---

Using B (branch always) instruction to implement subroutine ???

---

• Implement Subroutine
  • Consider:

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

    We can implement the subroutine call as follows:

    main: .... .... b A Ret: ... ... A: .... .... .... b Ret

  ---

  ---

  ---

  ---

• Why the "branch always" instruction 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: .... .... b A Ret1: ... ... b A Ret2: ... ... ... A: .... .... .... b Ret1

    Problem:

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

  ---

  ---

  ---

  ---

• 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

    Just like Hansel and Gretel ( click here ) who tracked into a forrest, they left a trail of bread crumbs to help them get back home: When you call a method/function, you must leave a return address to indicate where you need to go back to when the method/function has completed execution !!!