HC05 datasheet(150/232 Pages) MOTOROLA

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MOTOROLA

11-2

MC68HC05X16

Rev. 1

CPU CORE AND INSTRUCTION SET

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 MC68HC05X32 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)

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.

Figure 11-2 Stacking order

Condition code register

Accumulator

Index register

Program counter high

Program counter low

Stack

Unstack

Decreasing

memory

address

Increasing

memory

address

148

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