ST STM32L4+ Series Reference Manual page 180

Firewall (FW)
Bit 2 VDE: Volatile data execution
When VDS = 1, this bit has no meaning. The Volatile data segment can be executed whatever
the VDE bit value.
If VDS = 1, the code can be executed whatever the Firewall state (opened or closed)
If VDS = 0, the code can only be executed if the Firewall is opened or applying the "call gate"
entry sequence if the Firewall is closed.
Refer to
Bit 1 VDS: Volatile data shared
Refer to
Bit 0 FPA: Firewall prearm
Refer to
This register is protected in the same way as the Non-volatile data segment (refer to
Section 4.3.5: Firewall
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0: Volatile data segment cannot be executed if VDS = 0
1: Volatile data segment is declared executable whatever VDS bit value
Segment access depending on the Firewall
0: Volatile data segment is not shared and cannot be hit by a non protected executable code
when the Firewall is closed. If it is accessed in such a condition, a system reset will be
generated by the Firewall.
1: Volatile data segment is shared with non protected application code. It can be accessed
whatever the Firewall state (opened or closed).
Segment access depending on the Firewall
0: any code executed outside the protected segment when the Firewall is opened will
generate a system reset.
1: any code executed outside the protected segment will close the Firewall.
Closing the Firewall.
initialization)
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RM0432 Rev 6
state.
state.
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