IBM System/7 Functional Characteristics - All about the IBM 1130 ...

Interruption Request StackingInterruptions from devices are not serviced immediately if processing is taking place at thesame, or at a higher, priority level. The system stacks, or queues, such pending interruptionsfor servicing when possible. (See Figure 3-2.)Each of the four interruption levels has one interruption buffer in the processor module.The first request for any particular level is placed in its corresponding buffer. When theprocessor is able to service the request in the buffer (that is, no higher-priority interruptionsare pending and the processor has completed execution of an instruction), the request isremoved from the buffer and serviced. If another request is pending at that level, it entersthe buffer where it is available for servicing when the present program is completed and/orsystem control is returned to that level.When the buffer is full (occupied by a request not yet honored), another device assignedto the same interruption level requesting service cannot be recognized. This interruptionrequest is stacked by the I/O module or device and must compete with any otherinterruption requests assigned to that level when the buffer becomes available again.No matter how many interruptions are pending from devices on a particular prioritylevel, the next one to be serviced when control returns to that level is the request in thebuffer.Priority level 0Request bufferPriority level 0ProcessingRequestI Request 1 1INo. 1No. 2 III 1 II t 11 I Request No. 2 II[Request No. 1 serviceserviceIIv' 011.„...:,W. 4., .4.......4kke, '..' ..;;:s.:.:..":". :,,.:V7::'.:. MSESSIO$0.2.Mealniit ...*KA A:::::::::::a::::::::.„":. :::„, - :vim::. :, .....,.,......:- .. .,..,.:- ' 7..milea., ,,,,,A"rk ....