mc68hc908ap8cfa Freescale Semiconductor, Inc, mc68hc908ap8cfa Datasheet - Page 57

• Manufacturer Part Number: mc68hc908ap8cfa
• Description: M68hc08 Microcontrollers
• Manufacturer: Freescale Semiconductor, Inc

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.
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.
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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.
Reset:
Reset:
Read:
Read:
Write:
Write:
X = Indeterminate
Bit
15
Bit
15
0
0
14
14
0
0
13
13
0
0
MC68HC908AP Family Data Sheet, Rev. 4
Figure 4-3. Index Register (H:X)
12
12
0
0
Figure 4-4. Stack Pointer (SP)
11
11
0
0
10
10
0
0
NOTE
9
0
9
0
8
0
8
0
X
7
7
1
6
X
6
1
X
5
5
1
4
X
4
1
X
3
3
1
2
X
2
1
X
1
1
1
Bit 0
Bit 0
X
1
CPU Registers
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