MC68HC705P9 Datasheet, PDF (47/160 Pages) Motorola, Inc – HCMOS Microcontroller Unit

Freescale Semiconductor, Inc.
Central Processor Unit (CPU)
CPU Registers
4.6.3 Stack Pointer
The stack pointer is a 16-bit register that contains the address of the next
stack location to be used. 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.
Bit
Bit
15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
Read: 0 0 0 0 0 0 0 0 1 1
Write:
Reset: 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1
= Unimplemented
Figure 4-4. Stack Pointer (SP)
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.
MC68HC705P9 — Rev. 4.0
MOTOROLA
Central Processor Unit (CPU)
For More Information On This Product,
Go to: www.freescale.com
Technical Data
47