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Intro: using CPU registers to pass parameters and return values

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• Subroutine and registers
  • Important Facts:

    Registers can be used by any instruction in the computer program Therefore: Any register can be used by any subroutine !!!          Subroutines can be written by different programmers (people) Case in point: you use library methods in the Java library that was written by someone else !!! Therefore: There is no coordination between different subroutines on register assignments !!! The general practice is: Every subroutine will assume that it can use any register in the CPU in its execution

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• Pre-condition to using registers to store parameters and local variables
  • Safety condition:

    The subroutine must be a leaf function (that does not call any function)

    If the subroutine is not a leaf function, then its values in all or any register can be lost when it invokes another subroutine !!

    But if the subroutine does not call any subroutine, it can safely use all register to store the parameters and local variable.

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• Principle in using registers to pass parameters
  • Assumption:

    A method assumes that all registers are available for its own use         Reason: See above !!! Because different methods may be written by different programmers, you cannot know which registers will be used and updated by a method that is not written by yourself. So you can never leave any important value in a register when you call another method --- because when the call returns, values in any register may/could have been changed (and you will lose the value) !!! So when you write a subroutine, you will assume that the caller has saved all the important values in the registers and no register contains any value that will be needed. I.e.: you can use all general purpose registers.

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  • What happens when you use a register to pass a parameter:

    The called method (= the "callee") will reserve some registers to store parameters The caller method (= the "caller") must copy the parameter value(s) into the reserved register(s) The called method (= the "callee") must use the reserved register(s) as the specific parameter of the method

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• Returning the value of a methd/function
  • As you well know from CS170, Java methods can return values.

    Example: a method that returns an int typed value

    public int sum(int x, int y) { ... return (return_value); }

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  • Important fact:

    In Java, the value returned by a method is always a data type that can be stored in a register Example: The fundamental data types (int, short, byte, float, char) are equal to or less than 4 bytes and can be stored in a register (A double can also be stored in a floating point register of the ARM processor --- we did not discuss these registers for brevity When a method returns an object, it actually returns the address of an object Addresses can be stored in a register

    Therefore:

    Return values of methods can always be stored in a register

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  • What happens when you use a register to return a function value:

    The called method (= the "callee") must put the return value in the agreed register before the called method (= the "callee") returns The caller method (= the "caller") must use/copy the value from the agreed register in expressions that use the function call This agreed register is usually the register r0 of the ARM processor         (But in principle, you can use any general purpose register to return the function value)