MC908QT4MPE datasheet(62/184 Pages) FREESCALE

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Central Processor Unit (CPU)

MC68HC908QY/QT Family Data Sheet, Rev. 5

Freescale Semiconductor

Figure 7-1. CPU Registers

7.3.1 Accumulator

The accumulator is a general-purpose 8-bit register. The CPU uses the accumulator to hold operands and

the results of arithmetic/logic operations.

7.3.2 Index Register

The 16-bit index register allows indexed addressing of a 64-Kbyte memory space. H is the upper byte of

the index register, and X is the lower byte. H:X is the concatenated 16-bit index register.

In the indexed addressing modes, the CPU uses the contents of the index register to determine the

conditional address of the operand.

The index register can serve also as a temporary data storage location.

Bit 7

654321

Bit 0

Read:

Write:

Reset:

Unaffected by reset

Figure 7-2. Accumulator (A)

Bit

987654321

Bit

Read:

Write:

Reset:

00000000

XXXXXXXX

X = Indeterminate

Figure 7-3. Index Register (H:X)

ACCUMULATOR (A)

INDEX REGISTER (H:X)

STACK POINTER (SP)

PROGRAM COUNTER (PC)

CONDITION CODE REGISTER (CCR)

CARRY/BORROW FLAG

ZERO FLAG

NEGATIVE FLAG

INTERRUPT MASK

HALF-CARRY FLAG

TWO’S COMPLEMENT OVERFLOW FLAG

V1 1 H

N Z C

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