68HC705SP7 datasheet(43/230 Pages) FREESCALE

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Central Processor Unit (CPU)

Condition Code Register (CCR)

MC68HC705JJ7/MC68HC705JP7 — Rev. 3.0

General Release Specification

Central Processor Unit (CPU)

Half-Carry Flag (H)

The CPU sets the half-carry flag when a carry occurs between bits 3

and 4 of the accumulator during an ADD or ADC operation. The

half-carry flag is required for binary coded decimal (BCD) arithmetic

operations. Reset has no affect on the half-carry flag.

Interrupt Mask (I)

Setting the interrupt mask disables interrupts. If an interrupt request

occurs while the interrupt mask is a logic zero, the CPU saves the

CPU registers on the stack, sets the interrupt mask, and then fetches

the interrupt vector. If an interrupt request occurs while the interrupt

mask is set, the interrupt request is latched. The CPU processes the

latched interrupt as soon as the interrupt mask is cleared again.

A return from interrupt (RTI) instruction pulls the CPU registers from

the stack, restoring the interrupt mask to its cleared state. After a

reset, the interrupt mask is set and can be cleared only by a CLI

instruction.

Negative Flag (N)

The CPU sets the negative flag when an arithmetic operation, logical

operation, or data manipulation produces a negative result. Reset has

no affect on the negative flag.

Zero Flag (Z)

The CPU sets the zero flag when an arithmetic operation, logical

operation, or data manipulation produces a result of $00. Reset has

no affect on the zero flag.

Carry/Borrow Flag (C)

The CPU sets the carry/borrow flag when an addition operation

produces a carry out of bit 7 of the accumulator or when a subtraction

operation requires a borrow. Some logical operations and data

manipulation instructions also clear or set the carry/borrow flag. Reset

has no affect on the carry/borrow flag.

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