Mitsubishi DS5000TK, DS907x SIP manual 050396 94/173

USER'S GUIDE

cycle. If the interrupt acknowledge does not take place for one of the reasons cited above, the request flag will become subsequently inactive and the interrupt will have been lost and will not be serviced.

When an interrupt request is acknowledged, a long call is executed to the interrupt vector location and the 2±byte return address is pushed onto the stack. In addi- tion, an internal flag is set which indicates to the hard- ware the interrupt source that is being serviced. Execu- tion then proceeds from the interrupt vector location. At the conclusion of the interrupt service routine, an RETI instruction should be performed to return control to the main program. The RETI performs the same action as a RET instruction in terms of its operation on the stack and the Program Counter. In other words, two bytes of return address are popped off the stack and loaded into the Program Counter. However, the RETI performs the

additional operation of clearing the interrupt±in±service flag to inform the hardware that a service routine is no longer in progress. Therefore, an RETI should always be used to terminate an interrupt service routine. Failure to do so would indicate that the interrupt was still being serviced.

Higher priority interrupts, which are enabled, can inter- rupt lower priority interrupts. According to this rule, a higher priority interrupt could become pending just prior to machine cycle IA3 during an interrupt acknowledge of a lower priority interrupt. This would cause the hardware to vector to the higher priority service routine during the two machine cycles just after the long call to the lower priority interrupt so that no instruction within the lower priority interrupt service routine would have been executed.

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