I don't see an explanation of this comment at all. I don't think it's the context switching in general that we're not expected to understand, since it's pretty

I don't see an explanation of this comment at all. I don't think it's the context switching in general that we're not expected to understand, since it's pretty much the same in v7 but the comment is gone. Dmr actually explained the problem on his website (https://www.bell-labs.com/usr/dmr/www/odd.html https://www.bell-labs.com/usr/dmr/www/odd.html). savu is used to save the current call stack, retu is used to switch to a saved call stack. The problem is that the function which did the savu was not necessarily the same as the function that does the retu, so after retu the function could have the call stack of a different function. As dmr explained, this worked with the PDP-11 compiler but not with the interdata compiler. In V7 the stack switching was moved into separate functions, save and resume. save retured 0 and resume returned 1 so that an if statement could be used to check if the return from save was actually that of resume after the stack switch (the same trick as that of fork). This way the code that was to be executed after a stack switch was in the same function and stack frame as the one that did the save (as opposed to swtch). (EDIT: I was wrong about v7 save/resume at first) Note that Lions doesn't explain this either, he assumed that the difficulty was with with u_rsav and u_ssav, but those are still in v7 with the comment gone (he probably wasn't that wrong though, it really is confusing, but it's just not what the comment refers to)