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Call gate mechanism

The firewall is opened by calling a 'call-gate' mechanism: a single entry point that must be used to open the

gate, and to execute the code protected by the firewall. If the protected code is accessed without passing through

the call gate mechanism, then a system reset is generated. If any instruction is fetched outside the protected

area, the firewall is closed (see the figure below).

Figure 11. Firewall FSM

DT50954V1

IDLE

CLOSED OPEN

Call gate

Instruction fetch

out of prot. area

Reset

Firewall

enabled

Since the only way to respect the call gate sequence is to pass through the single call gate entry point,

a mechanism must be provided in order to support application calling multiple firewall-protected functions from

unprotected code area (such as encrypt and decrypt functions). A parameter can be used to specify which

function to execute (such as CallGate(F1_ID) or CallGate(F2_ID)). According to the parameter, the right

function is internally called. This mechanism is represented in the figure below.

Figure 12. Firewall application example

DT50961V1

Firewall code section

f1()

f1a()

f1b()

f2()

f2a()

f2b()

f3()

f3a()

f3b()

Function ID

Call gate single entry point

unprotected_code.c

AN5156

Firewall

AN5156 - Rev 8

page 36/56

ST STM32WB Series Application Note

Other manuals for ST STM32WB Series