Subroutines and the stack · 68000 assembly

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Subroutines and the stack

Following the stack, writing subroutines that preserve registers, passing parameters, and finding what unbalances the stack. Chapter 5.

Exercise · Following the stack

SP is $8000. D0.W is $0011, D1.W is $0022. Give SP and the contents of the stack after each line, then the values of D2 and D3 at the end.

1
2
3
4
5
6        MOVE.W  D0,-(SP)
        MOVE.W  D1,-(SP)
        MOVE.L  D0,-(SP)
        ADDQ.L  #4,SP
        MOVE.W  (SP)+,D2
        MOVE.W  (SP)+,D3

Exercise · Find the mistake

This subroutine is meant to return in D0.W twice the word pointed to by A0, while preserving D1. Why does the program crash on return? Fix it.

DOUBLE  MOVE.W  D1,-(SP)
        MOVE.W  (A0),D1
        ADD.W   D1,D1
        MOVE.W  D1,D0
        RTS