How can I finish program as below code continuously loops between __mainCode and funcOne subroutines?

__mainCode  PROC 
            MOV R0, 5                       ;0x0800 0008
            LDR R1, =0xA                    ;0x0800 000C
            BL funcOne                      ;0x0800 0010
            POP {R3}                        ;0x0800 0014
            ENDP ; end of function

            
funcOne     PROC
            MOV R2, #11                     ;0x0800 0018
            PUSH {R2}                       ;0x0800 001c
            BX LR                           ;0x0800 001e
            ENDP

            ALIGN ; fill rest of bytes with 0s
            END

CodePudding user response：

Bare metal code does not have any "exit" routine as it does not have anything to pass the control to.

When you write "normal" C program when you return from the main function the startup code simply disables the interrupts and enters the infinitive loop.

Below fragment of the startup code generated by the STMCube (they did not even care abut the interrupts)

/* Call the application's entry point.*/
    bl  main

LoopForever:
    b LoopForever

What is your code doing?

1. When BX LR is executed, it jumps to POP {R3} and continues execution
2. It reaches again BX LR (LR was not modified) and cycle repeats.

As you push R2 and next pop R3 the stack pointer remains the same and never reaches not legal addresses which could raise the BusError potentially breaking this dead loop.

But if you remove the pop instruction and eventually uC will access the illegal address, you do not have interrupt vectors set. It will try to jump to 0xffffffff address (flash is filled by default by 0xff values) and this address is also illegal. It will raise the BusFault again and your program will end up in the dead exception raising loop.

IMO learning ARM assembly if you do not perfectly know the architecture and do not proficiently write C code for ARM uCs, makes no sense at all. Assembly is used very infrequently (I personally I wrote 50 lines 10y ago when I was writing my own RTOS and needed few instructions to swap stack pointer values and access some system registers) - and programming (mainly ARM) uCs is my daytime job.