MC68030 Datasheet, PDF (76/602 Pages) Motorola, Inc – ENHANCED 32-BIT MICROPROCESSOR

Data Organization and Addressing Capabilities
2.8.2 User Program Stacks
The user can implement stacks with the address register indirect with postincrement and
predecrement addressing modes. With address register An (n = 0–6), the user can
implement a stack that is filled wither from high to low memory or from low to high memory.
Important considerations are:
• Use the predecrement mode to decrement the register before its contents are used as
the pointer to the stack.
• Use the postincrement mode to increment the register after its contents are used as the
pointer to the stack.
• Maintain the stack pointer correctly when byte, word, and long-word items are mixed in
these stacks.
To implement stack growth from high to low memory, use:
–(An) to push data on the stack,
(An)+ to pull data from the stack.
For this type of stack, after either a push or a pull operation, register An points to the top item
on the stack. This is illustrated as:
LOW MEMORY
An
(FREE)
TOP OF STACK
BOTTOM OF STACK
HIGH MEMORY
To implement stack growth from low to high memory, use:
(An)+ to push data on the stack,
–An to pull data from the stack.
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MC68030 USER’S MANUAL
MOTOROLA