MC68HC705JJ7 Datasheet, PDF (30/164 Pages) Freescale Semiconductor, Inc – Microcontrollers

Central Processor Unit (CPU)
3.3 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
3.4 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.
3.5 Program Counter
The program counter is a 16-bit register that contains the address of the next instruction or operand to be
fetched as shown in Figure 3-5. The three most significant bits of the program counter are ignored
internally and appear as 111 during stacking and subroutine calls.
Normally, the address in the program counter automatically increments to the next sequential memory
location every time an instruction or operand is fetched. Jump, branch, and interrupt operations load the
program counter with an address other than that of the next sequential location.
Bit
Bit
15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
Read:
111
Write:
Reset: 0 0 0
Loaded with vector from $1FFE and $1FFF
Figure 3-5. Program Counter (PC)
MC68HC705JJ7 • MC68HC705JP7 Advance Information Data Sheet, Rev. 4.1
30
Freescale Semiconductor