Central Processor Unit (CPU)
MC68HC908MR32 • MC68HC908MR16 Data Sheet, Rev. 6.1
82
Freescale Semiconductor
7.3.5 Condition Code Register
The 8-bit condition code register contains the interrupt mask and five flags that indicate the results of the
instruction just executed. Bits 6 and 5 are set permanently to 1. The following paragraphs describe the
functions of the condition code register.
V — Overflow Flag
The CPU sets the overflow flag when a two's complement overflow occurs. The signed branch
instructions BGT, BGE, BLE, and BLT use the overflow flag.
1 = Overflow
0 = No overflow
H — Half-Carry Flag
The CPU sets the half-carry flag when a carry occurs between accumulator bits 3 and 4 during an
add-without-carry (ADD) or add-with-carry (ADC) operation. The half-carry flag is required for
binary-coded decimal (BCD) arithmetic operations. The DAA instruction uses the states of the H and
C flags to determine the appropriate correction factor.
1 = Carry between bits 3 and 4
0 = No carry between bits 3 and 4
I — Interrupt Mask
When the interrupt mask is set, all maskable CPU interrupts are disabled. CPU interrupts are enabled
when the interrupt mask is cleared. When a CPU interrupt occurs, the interrupt mask is set
automatically after the CPU registers are saved on the stack, but before the interrupt vector is fetched.
1 = Interrupts disabled
0 = Interrupts enabled
NOTE
To maintain M6805 Family compatibility, the upper byte of the index
register (H) is not stacked automatically. If the interrupt service routine
modifies H, then the user must stack and unstack H using the PSHH and
PULH instructions.
After the I bit is cleared, the highest-priority interrupt request is serviced first.
A return-from-interrupt (RTI) instruction pulls the CPU registers from the stack and restores the
interrupt mask from the stack. After any reset, the interrupt mask is set and can be cleared only by the
clear interrupt mask software instruction (CLI).
N — Negative Flag
The CPU sets the negative flag when an arithmetic operation, logic operation, or data manipulation
produces a negative result, setting bit 7 of the result.
1 = Negative result
0 = Non-negative result
Bit 7
6
5
4
3
2
1
Bit 0
Read:
V
1
1
H
I
N
Z
C
Write:
Reset:
X
1
1
X
1
X
X
X
X = Indeterminate
Figure 7-6. Condition Code Register (CCR)
Содержание MC68HC908MR16
Страница 2: ......
Страница 6: ...List of Chapters MC68HC908MR32 MC68HC908MR16 Data Sheet Rev 6 1 6 Freescale Semiconductor...
Страница 78: ...Computer Operating Properly COP MC68HC908MR32 MC68HC908MR16 Data Sheet Rev 6 1 78 Freescale Semiconductor...
Страница 96: ...External Interrupt IRQ MC68HC908MR32 MC68HC908MR16 Data Sheet Rev 6 1 96 Freescale Semiconductor...
Страница 112: ...Input Output I O Ports PORTS MC68HC908MR32 MC68HC908MR16 Data Sheet Rev 6 1 112 Freescale Semiconductor...
Страница 114: ...Power On Reset POR MC68HC908MR32 MC68HC908MR16 Data Sheet Rev 6 1 114 Freescale Semiconductor...
Страница 180: ...Serial Communications Interface Module SCI MC68HC908MR32 MC68HC908MR16 Data Sheet Rev 6 1 180 Freescale Semiconductor...
Страница 194: ...System Integration Module SIM MC68HC908MR32 MC68HC908MR16 Data Sheet Rev 6 1 194 Freescale Semiconductor...
Страница 234: ...Timer Interface A TIMA MC68HC908MR32 MC68HC908MR16 Data Sheet Rev 6 1 234 Freescale Semiconductor...
Страница 281: ......