mc68hc05p18a Freescale Semiconductor, Inc, mc68hc05p18a Datasheet - Page 37

• Manufacturer Part Number: mc68hc05p18a
• Description: Mc68hc05p18a Hcmos Microcontroller Unit
• Manufacturer: Freescale Semiconductor, Inc

3.3.5 Condition Code Register
MC68HC05P18A
The condition code register (CCR) 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 flags that indicate the results of the
instruction just executed. The following paragraphs describe the
functions of the condition code register.
Half-Carry Flag
Interrupt Mask
Negative Flag
Reset:
Read:
Write:
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.
Setting the interrupt mask disables interrupts. If an interrupt request
occurs while the interrupt mask is 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. Normally, 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 any
reset, the interrupt mask is set and can be cleared only by a software
instruction.
The CPU sets the negative flag when an arithmetic operation, logical
operation, or data manipulation produces a negative result.
Freescale Semiconductor, Inc.
For More Information On This Product,
Bit 7
1
1
Central Processor Unit (CPU)
Figure 3-6. Condition Code Register (CCR)
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= Unimplemented
6
1
1
5
1
1
H
U
4
U = Unaffected
3
1
I
Central Processor Unit (CPU)
N
U
2
U
1
Z
Technical Data
CPU Registers
Bit 0
C
U