mc68hc908ap8a Freescale Semiconductor, Inc, mc68hc908ap8a Datasheet - Page 58

• Manufacturer Part Number: mc68hc908ap8a
• Description: M68hc08 Microcontrollers
• Manufacturer: Freescale Semiconductor, Inc

Central Processor Unit (CPU)
4.3.5 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 logic 1. The following paragraphs describe
the functions of the condition code register.
V — Overflow Flag
H — Half-Carry Flag
I — Interrupt Mask
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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.
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.
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 = Overflow
0 = No overflow
1 = Carry between bits 3 and 4
0 = No carry between bits 3 and 4
1 = Interrupts disabled
0 = Interrupts enabled
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.
Reset:
Reset:
Read:
Read:
Write:
Write:
X = Indeterminate
Bit
15
Bit 7
V
X
14
Figure 4-6. Condition Code Register (CCR)
13
MC68HC908AP A-Family Data Sheet, Rev. 3
6
1
1
Figure 4-5. Program Counter (PC)
12
11
Loaded with Vector from $FFFE and $FFFF
5
1
1
10
NOTE
9
H
4
X
8
7
3
1
I
6
5
N
X
2
4
3
X
1
Z
2
Freescale Semiconductor
1
Bit 0
C
X
Bit 0