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5.4.2
Interrupt Control Mode 0
Enabling and disabling of IRQ interrupts and on-chip supporting module interrupts can be set by
means of the I bit in the CPU's CCR. Interrupts are enabled when the I bit is cleared to 0, and
disabled when set to 1.
Figure 5-5 shows a flowchart of the interrupt acceptance operation in this case.
[1] If an interrupt source occurs when the corresponding interrupt enable bit is set to 1, an interrupt
request is sent to the interrupt controller.
[2] The I bit is then referenced. If the I bit is cleared to 0, the interrupt request is accepted. If the I
bit is set to 1, only an NMI interrupt is accepted, and other interrupt requests are held pending.
[3] Interrupt requests are sent to the interrupt controller, the highest-ranked interrupt according to
the priority system is accepted, and other interrupt requests are held pending.
[4] When an interrupt request is accepted, interrupt exception handling starts after execution of the
current instruction has been completed.
[5] The PC and CCR are saved to the stack area by interrupt exception handling. The PC saved on
the stack shows the address of the first instruction to be executed after returning from the
interrupt handling routine.
[6] Next, the I bit in CCR is set to 1. This masks all interrupts except NMI.
[7] A vector address is generated for the accepted interrupt, and execution of the interrupt handling
routine starts at the address indicated by the contents of that vector address.
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