User Stacks - Motorola CPU32 Reference Manual

Ell
To keep data on the system stack aligned properly, data entry on the stack is
restricted so that data is always put in the stack on a word boundary. Thus, byte
data is pushed on or pulled from the system stack in the high-order half of the
word; the low-order half is unchanged.
3.7.2 User Stacks
The user can implement stacks with the address register indirect with
postincrement and predecrement addressing modes. With address register An
(n
=
0 to 6), the user can implement a stack that is filled either from high to low
memory or from low to high memory. Important considerations are as follows:
• 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 SP 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 scheme is illustrated as follows
LOW MEMORY
(FREE)
An
~ TOP OF STACK
' 7
·
l
·
L
·
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.
In this case, after either a push or pull operation, register An points to the next
available space on the stack. This scheme is illustrated as follows:
MOTOROLA
3-20
DATA ORGANIZATION AND CPU32 REFERENCE MANUAL
ADDRESSING CAPABILITIES