mc68hc908jb16 Freescale Semiconductor, Inc, mc68hc908jb16 Datasheet - Page 78

• Manufacturer Part Number: mc68hc908jb16
• Description: M68hc08 Microcontrollers Microcontroller
• Manufacturer: Freescale Semiconductor, Inc

Central Processor Unit (CPU)
6.4.2 Index Register
6.4.3 Stack Pointer
Technical Data
78
The 16-bit index register allows indexed addressing of a 64K-byte
memory space. H is the upper byte of the index register, and X is the
lower byte. H:X is the concatenated 16-bit index register.
In the indexed addressing modes, the CPU uses the contents of the
index register to determine the conditional address of the operand.
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.
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
8
0
0
7
X
7
1
6
X
6
1
MC68HC908JB16
5
X
5
1
Freescale Semiconductor
4
X
4
1
3
X
3
1
2
2
X
1
—
Rev. 1.1
1
X
1
1
Bit
Bit
0
0
X
1