Advance Information MC68HC908MR16/MC68HC908MR32 — Rev. 4.0
76 Central Processor Unit (CPU) MOTOROLA
Central Processor Unit (CPU)
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 or ADC operation. The
half-carry flag is required for binary-coded decimal (BCD) arithmet ic
operations. The decimal adjust A (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 Bit
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 compatibility, the upper byte of the ind ex register (H)
is not stacked automatically. If the interrupt service routine modifies H,
then the user must stack and unstack H using the push H onto stack
(PSHH) and pull H from stack (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).