MC68HC05X4 Datasheet, PDF (29/156 Pages) Motorola, Inc – HCMOS Microcontroller Unit

Freescale Semiconductor, Inc.
CPU
CPU Registers
Reset:
Bit 7
6
5
4
3
2
Unaffected by reset
Figure 2. Accumulator
1
Bit 0
Index Register
In the indexed addressing modes, the CPU uses the byte in the index
register to determine the conditional address of the operand.
Reset:
Bit 7
6
5
4
3
2
Unaffected by reset
Figure 3. Index Register
1
Bit 0
The 8-bit index register can also serve as a temporary data storage
location.
Stack Pointer
The stack pointer 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
(RSP) instruction, the stack pointer is preset to $00FF. The address in
the stack pointer decrements as data is pushed onto the stack and
increments as data is pulled from the stack.
Bit 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 Bit 0
0000000011
Reset 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1
Figure 4. Stack Pointer
The ten most significant bits of the stack pointer are permanently fixed
at 000000011, 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.
3-cpu
MOTOROLA
CPU
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MC68HC05X4 Rev 1.0
29