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CHAPTER 3 INTERRUPT
3.5 Software Interrupt
3.5
Software Interrupt
The software interrupt function transfers control from the currently active program by
the CPU to the user-defined interrupt handling program in response to execution of the
software interrupt instruction (INT instruction).
The hardware interrupt stops during execution of a software interrupt.
■ Start of Software Interrupt
●
Start of software interrupt
A software interrupt is started by using the INT instruction. The software interrupt request has neither the
interrupt request flag nor enable flag, execution of the INT instruction always generates a software interrupt
request.
●
Hardware interrupt inhibition
Because the INT instruction has no interrupt level, the interrupt level mask register (ILM) is not updated.
During the execution of an INT instruction, "0" is set in the I flag of the condition code register (CCR) to
mask hardware interrupts. To enable hardware interrupts also during software interrupt handling, set the I
flag to "1" in the software interrupt handling routine.
●
Operation of software interrupt
Once the CPU fetches and executes the INT instruction, the software interrupt handling microcode is
activated. With this microcode, the registers and their related data in the CPU are automatically saved in the
system stack. After hardware interrupts are masked (CCR:I = 0), control branches to the associated
interrupt vector.
For allocations of interrupt numbers and interrupt vectors, see Section "3.2 Interrupt Cause and Interrupt
Vector".
■ Return from Software Interrupt
When the return interrupt (RETI instruction) instruction is executed in the interrupt handling program, the
12-byte data saved in the system stack returns to the dedicated registers. Control then returns to the process
which was being executed before the interrupt branch.
70
FUJITSU MICROELECTRONICS LIMITED
MB90335 Series
CM44-10137-6E
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