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

System/7 configurations can include a large number of devices and attachments capable ofsending and receiving data. Such a device or attachment can be an I/O module or an internalmachine checking circuit. Some operations are more important than others, so the System/7program must be able to recognize the more important requests for processing in the jobbeing performed. Such requests are presented from some external source to the processoras interruptions. That is, the current operation being performed by the processor isstopped, temporarily, so that a more important operation can be processed. The statusof the interrupted operation is saved in the machine registers, the higher-priorityoperation is performed, and then control of the system is returned to the interruptedoperation in the program.Two types of interruptions can occur on the System/7—priority interruptions and classinterruptions. Each type is explained further in the following sections.PRIORITY INTERRUPTIONSPriority LevelsThe priority interruption scheme establishes four priority levels (numbered 0 to 3) forinterrupting sources to obtain servicing by the processor. Each priority level has 16sublevels, and each sublevel can be assigned to a different processing routine, if desired.As long as no higher priority device requests attention, current program processing andcommunication with I/O devices continues to completion. Upon completion of the currentprocessing, the program must exit from the level. If no interruptions are pending at thistime, the machine goes into the wait state. In the wait state no processing is performed,but the machine can recognize and accept interruptions that are expected to occur.If interruptions are pending on the same or lower priority levels, when the currentprogram level exit occurs, the machine does not enter the wait state, but immediatelybegins servicing the highest pending priority interruption. The machine will execute oneinstruction on the highest level in the backup register before accepting the new interrupt.Current processing is interrupted only by requests on a higher priority level, not for requestson a higher sublevel within the current priority level. For example, if the machineis processing sublevel 9 on priority level 2, an interruption on sublevel 5, priority level 2will not interrupt the processing for sublevel 9.If an interruption occurs that is assigned to a higher priority level than the currentlyexecuting program, the current program's status is automatically saved in the machineregisters assigned to the level that was interrupted, and processing transfers to theroutine that services the higher priority request. This routine can, in turn, beinterrupted by a still higher priority request. The original program is not resumeduntil all higher priority interruptions are serviced. Figure 3-1 illustrates the priorityinterruption scheme just discussed.3-2 GA34-00003