Cortex R5 ARM processor

In your write-back instruction, you use r0, not r1. Is this the issue?

In your write-back instruction, you use r0, not r1. Is this the issue?

HI Synnott,

I applogize this print mistake.

But actual case is 

.def _CPU_system_mode_Switch
.asmFunc
_CPU_system_mode_Switch

MRS r0, CPSR          ;read cpsr into r0
orr r0, r0, #0x1f          ; set mode to SYS
MSR CPSR, r0          ; write r0 back to cpsr
bx lr ; return

.endasmfunc

After execution of this instruction system is going to reset condition and MCU System register shows SWRST.

When you switch to SYS mode, a different LR (and SP) will be banked in, and so the return address is corrupt.

Do you have a debugger attached? You should be able to step through the code and see what occurs.

HI Synnott,

thanks for your quick reply.

Yes you are right the LR register and SP register is clearing. and system is going to reset.

I am attaching few images for reference

This is before calling switching function

This is before MSR instruction

This is after MSR instruction

Can you please let me know why this happen?

How we solve this problem?

Thanks 

Pankaj Verma

This seems correct to me.

The MSR instruction switches to SYS mode, and so new LR and SP are banked in. The LR of SYS mode is 0x0, and so when the BX instruction executes, you branch back to that address, which is most likely also your reset vector.

HI Synnott,

thanks again for your quick reply.

Yes you are correct 

After execution of MSR instruction Mode is showing system mode and than the value at LR and  SP register is 0x00000000.

If BX instruction is calling it is going to reset vector.

How we solve this problem?

Is It possible?

"can i store LR and SP register value to R1 register and than update data of R1 to LR again before BX instruction".

Thanks 

Pankaj Verma

I would not describe it as a 'problem'. the processor is doing exactly what it is supposed to do.

I don't know the larger code block you are integrating this into, but generally such code would be inline rather than a function call (for exactly the reasons you are seeing).

What does your code do after switching to SYS? From your initial posting, it seems you just need to Branch to __main (or _start), and go to the start of your C application.

HI Synnott,

I tell you whole application scenario.

I am using TMS570LC43 MCU with RTOS. Here we are also using safeTI diagnostic library for MCU self test condition.

So initially when the system power ON the system work on Supervisor mode "0x13". 

After initialization of all the CPU core registers, clock, RAM, Flash, other memory, peripherals etc.

The code going inside main loop. After that we need to initialize RTOS Scheduler.

For initialization of RTOS Scheduler we need to change mode from supervisor to system mode otherwise the schedular will not successfully initialize. The scheduler is fail.

And for safeTI diagnostic we need to run all the safety test cases in supervisor mode only.

That is the reason we need to change from supervisor to system mode and after that we need to change it again in supervisor mode when the safety test will run again.

Thanks 

Pankaj Verma

It is standard for an RTOS to run in SVC mode, and its applications to run in USR mode.

Applications running in SYS mode use the same physical registers as USR, but with privileged permissions.

I don't know why the scheduler init code would need to run in SYS specifically.

Assuming what you wish to do is valid... I've written (but not tested) code to switch to SYS. Would this work for you?

	push {r0, r1, lr}	// preserve r0, r1, lr on SVC stack
	
	mov r1, sp			// copy sp_svc into r1
	
    mrs r0, cpsr		// read cpsr into r0
    orr r0, r0, 0x1f	// set mode bits to SYS
    msr cpsr, r0		// switch to SYS mode
	
	mov sp, r1			// copy sp_svc (r1) into sp_sys
	
	pop {r0, r1, pc}	// return in SYS mode using SVC stack

HI Synnott,

Yes you are right and i have tried below code as per your suggestion.

It is jump to the supervisor mode to system mode by using below instruction.

.def _CPU_system_mode_Switch
.asmFunc
_CPU_system_mode_Switch

MOV R1, LR
MOV R2, SP
MRS r0, CPSR                     //read cpsr into r0
orr r0, r0, #0x1f                       //set mode to SYS
MSR cpsr, r0                           //write r0 back to cpsr
MOV LR, R1
MOV SP, R2
bx lr                                         //return
.endasmfunc

Is it correct?

I will also try your code.

Can you tell me that, if a application jump to system mode to supervisor mode at any time and vise versa  . It will not effect to running application and RTOS behavior's.

Thanks for your great help, Have a nice day.

Pankaj Verma

The code looks correct to me, but as I said, I don't believe this is really what you want to do...

Your application should run in SYS mode (ideally USR mode), and the RTOS in SVC mode, with independent stacks. The RTOS will manage switching between applications.

Regards. Ronan