MC68HC705JP7 Datasheet, PDF (47/242 Pages) Motorola, Inc – HCMOS Microcontroller Unit

Freescale Semiconductor, Inc.
Central Processor Unit (CPU)
Index Register
3.4 Index Register
The index register is a general-purpose 8-bit register as shown in
Figure 3-3. In the indexed addressing modes, the CPU uses the byte in
the index register to determine the conditional address of the operand.
The 8-bit index register can also serve as a temporary data storage
location.
Bit 7
Read:
Write:
Reset:
6
5
4
3
2
Unaffected by reset
Figure 3-3. Index Register (X)
1
Bit 0
3.5 Stack Pointer
The stack pointer is a 16-bit register that contains the address of the next
location on the stack as shown in Figure 3-4. During a reset or after the
reset stack pointer (RSP) instruction, the stack pointer initializes 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
Bit
15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
Read:
0000000011
Write:
Reset: 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1
Figure 3-4. 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.
MC68HC705JJ7 • MC68HC705JP7 — REV 4
MOTOROLA
Central Processor Unit (CPU)
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