M68ICS08GR datasheet(142/408 Pages) FREESCALE

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Central Processing Unit (CPU)

Technical Data

MC68HC908GR8 — Rev 4.0

142

Central Processing Unit (CPU)

MOTOROLA

10.4.3 Stack pointer (SP)

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 zero ($0000 to $00FF) frees direct

address (page zero) space. For correct operation, the stack pointer must

point only to RAM locations.

10.4.4 Program counter (PC)

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.

Bit

8765

4321

Bit

Read:

Write:

Reset:

000

0000

0111

1111

Figure 10-4. Stack pointer (SP)

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