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Change from the privileged mode to user mode

daniel.dueringer
Intellectual 365 points


At the end of the start-up I would like to change from the privileged mode to user mode. I use the following code:
          mrs r0, CPSR      ; take a copy of the CPSR
          bic r0, r0, #0x1F ; clear the mode bits
          orr r0, r0, #0x10 ; select new mode
          msr CPSR_c, r0    ; write back the modified CPSR

As the last line is executed, I get a Data Abort Interrupt!

What happens with the stack pointer as I change the mode? In privileged mode the SP starts at 0x08001100, but in user mode at 0x08001000.

  • 0
    TI__Expert 7650 points

    HI Daniel,

    We received your post.  I'll ask one of our experts on start-up/initialization to follow up with you.

  • 0
    TI__Mastermind 49120 points

    Hi Daniel,

    There is a simpler way to change the processor's operating mode. Use the CPS (Change Processor State) instruction. For example, to change to user mode from a privileged mode,

    CPS #0x10

    This instruction executes in a single cycle versus the 5 cycles taken by the MSR (plus those taken for the complete read-modify sequence). This is also safer as it won't affect other bits in the CPSR which is possible using the read-modify-write sequence.

    As for the cause of the data abort, there is nothing in the above instruction sequence that would cause a data abort. Can you check the link register (R14_abt) contents while the CPU is in the abort handler?

    There is a separate stack pointer for each of the CPU modes with one exception: the System mode and the User mode use the same stack and hence the same stack pointer.

    Regards,

    Sunil

  • 0 in reply to Sunil Oak
    Intellectual 365 points

    Hi Sunil

    I replaced my swap from privileged to user mode with:
      asm(" CPS #0x10");

    I get a data abort: R14_ABT = 0x0000BF18.

    Thanks Daniel

  • 0 in reply to daniel.dueringer
    TI__Mastermind 49120 points

    Hi Daniel,

    I would need to see the code at addresses 0xBF10 through 0xBF20 with explanation for where the code is trying to read from or write to. Only then can I discern the cause of the data abort.

    Regards, Sunil

  • 0 in reply to daniel.dueringer
    TI__Mastermind 49120 points

    Hi Daniel,

    I would need to see the code at addresses 0xBF10 through 0xBF20 with explanation for where the code is trying to read from or write to. Only then can I discern the cause of the data abort. You could also include a screen shot that shows the disassembly along with the source code near this address as well as the CPU registers.

    Regards, Sunil

  • 0 in reply to Sunil Oak
    TI__Mastermind 49120 points

    This post is being closed for lack of information necessary to help with the debug. Please start a new thread if you are still having problems with the data abort.

    Regards, Sunil