XC908AS60ACFU Motorola Semiconductor Products, XC908AS60ACFU Datasheet - Page 132

• Manufacturer Part Number: XC908AS60ACFU
• Description: MC68HC908AZ60A, MC68HC908AS60A Hcmos Microcontroller Unit Technical Data
• Manufacturer: Motorola Semiconductor Products

Central Processor Unit (CPU)
8.4.3 Stack pointer (SP)
8.4.4 Program counter (PC)
Technical Data
132
NOTE:
SP
Reset:
Read:
Write:
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.
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.
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
15
0
14
0
Central Processor Unit (CPU)
13
0
Figure 8-4. Stack pointer (SP)
12
0
11
0
10
0
9
0
8
0
7
1
6
1
MC68HC908AZ60A — Rev 2.0
5
1
4
1
3
1
2
1
MOTOROLA
1
1
Bit
0
1