Assigning VME Data Bus and Interrupt PrioritiesE-9014–001867Introduction to Interrupt PriorityAny board on a VMEbus can interrupt the host to gain attention. A controllerusually wants the host’s attention because it has finished a command and needs tocommunicate this information, along with completion status, back to the processorthat initiated the action. Even though interrupts are usually related to commandmanagement, a controller can request the host’s attention whenever it has a reasonto do so. The returned interrupt status will indicate to the host what type of servicethe board requires.Interrupt requests are usually less frequent than requests for use of the data bus.However, the ability of a controller to gain access to host services, when required,may be as important to system performance as the ability to transfer data ondemand.The VMEbus provides a two–dimensional interrupt priority scheme similar to thatpreviously described for prioritized DTB arbitration. The higher order hierarchy iscontrolled by any one of seven interrupt request lines, IRQ1 through IRQ7. IRQ7has the highest priority and IRQ1 the lowest. Each controller on the bus is assignedto drive one of the seven request lines.Once a host receives one or more interrupt requests, it acknowledges the highestpriority interrupt request line first. If two or more controllers on the bus haverequests at the same priority level, the one closest to the interrupt handler (slot 1)receives service first.DG/UX defines what the IRQ priority level will be on all VMEbus controllers DataGeneral sells for use in AViiON computer systems. If you suspect that the VMEbusinterrupt priority settings are causing a system problem, or if you are installing anon–DGC VME controller and you require information on setting the interruptpriority level, contact your Data General Customer Support Center.For a detail description of the VMEbus interrupt priority system, refer to theMotorola publication, The VMEbus Specification, revision C.1, or the ANSI/IEEEspecification 1014–1987.End of Appendix
