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I cannot write the sp register in the monitor mode

Takumi Shimada

I use a Cortex-A7 board and write start up code.

I try to use Security Extension.

I use `smc` instruction and make cpu mode monitor mode.

In the monitor handler, I tried to changed stack pointer value for calling other functions.

But after execute `ldr sp, =_stack_start`, sp value is 0x0 (Of course, _stack_start is not 0x0 and I check it in the assembler code).

Therefore, after `push` some registers and `pop` those registers, the data abort excetpions occurred!

How can I change the sp value?

  • 0 in reply to Takumi Shimada

    I do not use OpenOCD with my CubieBoard2, as I'm just running Linux from a SD card on this one.

    Looking further into this, I'm not sure the LS1021A is supported in OpenOCD yet (eg. I do not think there's a driver for it).

    To see the currently supported boards, list the contents of /usr/local/share/openocd/scripts/board/

    To see the possible targets, list the contents of /usr/local/share/openocd/scripts/target/

    If you feel like writing a driver for it, let me know, and I'll point you in the right direction - but it may be something that requires a fair amount of work.

    Another way might be to try a differnt IDE. Which one are you using right now ?

  • 0 in reply to Jens Bauer

    I think that means sp actually has the right value and it's the debugger that does not know how to read the registers properly.

    I think so too, because following push and pop instructions correctly worked.

    But I don't know how to configure OpenOCD for my board.

    Could you show me your configuration file for CubieBoard2?

  • 0 in reply to Takumi Shimada

    I think that means sp actually has the right value and it's the debugger that does not know how to read the registers properly.

    Have you tried using OpenOCD ?

  • 0 in reply to Takumi Shimada

    I try to do another test:

         ldr r0, =_stack_start

         mov sp, r0

         mov r1, sp

    I checked the value with J-Link debugger, and r0 is _stack_start value and sp is 0x0.

    But r1 is _stack_start value correctly.

    Therefore, it seems to the J-Link problem.

  • 0 in reply to Jens Bauer

    It sounds like you're using a board without an operating system, is this correct ?

    Yes, I'm using a board without an OS.

    I use TWR-LS1021A board.

    I tried to load the value to r0 at first and then move sp like this:

         ldr r0, =_stack_start

         mov sp, r0

         mov r0, #0x1000

         mov sp, r0

    These code cannot change the sp value though the r0 value is not 0x0.

    I checked the manual of ARMv7-A, but I cannot find such a description that we cannot change the sp value in the monitor mode.

  • 0

    It sounds like you're using a board without an operating system, is this correct ?

    If it's a CubieBoard2, then I may be able to do some tests (I'm running Cubian on mine)

    I do not know why this is happening (it does not sound logical that this should happen at all, but there must be a reason somewhere).

    Try doing a few tests...

      ldr r0,=_stack_start

      mov sp,r0

      mov r0,0x01000000

      mov sp,r0

      ldr r0,new_stack

    mov sp,r0

      (other code here)

      b main

    newstack:

    .word 0x02000000

    ...etc.

    The first snippet I mentioned may give you exactly the same result as you had before, but at least you can see the value of r0 first.