MC68HC705J MOTOROLA [Motorola, Inc], MC68HC705J Datasheet - Page 49

• Manufacturer Part Number: MC68HC705J
• Description: Microcontrollers
• Manufacturer: MOTOROLA [Motorola, Inc]

3.5.3 Stack Pointer
MC68HC705J1A — Rev. 4.0
MOTOROLA
Reset:
Read:
Write:
The stack pointer (SP) is a 16-bit register that contains the address of
the next location on the stack. During a reset or after the reset stack
pointer instruction (RSP), the stack pointer is preset to $00FF. The
address in the stack pointer decrements after a byte is stacked and
increments before a byte is unstacked.
The 10 most significant bits of the stack pointer are permanently fixed at
0000000011, so the stack pointer produces addresses from $00C0 to
$00FF. If subroutines and interrupts use more than 64 stack locations,
the stack pointer wraps around to address $00FF and begins writing
over the previously stored data. A subroutine uses two stack locations;
an interrupt uses five locations.
Bit
15
0
0
= Unimplemented
14
0
0
Central Processor Unit (CPU)
13
0
0
12
0
0
Figure 3-4. Stack Pointer (SP)
11
0
0
10
0
0
9
0
0
8
0
0
7
1
1
6
1
1
Central Processor Unit (CPU)
5
1
4
1
3
1
CPU Registers
Technical Data
2
1
1
1
Bit
0
1
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