.TITLE   AddVersion2
;  Program to calculate the expression S = A + B
A:      .LONG    3                ; Initialize A
B:      .LONG    7                ; Initialize B
S:      .BLKL    1                ; Save room for S

;  Subroutine Add(X, Y, Sum) for calculating Sum = X + Y
;    Input:    X, Y;  Passed by reference on stack
;    Output:   Sum;   Passed by reference on stack
;    Destroys contents of R0, R1, R2, and R3

Add:    POPL     R3               ; Pop return address into R3
        POPL     R0               ; Move address of X into R0
        POPL     R1               ; Move address of Y into R1
        POPL     R2               ; Move address of Sum into R2
        ADDL3    (R0), (R1), (R2) ; Sum <-- X + Y
        PUSHL    R3               ; Push return address
        RSB                       ; Return from subroutine

;  The main program:

        .Entry   ByReferenceInStack, 0
;    Call Add (A, B, S)
        PUSHAL   S                ; Push address of S
        PUSHAL   B                ; Push address of B
        PUSHAL   A                ; Push address of A
        BSBB     Add              ; Call the subroutine
        $EXIT_S
        .END     ByReferenceInStack