M68HC08 datasheet(39/172 Pages) FREESCALE

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CPU Registers

MC68HC908JL3E Family Data Sheet, Rev. 3

Freescale Semiconductor

4.3.3 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.

4.3.4 Program Counter

The program counter is a 16-bit register that contains the address of the next instruction or operand to be

fetched.

Normally, the program counter automatically increments to the next sequential memory location every

time an instruction or operand is fetched. Jump, branch, and interrupt operations load the program

counter with an address other than that of the next sequential location.

During reset, the program counter is loaded with the reset vector address located at $FFFE and $FFFF.

The vector address is the address of the first instruction to be executed after exiting the reset state.

Bit

987654321

Bit 0

Read:

Write:

Reset:

00000000

XXXXXXXX

X = Indeterminate

Figure 4-3. Index Register (H:X)

Bit

987654321

Bit 0

Read:

Write:

Reset:

0000000011111111

Figure 4-4. Stack Pointer (SP)

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