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Prelude to using register to pass parameters and store local variables

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• The main caveat in using registers to store parameters and local variables
  • Introduction:

    Although a computer program is one unit, but:

    a computer program is logically partitioned into multiple units called subroutines/methods to reduce the complexity (Humans can't handle things that are too large) By dividing up a complex program into units (subroutines) that perform specific (and more simpler) tasks, we can reduce the program complexity and write bug-free programs

    So, although subroutines are in the same program, you must view each subroutine as a separate and independent execution unit

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  • All subroutines (must) use in their computations:

    Each subroutine will need to:

    Use the CPU register to perform computations (because that's how the CPU operates !)

    It is important that you understand that the CPU registers are resources shared by all subroutines:

    Each subroutine can access all the CPU registers

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  • Main caveat in using registers:

    The value in any register used by a subroutine A can be changed after A calls a subroutine B !!!

    Example:

    main set the register r0 to 4 and call subroutine subroutine1 that changes register r0: When subroutine subroutine1 returns to main, the register r0 will have changed !!!

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    This can happen with any subroutine call.

    Another example:

    subroutine1 set the register r0 to 7 and call subroutine subroutine2 that changes register r0: When subroutine subroutine2 returns to subroutine1, the register r0 will have changed !!!

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  • Moral (programming rule) of this story:

    If a sub-program has an important value stored in a register, then: This sub-program cannot call make any subroutine call without first saving the value in the register (Otherwise, the value will be lost !!!)

    We will learn how to save value in registers later

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  • But the above rule can help use understand that we can use registers to store parameters and local variables for leaf functions (that do not call any function)

    storing parameters and local variables is simple and efficient, we will discuss this technique first.