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Of exceptions and aborts..

Felix Varghese
Note: This was originally posted on 21st January 2009 at http://forums.arm.com

The ARM reference manual specifies sometimes MOV PC,R14 and sometimes SUBS PC,R14, #4 for returning from exceptions. But they have not mentioned why. Can somebdy expain?
Also, when exactly does the PC get incremented? During the Fetch ?
And exactly when (Fetching / Decoding / Executing) does an exception get generated?
Does an undefined instruction get generated when the instruction is decoded or when it is executed?
What about Data and Prefetch aborts?
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  • 0
    Note: This was originally posted on 9th February 2009 at http://forums.arm.com

    I was wrong about the effect of the instruction, and for some exceptions the instruction will return to the instruction that caused the exception allowing it to be reexecuted.

    Exactly. The value saved in LR seems to be inconsistent across exceptions. Which is the source of all my confusion!
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  • 0
    Note: This was originally posted on 9th February 2009 at http://forums.arm.com

    I was wrong about the effect of the instruction, and for some exceptions the instruction will return to the instruction that caused the exception allowing it to be reexecuted.

    Exactly. The value saved in LR seems to be inconsistent across exceptions. Which is the source of all my confusion!
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