difference between user and supervisor mode?

[JeorgeB]

Hi, 

what is difference between user and supervisor mode?

e.g. page 542 (S32K1xx RM):

" The RCM registers can be written only in supervisor mode. Write accesses in user mode are blocked and will result in a bus error. "

can we switch between them in runtime dynamically?

Thanks.

[Robin_Shen]

Hi JeorgeB,

The device is running in Supervisor (privileged) mode out of reset. It is controlled by nPRIV bit in CONTROL register (it is core register, see ARM documentation for more details).

If you need to switch to User mode, just set the nPRIV bit in CONTROL register

// switch to User (unprivileged) mode
 __asm__("mov r0, #0x01");
 __asm__("msr control, r0"); // nPRIV = 1‍‍‍

And to switch it back, you need to do it in SVC handler

// make a supervisor call (SVC) to transfer control to privileged software
 __asm__("svc #0x00");

void SVC_Handler(void){
 // switch to Supervisor (privileged) mode
 __asm__("mov r0, #0x00");
 __asm__("msr control, r0"); // nPRIV = 0
}‍‍‍‍‍‍‍‍

Best Regards,
Robin
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[JeorgeB]

Thanks for your kind reply, where is benefit or application of using supervisor mode? (assume we do not implement any OS on MCU)

[Robin_Shen]

Sorry for my late reply!
I am not very familiar with this part, you can refer: Processor mode and privilege levels for software execution
ARM Cortex related question can also ask in community.arm.com