Storage protection

For each 4-kilobyte block of central storage, there is a 7-bit control field, called a storage key. This key is used as follows:

Access control bits: Bits 0-3 are matched against the 4-bit protection key in the program status word (PSW) whenever information is stored, or whenever information is fetched from a location that is protected against fetching.

The 16 protection keys provided by the PSW (and matched against the access control bits) are assigned as follows:

Key: Assigned to:
0: Supervisor and other system functions that require access to all areas of storage
1: Job scheduler, job entry subsystem (JES), APPC, and TSO/E
2: Reserved
3: Availability manager (AVM)
4: Reserved
5: Data management, including Open/Close/EOV
6: VTAM®
7: IMS™ and DB2®
8-9: All V=V problem programs
10-15: V=R problem programs (each protected by a unique protection key)

Fetch protection bit: Bit 4 indicates whether protection applies to fetch-type references. A zero indicates that only store-type references are monitored, and that fetching with any protection key is permitted; a one indicates that protection applies to both fetching and storing. No distinction is made between the fetching of instructions and the fetching of operands.

Reference bit: Bit 5 is associated with dynamic address translation (DAT). It is normally set to one whenever a location in the related 4-kilobyte storage block is referred to for either storing or fetching of information.

Change bit: Bit 6 is also associated with DAT. It is set to one each time that information is stored into the corresponding 4-kilobyte block of storage.