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run standalone software in user mode

XNoOp

Hello,

I am using a standalone software which is current run in SYSTEM mode. But I would like to run it in USER mode instead, in order to be in non-privileged mode and be able to test my MMU fault handler (in particular permission faults).

In the linker script I have declared the USER stack within the stack section, and defined its size as 1024B.

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_STACK_SIZE = DEFINED(_STACK_SIZE) ? _STACK_SIZE : 0x2000;
_ABORT_STACK_SIZE = DEFINED(_ABORT_STACK_SIZE) ? _ABORT_STACK_SIZE : 1024;
_SUPERVISOR_STACK_SIZE = DEFINED(_SUPERVISOR_STACK_SIZE) ? _SUPERVISOR_STACK_SIZE : 2048;
_IRQ_STACK_SIZE = DEFINED(_IRQ_STACK_SIZE) ? _IRQ_STACK_SIZE : 1024;
_FIQ_STACK_SIZE = DEFINED(_FIQ_STACK_SIZE) ? _FIQ_STACK_SIZE : 1024;
_UNDEF_STACK_SIZE = DEFINED(_UNDEF_STACK_SIZE) ? _UNDEF_STACK_SIZE : 1024;
_USR_STACK_SIZE = DEFINED(_USR_STACK_SIZE) ? _USR_STACK_SIZE : 1024;
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

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.stack (NOLOAD) : {
. = ALIGN(16);
_stack_end = .;
. += _STACK_SIZE;
. = ALIGN(16);
_stack = .;
__stack = _stack;
. = ALIGN(16);
_irq_stack_end = .;
. += _IRQ_STACK_SIZE;
. = ALIGN(16);
__irq_stack = .;
_supervisor_stack_end = .;
. += _SUPERVISOR_STACK_SIZE;
. = ALIGN(16);
__supervisor_stack = .;
_abort_stack_end = .;
. += _ABORT_STACK_SIZE;
. = ALIGN(16);
__abort_stack = .;
_fiq_stack_end = .;
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

Then in the __boot assembly code I added the code to initialize the USER stack and switch to USER mode (after SYS mode init):

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mrs r0, cpsr /* get the current PSR */
mvn r1, #0x1f /* set up the system stack pointer */
and r2, r1, r0
orr r2, r2, #0x1F /* SYS mode */
msr cpsr, r2
ldr r13,=SYS_stack /* SYS stack pointer */
bic r2, r2, #(0x1 << 9) /* Set EE bit to little-endian */
msr spsr_fsxc,r2
mrs r0, cpsr /* get the current PSR */
mvn r1, #0x1f /* set up the user stack pointer */
and r2, r1, r0
orr r2, r2, #0x10 /* USER mode */
msr cpsr, r2
ldr r13,=USR_stack /* USER stack pointer */
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

But when I try to read the current CPSR mode when the standalone software is executing, I find out it is still in SYSTEM mode (0x1f)...

Could anyone tell me if the stack is setup properly? 

Thank you a lot.

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