Freescale Semiconductor, Inc.
Central Processor Unit (CPU)
CPU Registers
3.3.5 Condition Code Register
The condition code register (CCR) shown in Figure 3-6 is an 8-bit
register whose three most significant bits are permanently fixed at 111.
The condition code register contains the interrupt mask and four bits that
indicate the results of prior instructions.
Bit 7
1
6
1
5
1
4
3
I
2
1
Bit 0
C
Read:
Write:
Reset:
H
U
N
U
Z
U
1
1
1
1
U
= Unimplemented
U = Unaffected
Figure 3-6. Condition Code Register (CCR)
H — Half-Carry Bit
The CPU sets the half-carry flag when a carry occurs between bits 3
and 4 of the accumulator during an add without carry (ADD) or add
with carry (ADC) operation. The half-carry bit is required for
binary-coded decimal (BCD) arithmetic operations. Reset has no
affect on the half-carry flag.
I — Interrupt Mask Bit
Setting the interrupt mask (I) disables interrupts. If an interrupt
request occurs while the interrupt mask is a logic 0, 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,
STOP, or WAIT instruction.
MC68HC705C8A — Rev. 3
MOTOROLA
Technical Data
Central Processor Unit (CPU)
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