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Central Processor Unit (CPU)
7.4.2 Index Register
7.4.3 Stack Pointer
Technical Data
92
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.
Bit
14
13
12
15
Read:
Write:
Reset:
0
0
0
0
X = Indeterminate
Figure 7-3. Index Register (H:X)
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.
Central Processor Unit (CPU)
11
10
9
8
7
6
0
0
0
0
X
X
Bit
5
4
3
2
1
X
X
X
X
X
MC68HC908AB32
Rev. 1.0
—
MOTOROLA
0
X
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