Motorola M68000 Betriebsanweisung PDF herunterladen (Seite 110)

96 Assembly Language Programming for the 68000 Family

What happens if we do use a character code that appears in the string?

When we try to reverse the string, we will terminate the reverse loop

prematurely. This will leave data on the stack that should have been

removed. The consequences of this bad stack depend on the rest of the

program. The very next pop that is performed will result in the wrong

value. Since returns from procedures also use the stack, we can have

disastrous results.

In the next chapter we will discuss subroutines. The stack will be an

extremely important element in the implementation and use of subrou

tines.

Exercises

1. Which register is used as the hardware stack pointer?

2. Do some instructions implicitly reference the stack?

3. Does the 68000 stack grow upward or downward in memory?

4. What addressing mode is used to push data onto the stack?

5. What addressing mode is used to pop data from the stack?

6. Write the instructions necessary to push the value 100 onto the stack.

7. Write the instructions necessary to push the DO register onto the stack

and pop it into the Dl register.

8. Can you access data below the stack pointer?

9. Write the instruction to place the address of variable COUNT onto

the stack.

10. Write the instructions necessary to push the values 1 to 10 onto the

stack.

Answers

1. A7.

2. Yes, the JSR is one example.

3. Downward.

4. Address register indirect with predecrement.

5. Address register indirect with postincrement.

6. h o v e .L #100,-(SP)

7. HOVE• L DO r - (SP)

HOVE.L (SP)+,D1

8. Yes, but you can’t be sure of what it is.

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Motorola M68000 Betriebsanweisung Seite 110