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Chapter 7 Central Processor Unit (S08CPUV2)

MC9S08QD4 Series MCU Data Sheet, Rev. 6

Freescale Semiconductor

7.2

Programmer’s Model and CPU Registers

Figure 7-1 shows the five CPU registers. CPU registers are not part of the memory map.

Figure 7-1. CPU Registers

7.2.1

Accumulator (A)

The A accumulator is a general-purpose 8-bit register. One operand input to the arithmetic logic unit

(ALU) is connected to the accumulator and the ALU results are often stored into the A accumulator after

arithmetic and logical operations. The accumulator can be loaded from memory using various addressing

modes to specify the address where the loaded data comes from, or the contents of A can be stored to

memory using various addressing modes to specify the address where data from A will be stored.

Reset has no effect on the contents of the A accumulator.

7.2.2

Index Register (H:X)

This 16-bit register is actually two separate 8-bit registers (H and X), which often work together as a 16-bit

address pointer where H holds the upper byte of an address and X holds the lower byte of the address. All

indexed addressing mode instructions use the full 16-bit value in H:X as an index reference pointer;

however, for compatibility with the earlier M68HC05 Family, some instructions operate only on the

low-order 8-bit half (X).

Many instructions treat X as a second general-purpose 8-bit register that can be used to hold 8-bit data

values. X can be cleared, incremented, decremented, complemented, negated, shifted, or rotated. Transfer

instructions allow data to be transferred from A or transferred to A where arithmetic and logical operations

can then be performed.

For compatibility with the earlier M68HC05 Family, H is forced to 0x00 during reset. Reset has no effect

on the contents of X.

CONDITION CODE REGISTER

CARRY

ZERO

NEGATIVE

INTERRUPT MASK

HALF-CARRY (FROM BIT 3)

TWO’S COMPLEMENT OVERFLOW

ACCUMULATOR

INDEX REGISTER (LOW)

INDEX REGISTER (HIGH)

STACK POINTER

PROGRAM COUNTER

16-BIT INDEX REGISTER H:X

CCR

V1 1 H I N Z

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