This disclosure improves data processing system integrity by assigning content types to virtual pages and using the assigned content types to enforce special access rules. The page content types are: (a) any changeable data and/or any executable instructions (current S/370); (b) SCP executable instructions and/or unchangeable data (SENC); (c) SCP restrictively changeable data only (system DO); and optionally (d) application data only (user DO). Page content type designation is done by providing in each PTE two integrity control flag bits called herein SENC and DO. In the first embodiment, the SENC and DO bits are separately coded to respectively control access to SENC and system DO page types. In a second embodiment the SENC and DO bits are combinatorially encoded to obtain four types. A new system integrity state, the SCP state, is provided to protect SENC and system DO page usage. The SCP state is initiated by a hardware interrupt, i.e. a swap of PSWs, and exists only as long as the CPU is fetching instructions from SENC pages. The following new program interrupts are provided to identify when the CPU tries: (1) to execute an instruction fetched from a SENC page while the system is not in the SCP (2) to execute an instruction fetched from a system or application DO page; or (3) to store data into a system DO page using an instruction not fetched from a SENC page. Also while in the SCP state, key zero users are forced to change their access key, whenever necessary, to match the storage key of any page frame prior to changing it.