[Cortex-M33 FVP]:SecureFault with SAU disable


I am using Cortex-M33 FVP model to run the Keil RTX TrustZoneV8M RTOS example.
Have also tried to experiment with the memory map configuration and the corresponding SAU region programming for NS/S patitioning and the behaviour is as expected.

However when i try to disable the SAU usage in the system, I am seeing SECUREFAULT when switching from NS to S.
Here is the procedure i followed:

1) Configure SAU_INIT_CTRL=0 in TrustZoneV8M\RTOS\CM33_s\RTE\Device\ARMCM33_DSP_FP_TZ\partition_ARMCM33.h

=> Not attempting to program the SAU.

2) Disable SAU in the model,
cpu0.SAU=0x0                                          # (int   , init-time) default = '0x4'    : Number of SAU regions (0 => no SAU) : [0x0..0x8]

cpu0.SAU_CTRL.ENABLE=0                                # (bool  , init-time) default = '0'      : Enable SAU at reset

cpu0.SAU_CTRL.ALLNS=1                                 # (bool  , init-time) default = '0'      : At reset, the SAU treats entire memory space as NS when the SAU is disabled if this is set

VECTOR=SECUREFAULT FaultCause=SecureFault invalid transition SecurityState=SECURE

I also noticed that, If i enable the SAU but set ALLNS=1 ( both in SW and FVP config), the behaviour is still the same.

Note; I am not sure if this is even a valid experiment. Our target HW platform will not have an SAU.