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Saving registers in an interrupted computation in the program stack

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• Review: interrupted computation
  • Recall this program (found on this webpage: click here) that computes:

    result = square(3) + square(4);

    The following naive code does not work:

    // result = square(3) /* ------------------------------------------------- Call square(3) ------------------------------------------------- */ mov r0, #3 bl square mov r1, r0 // Save 32 in r1 *** FAILED !!! /* ------------------------------------------------- Call square(4) ------------------------------------------------- */ mov r0, #4 bl square add r0, r0, r1 // Computes 32+ 42 /* ------------------------------------------------- Store in result ------------------------------------------------- */ movw r1, #:lower16:result movt r1, #:upper16:result str r0, [r1]

    Because: the addition was interrupted by a function call

    This naive code assumes that the square subroutine will preserve the saved value in register r1 !!! Consequently: This code will not work if the square subroutine uses register r1 in its computation

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• Solution: save the value in a place where other function will not change
  • Fact:

    Values saved on the program stack are used only by the function that has allocated (= reserve) the space in the stack

    Therefore:

    We can save register values on the stack and no function will change the value But I must add that: unless your program contains a bug, (in that case, your bug in the program can change values store in the program stack....

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  • How I can show you the solution to compute:

    result = square(3) + square(4);

    The following modified code save register r0 on the program stack and retrieve the value after the function call to continue with the (interrupted) computation:

    // result = square(3) /* ------------------------------------------------- Call square(3) ------------------------------------------------- */ mov r0, #3 bl square push {r0} // Save 32 in stack *** !!! /* ------------------------------------------------- Call square(4) ------------------------------------------------- */ mov r0, #4 bl square pop {r1} // Retrieve 32 from stack *** !!! add r0, r0, r1 // Computes 32+ 42 /* ------------------------------------------------- Store in result ------------------------------------------------- */ movw r1, #:lower16:result movt r1, #:upper16:result str r0, [r1]

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  • Example Program: (Demo above code)                                                
    • Use the prog file: /home/cs255001/demo/asm/reg-problem.s

      Change the program into the above code

    How to run the program:

    To compile:   as255 reg-problem          To run: use EGTAPI