content: Privilege levels are used to separate one program from the infrastructure that supports it. A program can make a lot of accesses to the infrastructure, so we have to make privilege changes efficient. That's what my article is about. To change between programs we clear out everything and start from scratch with the new program. The new program has the same privilege level as the old one, but the memory mappings are changed. Here's an analogy with hotels. Privilege changes are like calling room service. You can't cook something in the kitchen yourself, so you call on someone to do it for you. The call needs to be fast, because you don't want to wait. Program changes are like when you check out and a new guest checks in. It takes a long time for you to pack and the next guest to unpack. And then the next guest needs the same fast, efficient calls to room service, billed to their own account.