HCS12 datasheet(26/452 Pages) FREESCALE

26 / 452 page

S12CPUV2 Reference Manual, Rev. 4.0

Freescale Semiconductor

pending and are remembered, but operation continues uninterrupted until

the I bit is cleared.

When an interrupt occurs after interrupts are enabled, the I bit is

automatically set to prevent other maskable interrupts during the interrupt

service routine. The I bit is set after the registers are stacked, but before the

first instruction in the interrupt service routine is executed.

Normally, an RTI instruction at the end of the interrupt service routine

restores register values that were present before the interrupt occurred.

Since the CCR is stacked before the I bit is set, the RTI normally clears the

I bit, and thus re-enables interrupts. Interrupts can be re-enabled by clearing

the I bit within the service routine.

2.2.5.5 N Status Bit

The N bit shows the state of the MSB of the result. N is most commonly used

in two’s complement arithmetic, where the MSB of a negative number is 1

and the MSB of a positive number is 0, but it has other uses. For instance,

if the MSB of a register or memory location is used as a status flag, the user

can test status by loading an accumulator.

2.2.5.6 Z Status Bit

The Z bit is set when all the bits of the result are 0s. Compare instructions

perform an internal implied subtraction, and the condition codes, including

Z, reflect the results of that subtraction. The increment index register X

(INX), decrement index register X (DEX), increment index register Y (INY),

and decrement index register Y (DEY) instructions affect the Z bit and no

other condition flags. These operations can only determine = (equal) and

≠

(not equal).

2.2.5.7 V Status Bit

The V bit is set when two’s complement overflow occurs as a result of an

operation.

2.2.5.8 C Status Bit

The C bit is set when a carry occurs during addition or a borrow occurs

during subtraction. The C bit also acts as an error flag for multiply and divide

HCS12 데이터시트(PDF) 26 Page - Freescale Semiconductor, Inc