Protection key

Proection keys are a mechanism used in memory management on multi-user operating systems to prevent one user's buggy or malevolent program from damaging another user's data.

Generally, main memory is divided up into blocks, and each block is assigned a key; a user (generally a process) is assigned one or more keys. References to memory are checked to make sure the memory's key, and the user's key, match; if not, generally a trap will result.