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Prefetch Abort due to unaligned PC.

Nikhil Girdhar
Note: This was originally posted on 22nd August 2011 at http://forums.arm.com

Hi,

I am working on ARM1136 processor in Thumb mode.

Problem

I am getting a prefetch abort while executing an instruction. Program Counter at this instant is odd.



Description

My source code has few instructions as below...

                            ....
                        [color=#FF0000]    [/color]bx lr   [color=#FF0000]       [/color];lr = 0xB76B2A39 i.e. some odd value


After executing this instruction the code jumps to the  location 0xB76B2A38 (pc = lr & 0xFFFFFFFE) which has the instruction as below

Address 0xB76B2A38:   [color=#FF0000]  [/color]ldr r0, #0xB76B5DAC [color=#FF0000]    [/color];prefetch abort.

The above instruction generates a prefetch abort as the pc value is odd 0xB76B2A39.



BX Instruction description

BX Rm instruction ensures that the location that you jump to is half a word aligned. i.e. pc = lr & 0xFFFFFFE

ARM thumb mode requires that pc should always be half a word aligned.



In this case pc alignment (to be done by the processor) did not take place. How can I figure out whether the processor is goofing up or there is some software issue?

Thank you.
Nikhil.
Parents
  • Arm Employee Badge
    Note: This was originally posted on 22nd August 2011 at http://forums.arm.com

    The bottom bit in the address of a BX instruction is used to encode the "eXchange" - the state change from ARM to Thumb. So the address you feed the BX instruction looks fine.

    I would check the following:
    • Does the destination address exist (i.e. is it mapped in the MMU)?
    • Does the destination address contain code (i.e. is it mapped as executable in the MMU)?
    • Does the destination address contain an instruction which is valid encoding for the current instruction set?
    • If you are using ARM and Thumb code make sure it is all compiled for "interworking"
    • Check the Fault Status Register in CP15 - this will give more info about why an abort was raised.
    What platform are you running on, and which core revision?

    Cheers,
    Iso
Reply
  • Arm Employee Badge
    Note: This was originally posted on 22nd August 2011 at http://forums.arm.com

    The bottom bit in the address of a BX instruction is used to encode the "eXchange" - the state change from ARM to Thumb. So the address you feed the BX instruction looks fine.

    I would check the following:
    • Does the destination address exist (i.e. is it mapped in the MMU)?
    • Does the destination address contain code (i.e. is it mapped as executable in the MMU)?
    • Does the destination address contain an instruction which is valid encoding for the current instruction set?
    • If you are using ARM and Thumb code make sure it is all compiled for "interworking"
    • Check the Fault Status Register in CP15 - this will give more info about why an abort was raised.
    What platform are you running on, and which core revision?

    Cheers,
    Iso
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