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Increasing
11.1.2
The index register is an 8-bit register, which can contain the indexed addressing value used to
create an effective address. The index register may also be used as a temporary storage area.
11.1.3
The program counter is a 16-bit register, which contains the address of the next byte to be fetched.
11.1.4
The stack pointer is a 16-bit register, which contains the address of the next free location on the
stack. During an MCU reset or the reset stack pointer (RSP) instruction, the stack pointer is set to
location $00FF. The stack pointer is then decremented as data is pushed onto the stack and
incremented as data is pulled from the stack.
When accessing memory, the ten most significant bits are permanently set to 0000000011. These
ten bits are appended to the six least significant register bits to produce an address within the
range of $00C0 to $00FF. Subroutines and interrupts may use up to 64 (decimal) locations. If 64
locations are exceeded, the stack pointer wraps around and overwrites the previously stored
information. A subroutine call occupies two locations on the stack; an interrupt uses five locations.
11
11.1.5
The CCR is a 5-bit register in which four bits are used to indicate the results of the instruction just
executed, and the fifth bit indicates whether interrupts are masked. These bits can be individually
tested by a program, and specific actions can be taken as a result of their state. Each bit is
explained in the following paragraphs.
Half carry (H)
This bit is set during ADD and ADC operations to indicate that a carry occurred between bits 3 and 4.
MOTOROLA
11-2
7
memory
address
Unstack
Figure 11-2 Stacking order
Index register (X)
Program counter (PC)
Stack pointer (SP)
Condition code register (CCR)
CPU CORE AND INSTRUCTION SET
Condition code register
Accumulator
Index register
Program counter high
Program counter low
Stack
0
Decreasing
memory
address
MC68HC05L1
TPG
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