Hello
This is another example fit to your needs
It used only from inside an ISR, to jump to an address in thread mode.
Use it like this: jumpfromISR(topOfStack,(uint32_t)main);// first parameter is the new stackpointer in MSP, main is the function to jump.
The bit 0 of the second parameter must be always 1
clear also all pending interrupts before call this function.
It just a working (tested in M4) example and needs improvements , dont rely on it until you understand how it works.
In general , install the new stack, make the stack frame with new address to jump, load PC with proper exec return value, the stack pointer returns to its set position after jump.
__attribute__((naked)) __attribute__((noreturn)) void jumpfromISR(uint32_t new_stack_pointer,uint32_t jump_to_address);// remember jumptoaddress bit0 must be 1
__attribute__((naked)) __attribute__((noreturn)) void jumpfromISR(uint32_t new_stack_pointer,uint32_t jump_to_address)
{
//_____________________________________________________________________________
// this example uses MSP for both thread and handler modes.
__asm("LDR.W R2, =0xE000ED14");// Read SCB->CCR that located at address 0xE000ED14
__asm("LDR R3, [R2]");//Take the value of register
__asm("ORR R3, R3, #1");//Set bit 0 to enable NONBASETHRDENA
__asm("STR R3, [R2]");// Write back the value to the SCB->CCR
// take only the xPSR from stacked frame and discard the rest
__asm("POP {r2-r8}");// POP R0,ri,r2,r3,r12,lr,pc
__asm("POP {r2}");// POP xPSR to use it
__asm("MSR MSP, R0");//set the new stack pointer
__asm("DSB");// wait for store to complete
__asm("ISB");// reset pipeline
//Make a new stack frame
__asm("PUSH {R2}");//xPSR
__asm("PUSH {R1}");// new address to go
__asm("PUSH {R0-R5}");// just complete the next of stack frame with "dont care" values. But if you care to return something , just modify it
// this is the "jump" of ISR , dont forget to disable the NONBASETHRDENA bit when start the new program(thread mode)
//load the PC with one of EXC_RETURN values.. Exception return values,described in cortex Generic User Guide
__asm("LDR R0, =0xFFFFFFF9");//Return to Thread mode, exception return uses non-floating-point state from MSP and execution uses MSP after return.
__asm("BX R0");// load PC with EXC_RETURN value.. will go to thread mode, after xPSR(8th word) poped from stack
// normaly the execution will not reach this line
__asm("B .");//trap
//_____________________________________________________________________________
}
