M68HC08 datasheet(78/332 Pages) FREESCALE

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Central Processor Unit (CPU)

Technical Data

MC68HC908JB16 — Rev. 1.1

Central Processor Unit (CPU)

Freescale Semiconductor

6.4.2 Index Register

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.

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.

Bit

987654321

Bit

Read:

Write:

Reset:

00000000

XXXXXXXX

X = Indeterminate

Figure 6-3. Index Register (H:X)

Bit

987654321

Bit

Read:

Write:

Reset:

0000000011111111

Figure 6-4. Stack Pointer (SP)

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