Sorry, you're right, "protection domain crossing" probably isn't common jargon. It's a holdover from my time as a processor architect on IA-64. It means privile

Sorry, you're right, "protection domain crossing" probably isn't common jargon. It's a holdover from my time as a processor architect on IA-64. It means privilege escalation. Anyway, in any common parlance of the term "context switch" I've ever seen, the amount of state switched to take a system call is much less (edit: oops, originally said 'greater' ;) than the amount of state switched in a context switch. At the very least, a context switch should mean the save and restore of user-level register state, which isn't necessary for a system call. You certainly wouldn't want to do that just to make a system call on a typical RISC architecture with 30-something registers! After all, from the perspective of the caller, it's just a special procedure call. The compiler and system call entry point can even use the same argument-passing convention. I wrote at least part of (maybe most, can't remember anymore :) the "recommended" context switch handler for IA-64. It was significantly more expensive than the system call handler. IA-64 dedicated quite a bit of hardware to making system calls cheap. If I recall correctly, besides a few privileged registers, the only thing we changed was the location of the register stack engine's backing store. That's just a single user-level register. (You don't want to spill registers with potentially privileged state into user space.) Anyway, this is way off-topic now and I don't think anybody's interested other than the three of us, so that's the last I'll say about it.