Motorola M68000 User's Guide download pdf (Page 106)

92 Assembly Language Programming for the 68000 Family

To retrieve elements from the stack, we use a MOVE instruction with ad

dress register indirect with postincrement addressing. The general form

of this addressing mode is (An)+ and (SP)+ for the stack pointer. The

use of this addressing mode is the reverse of register indirect with pre

decrement addressing and results in popping the topmost value off the

stack. The data element pointed to by the stack pointer, SP, is obtained

from the stack and placed into the destination operand of the MOVE

instruction. The stack pointer is then incremented by 2 or 4. If we perform a

MOVE.W (SP)+,D0

on the above stack, the value 5555 is placed in the DO register. The stack

then looks like this:

HIGH ADDRESS

D0 <

5555

LOW ADDRESS

Notice that the value 5555 has not been changed on the stack. However, it

is below the stack pointer and is therefore no longer relevant. You might

be tempted to try to access this value by modifying the stack pointer

directly, perhaps with a SUB instruction. This is not a good idea. In fact, if

hardware interrupts are enabled, it might be changed without your being

able to predict just when (hardware interrupts will be covered in Chapter

12). Always regard data below the stack pointer as lost forever.

What happens if we pop one too many elements from the stack? Since

SP-

1234

5555

1 2 ... 101 102 103 104 105 106 107 108 109 110 111 ... 255 256

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Motorola M68000 User's Guide Page 106