mc68hc705j2 Freescale Semiconductor, Inc, mc68hc705j2 Datasheet - Page 22

• Manufacturer Part Number: mc68hc705j2
• Description: 8-bit Microcontroller
• Manufacturer: Freescale Semiconductor, Inc

4.1.4 Program Counter
4.1.5 Condition Code Register
4.1.5.1 Half-Carry Flag
4.1.5.2 Interrupt Mask
MC68HC705J2
The program counter is a 16-bit register that contains the address of the next
instruction or operand to be fetched. The four most significant bits of the
program counter are permanently fixed at 0000. In MC68HC05J1 emulation
mode, the five most significant bits are fixed at 00000.
Normally, the address in the program counter automatically increments to the
next sequential memory location every time an instruction or operand is fetched.
Jump, branch, and interrupt operations load the program counter with an
address other than that of the next sequential location.
The condition code register 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.
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.
required for binary-coded decimal (BCD) arithmetic operations.
Setting the interrupt mask disables interrupts. If an interrupt request occurs
while the interrupt mask is zero, 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.
CENTRAL PROCESSOR UNIT
The half-carry flag is
MOTOROLA
4-3