MC68HC705KJ1 Datasheet, PDF (35/108 Pages) Motorola, Inc – HCMOS Microcontroller Unit

4.5 CPU Registers
The M68HC05 CPU contains five registers that control and monitor MCU operation:
• Accumulator
• Index register
• Stack pointer
• Program counter
• Condition code register
CPU registers are not memory mapped.
CPU Registers
4.5.1 Accumulator
The accumulator is a general-purpose 8-bit register. The CPU uses the accumulator to hold operands and
results of ALU operations.
Read:
Write:
Reset:
Bit 7
6
5
4
3
2
Unaffected by reset
Figure 4-2. Accumulator (A)
1
Bit 0
4.5.2 Index Register
In the indexed addressing modes, the CPU uses the byte in the index register to determine the conditional
address of the operand. The index register also can serve as a temporary storage location or a counter.
Read:
Write:
Reset:
Bit 7
6
5
4
3
2
Unaffected by reset
Figure 4-3. Index Register (X)
1
Bit 0
4.5.3 Stack Pointer
The stack pointer is a 16-bit register that contains the address of the next location on the stack. During a
reset 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)
MC68HC705KJ1 • MC68HRC705KJ1 • MC68HLC705KJ1 Data Sheet, Rev. 4.1
Freescale Semiconductor
35