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Using the stack to pass parameters and accessing the variable using the stack pointer

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• Example: passing parameter on the system stack
  • Example "main" and subroutine "sum" in high-level language:

    main: int a, b, c; c = sum(a, b);

    int sum(int x, int y) { x = x + 1; y = y + 1; return ( x*x + y*y ); }

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  • "main" in assembler language:

    main: move.l a, -(a7) * Pass a on stack move.l b, -(a7) * Pass b on stack bsr sum * Call subroutine sum adda.l #8, a7 * remove a and b from stack move.l d0, c * put return value in c

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  • Stack content when subroutine "sum" begins execution:

    +---------------+ | free space | +---------------+ <----- a7 (stack pointer) 0(a7) | return address| +---------------+ 4(a7) | b | parameter y (has the value b) +---------------+ 8(a7) | a | parameter x (has the value a) +---------------+ | ..... |

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  • Subroutine "sum" in assembler language:
    Pay special attention to how a7 is used to access the parameters on the stack !!!

    The subroutine returns the value in register D0.

    sum: move.l 8(a7), d0 * puts first parameter x in d0 add.l #1, d0 move.l d0, 8(a7) * puts x+1 back into the param var x move.l 4(a7), d0 * puts 2nd parameter y in d0 add.l #1, d0 move.l d0, 4(a7) * puts y+1 back into the param var y move.l 8(a7), d0 * puts first parameter x in d0 muls d0, d0 * now d0 = x*x move.l 4(a7), d1 * puts second parameter y in d1 muls d1, d1 * now d1 = y*y add.l d1, d0 * now d0 = x*x + y*y, in agreed return location rts

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  • Example Program: (Demo above code)                                                
    • Prog file: click here

    NOTE:

    When you run the program, pay attention to the fact that the parameters (a and b) are still on the stack when the subroutine sum returns.

    That's why we need:

    main: move.l a, -(a7) move.l b, -(a7) bsr sum adda.l #8, a7 * remove (pop) a and b from stack move.l d0, c * put return value in variable c

    • The instruction "adda.l #8, a7" remove the parameters off the stack