Datasheet

MC68HC908QY4•MC68HC908QT4•MC68HC908QY2•MC68HC908QT2•MC68HC908QY1•MC68HC908QT1
62 Central Processor Unit (CPU) MOTOROLA
Central Processor Unit (CPU)
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The index register can serve also as a temporary data storage location.
6.4.3 Stack Pointer
The stack pointer is a 16-bit register that contains the address of the next
location on the stack. During a reset, the stack pointer is preset to
$00FF. The reset stack pointer (RSP) instruction sets the least
significant byte to $FF and does not affect the most significant byte. The
stack pointer decrements as data is pushed onto the stack and
increments as data is pulled from the stack.
In the stack pointer 8-bit offset and 16-bit offset addressing modes, the
stack pointer can function as an index register to access data on the
stack. The CPU uses the contents of the stack pointer to determine the
conditional address of the operand.
NOTE: The location of the stack is arbitrary and may be relocated anywhere in
RAM. Moving the SP out of page 0 ($0000 to $00FF) frees direct
address (page 0) space. For correct operation, the stack pointer must
point only to RAM locations.
Bit
15
1413121110987654321
Bit
0
Read:
Write:
Reset:00000000XXXXXXXX
X = Indeterminate
Figure 6-3. Index Register (H:X)
Bit
15
1413121110987654321
Bit
0
Read:
Write:
Reset:0000000011111111
Figure 6-4. Stack Pointer (SP)