Central Processor Unit (CPU)
Bit
15
14 13 12 11 10
9
8
7
6
5
4
3
2
1
Bit 0
Read:
Write:
Reset:
Loaded with Vector from $FFFE and $FFFF
Figure 4-5. Program Counter (PC)
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.
Bit 7
V
6
1
1
5
1
1
4
H
X
3
I
2
N
X
1
Z
X
Bit 0
C
Read:
Write:
Reset:
X
1
X
X = Indeterminate
Figure 4-6. Condition Code Register (CCR)
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.
MC68HC908AP Family Data Sheet, Rev. 4
58
Freescale Semiconductor
FREESCALE [ Freescale ]
