABLE bit.

1 = the MPU is enabled during HardFault and NMI handlers.

When the MPU is disabled, if this bit is set to 1 the behavior is Unpredictable.

[0]ENABLE

Enables the MPU:

0 = MPU disabled.

1 = MPU enabled.


When ENABLE and PRIVDEFENA are both set to 1:

  • For privileged accesses, the default memory map is as described in Memory model. Any access by privileged software that does not address an enabled memory region behaves as defined by the default memory map.

  • Any access by unprivileged software that does not address an enabled memory region causes a HardFault.

XN and Strongly-ordered rules always apply to the System Control Space regardless of the value of the ENABLE bit.

When the ENABLE bit is set to 1, at least one region of the memory map must be enabled for the system to function unless the PRIVDEFENA bit is set to 1. If the PRIVDEFENA bit is set to 1 and no regions are enabled, then only privileged software can operate.

When the ENABLE bit is set to 0, the system uses the default memory map. This has the same memory attributes as if the MPU is not implemented, see Table 2.9. The default memory map applies to accesses from both privileged and unprivileged software.

When the MPU is enabled, accesses to the System Control Space and vector table are always permitted. Other areas are accessible based on regions and whether PRIVDEFENA is set to 1.

Unless HFNMIENA is set to 1, the MPU is not enabled when the processor is executing the handler for an exception with priority -1 or -2. These priorities are only possible when handling a HardFault or NMI exception. Setting the HFNMIENA bit to 1 enables the MPU when operating with these two priorities.

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