A protection boundary protects one software subsystem on a

computer from another, in such a way that only data that is

explicitly shared across such a boundary is accessible to

the entities on both sides. In general, all code within a

protection boundary will have access to all data within

that boundary.

The canonical example of a protection boundary on most

modern systems is that between processes and the kernel.

The kernel is protected from processes, so that they can

only examine or change its internal state in certain

strictly-defined ways.

Protection boundaries also exist between individual

processes on most modern systems. This prevents one buggy

or malicious process from wreaking havoc on others.