MC68HC908QY2 MOTOROLA [Motorola, Inc], MC68HC908QY2 Datasheet - Page 62

• Manufacturer Part Number: MC68HC908QY2
• Description: Microcontrollers
• Manufacturer: MOTOROLA [Motorola, Inc]

Central Processor Unit (CPU)
6.4.3 Stack Pointer
62
MC68HC908QY4•MC68HC908QT4•MC68HC908QY2•MC68HC908QT2•MC68HC908QY1•MC68HC908QT1
NOTE:
The index register can serve also as a temporary data storage location.
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.
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.
Reset:
Reset:
Read:
Read:
Write:
Write:
X = Indeterminate
Bit
15
Bit
15
0
0
14
14
Central Processor Unit (CPU)
0
0
13
13
0
0
Figure 6-3. Index Register (H:X)
Figure 6-4. Stack Pointer (SP)
12
12
0
0
11
11
0
0
10
10
0
0
9
0
9
0
8
0
8
0
7
X
7
1
6
X
6
1
5
X
5
1
X
4
4
1
X
3
3
1
MOTOROLA
X
2
2
1
X
1
1
1
Bit
Bit
X
0
0
1