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.
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.