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7721 Group User’s Manual

INTERRUPTS

7–16

7.8 Return from interrupt routine

When the RTI instruction is executed at the end of the interrupt routine, the contents of the program bank

area just before the INTACK sequence, are automatically pulled. After this, the control returns to the original

routine. And then, the suspended processing, which was in progress before the acceptance of the interrupt

request, is resumed.

Before the RTI instruction is executed, pull registers which were pushed by software in the interrupt routine,

using the PUL instruction, etc.

7.9 Multiple interrupts

Just after a branch is made to an interrupt routine, the following occur:

•Interrupt disable flag (I) = “1” (Interrupts are disabled)

•Interrupt request bit of accepted interrupt = “0”

•Processor interrupt priority level (IPL) = Interrupt priority level of accepted interrupt

Accordingly, as long as the IPL remains unchanged, an interrupt request whose priority level is higher than

that of the interrupt which is in progress can be accepted by clearing the interrupt disable flag (I) to “0” in

an interrupt routine. In this way, multiple interrupts are processed.

Figure 7.9.1 shows the processing for multiple interrupts.

An interrupt request which has not been accepted because its priority level is lower is held. When the RTI

instruction is executed, the interrupt priority level of the routine which was in progress at acceptance of an

interrupt request is pulled into the IPL. Therefore, if the following relationship is satisfied when interrupt

priority level detection is performed next, the held interrupt request is accepted.

Held interrupt request’s priority level > Processor interrupt priority level (IPL)

7.8 Return from interrupt routine, 7.9 Multiple interrupts

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