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Using BRA instruction to implement subroutine

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• Implement Subroutine
  • Consider:

    main( ) { ... A( ); // Execute instructions in A and continue ... } A( ) { .... }

    We can implement the subroutine call as follows:

    main: .... .... BRA A Ret: ... ... A: .... .... .... BRA Ret

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• WHy BRA will not work !
  • Consider now:

    main( ) { ... A( ); // Execute instructions in A and continue ... A( ); // Execute instructions in A and continue } A( ) { .... }

    The implementation would be as follows:

    main: .... .... BRA A Ret1: ... ... BRA A Ret2: ... ... ... A: .... .... .... BRA Ret1

    Problem:

    BRA Ret1 will jump to the wrong continuation location in the second invocation !!!

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• Lesson learned
  • Lesson:

    The subroutine call (to run code in the subroutine) must remember the location of the subroutine invocation !!! The subroutine must return (= go back) to the location where the subroutine call instruction was executed

    Analogy: Hansel and Gretel