mc68hc08ln56 Freescale Semiconductor, Inc, mc68hc08ln56 Datasheet - Page 35

• Manufacturer Part Number: mc68hc08ln56
• Description: M68hc08 Microcontrollers
• Manufacturer: Freescale Semiconductor, Inc

3.3.3 Stack Pointer
The stack pointer (SP) 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.
3.3.4 Program Counter
The program counter (PC) 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.
Freescale Semiconductor
Reset:
Reset:
Read:
Read:
Write:
Write:
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.
PC
SP
MC68HC08LN56 • MC68HC708LN56 General Release Specification, Rev. 2.1
Bit
15
Bit
15
0
14
14
0
13
13
0
Figure 3-5. Program Counter (PC)
12
12
0
Figure 3-4. Stack Pointer (SP)
Loaded with vector from $FFFE and $FFFF
11
11
0
10
10
0
9
0
9
NOTE
8
0
8
7
1
7
6
1
6
5
1
5
4
1
4
3
1
3
2
1
2
1
1
1
Bit
Bit
0
1
0
CPU Registers
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