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

The accumulator associated with the level contains the address of the I/O devicecausing the interruption, in the following format:0 4 10 15Sublevel1 1 1 1Deviceaddress1 1 1 1 1Moduleaddress1 1 I I0-F X X XDuring interruption presentation, an I/O device also presents a summary status bit (S),which is placed in the carry indicator. The carry indicator is set to 0 if a normal interruptioncondition occurred, or to 1 if an exception interruption condition occurred. Exceptionconditions that set the summary status bit are noted in descriptions of I/O commandsfor I/O devices, later in this manual.CLASS INTERRUPTIONSInternal machine error conditions can cause three types of class interruptions:1. Machine check, caused by a machine error or by program execution.2. Program check, caused by an invalid machine instruction.3. Power/thermal warning, caused by a power or thermal irregularity.Class interruptions alert the system to error conditions which may prevent furtherprocessing. Machine checks and program checks cannot be prevented from interruptingthe system. Power/thermal warning class interruptions, however, are controlled by thesummary mask and are inhibited during execution of the supervisor call (SVC) instruction.(Supervisor call is discussed in more detail in Chapter 4.)Because of their nature, class interruptions immediately disable all four priority interruptionlevels and cause a branch via one of the main storage locations reserved for class interruptionstart addresses. (See "Reserved Storage Locations" in this chapter.) Machine checks andprogram checks turn on the summary mask function. A power/thermal warning that occurswhen the summary mask function is off will also turn it on.A class interruption does not cause a change in priority level; the interruption isserviced on the level that is active when the error condition occurs. If no level isactive when a power/thermal warning interruption occurs, the interrupt is serviced onpriority level 3. When the class interruption is serviced, the servicing program shouldsave any register contents and/or status information needed to restart the program.Lights on the operator console indicate the cause of a class interruption when theprocessor is in the stop state. In addition, a machine instruction (load processorstatus) can check the processor status to determine the specific cause of the interruption.This instruction is described in more detail in Chapter 4.Priority of Class InterruptionsAlthough class interruptions are serviced on the current priority level, they are servicedaccording to class priority hierarchy. A machine check has the highest priority, aprogram check has the next priority, and a power/thermal warning has the lowestpriority. The load processor status instruction resets all class interruption indicatorsexcept for power/thermal warnings.System/7 Interruptions 3-9