ST STM32L4+ Series Reference Manual page 172

Firewall (FW)
Volatile data segment
Volatile data used by the protected code located into the code segment must be defined into
the SRAM1 memory. The access to this segment is defined into the
accesses and
Firewall must be opened or not before accessing this segment area. The segment length
and the base address of the segment as well as the segment options must be configured
before enabling the Firewall (refer to
The Volatile data segment can also be defined as executable (for the code execution) or
shared using two bit of the Firewall configuration register (bit VDS for the volatile data
sharing option and bit VDE for the volatile data execution capability). For more details, refer
to Table
4.3.4 Segment accesses and properties
All DMA accesses to the protected segments are forbidden, whatever the Firewall state, and
generate a system reset.
Segment access depending on the Firewall state
Each of the three segments has specific properties which are presented in
Table 21. Segment accesses according to the Firewall state
Segment
Code segment
Non-volatile data
segment
Volatile data
Execute if VDE = 1 and
segment
VDS = 0 into the Firewall
configuration register
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properties. Depending on the Volatile data segment configuration, the
21.
Firewall opened
access allowed
Read and execute
Read and write
Read and Write
Section 4.3.5: Firewall
Firewall closed
access allowed
No access allowed.
Any access to the segment
(except the "call gate" entry)
generates a system reset
No access allowed
No access allowed if VDS = 0
and VDE = 0 into the Firewall
configuration register
Read/write/execute accesses
allowed if VDS = 1 (whatever
VDE bit value)
Execute if VDE = 1 and VDS = 0
but with a "call gate" entry to
open the Firewall at first.
RM0432 Rev 6
Section 4.3.4: Segment
initialization).
Table 21.
Firewall disabled
access allowed
All accesses are allowed
(according to the Flash page
protection properties in which
the code is located)
All accesses are allowed
(according to the Flash page
protection properties in which
the code is located)
All accesses are allowed
RM0432