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

The storage protect mechanism is disabled (turned off) by power-on reset, system reset, thereset caused by check restart, initial program load, the occurrence of a class interruption,and the execution of an SVC instruction. The storage and protect keys are not changed by anyof these actions. The keys are changed only by the appropriate alter storage or protect keyinstructions.The storage protect mechanism is enabled (turned on) by the execution of an alter storagekey or an alter protect key instruction. If these instructions are executed when storage protectis already enabled, it remains enabled.The protect key is read using the read protect key instruction. If any of the storage protectinstructions are issued on the 5010 Processor Module models A or B, a program checkcondition results and the instruction is not executed.To write into main storage, at least one of the following conditions must be true:1. The storage key of the addressed block must be 111.2. The protect key currently in use must be 000.3. The storage key of the addressed block must match the current protect key.The first two conditions above indicate that a storage key of 111 permits writing into anystorage location in that block regardless of the protect key used, and that a protect key of 000authorizes writing into any location in storage regardless of the storage key value of theaddressed block.If none of the three conditions specified above is true, the attempt to write is prevented, thecontents of main storage are not changed and a program check interruption occurs with invalidstorage address set in the processor status word.For certain hardware functions that involve accessing main storage, the key comparisonmechanism is suppressed. For these cases, no storage protect checking is performed until thehardware function is completed. Because of this:1. No storage protection is provided during an initial program load, since it is preceded by ahardware reset, and no instructions are executed until the initial program load is complete.2. No storage protect checking is performed while the system is in the stop state and storeoperations are being performed from the System/7 console.3. No storage protect checking is provided during a cycle steal operation.4. The hardware controlled storing of old instruction addresses during class interruptions is notaffected by storage protect.