MC68HC05L28 Datasheet, PDF (112/156 Pages) Motorola, Inc – Flexible general-purpose microcomputer

7
Increasing
memory
address
Unstack
Condition code register
Accumulator
Index register
Program counter high
Program counter low
0 ^Stack
Decreasing
memory
address
Figure 11-2 Stacking order
11.1.2 Index register (X)
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 Program counter (PC)
The program counter is a 16-bit register, which contains the address of the next byte to be fetched.
Although the M68HC05 CPU core can address 64K bytes of memory, the actual address range of
the MC68HC05L28 is limited to 16K bytes. The two most significant bits of the program counter
are therefore not used and are permanently set to zero.
11.1.4 Stack pointer (SP)
11
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.
11.1.5 Condition code register (CCR)
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.
MOTOROLA
11-2
CPU CORE AND INSTRUCTION SET
TPG
MC68HC05L28