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Sequencing of events in function call + return with parameters and return values

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• Review: how to call a leaf-function
  • Now that we have established that we can only use registers to pass parameters and return values in a leaf function, let's recall how we call a leaf function:

    main: .... .... bl A // Call leaf function A .... .... Stop: /* =============================== Leaf function =============================== */ A: .... .... mov pc, lr

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• When do we pass the parameters ?
  • The parameters must be passed:

    By the caller method        and        Before the caller executes the bl instruction !!!

    Schematically:

    main: .... pass parameters here bl A // Call leaf function A .... .... Stop: /* =============================== Leaf function =============================== */ A: .... .... mov pc, lr

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• When do we return a value to the caller ?
  • The return value must be returned (= saved):

    By the callee method        and        Before the callee executes the function return instruction (mov pc, lr or pop {pc}) !!!

    Schematically:

    main: .... pass parameters here bl A // Call leaf function A .... .... Stop: /* =============================== Leaf function =============================== */ A: .... .... store return value in register here mov pc, lr

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• When do we use the return value in the caller ?
  • The return value must be used:

    By the caller method        and        As soon as possible after the callee has returned to the caller !!

    Schematically:

    main: .... pass parameters here bl A // Call leaf function A use the return value here .... .... Stop: /* =============================== Leaf function =============================== */ A: .... .... store return value in register here mov pc, lr

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• Skeletal example (empty functions) on how to pass parameters and return values with registers (interactive demo in class)
  • The following example shows how to pass parameters and return values using empty functions (functions with no statements).

  • I will pass:

    Pass value 1111 as the first parameter in register r0 Pass value 2222 as the first parameter in register r1           Pass return value 9999 in register r0

    Example:

    main: mov r0, #1111 // Pass 1111 as parameter 1 in r0 mov r1, #2222 // Pass 2222 as parameter 2 in r1 bl A // Call leaf function A // When A returns, you will see the return value in r0 = 9999 nop nop Stop: nop /* =============================== Leaf function =============================== */ A: // When run in EGTAPI - you will see r0 = 1111 and r1 = 2222 nop nop mov r0, #9999 // Pass return value in register r0 mov pc, lr

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  • Example Program: (Demo above code)                                                
    • Prog file: /home/cs255001/demo/asm/8-sub/skel-reg-param.s

    How to run the program:

    To compile:   as255 skel-reg-param          To run: use EGTAPI