I have recently become interested in operating system, and a friend of mine lent me a book called Operating Systems: Internals and Design Principles (I have the third edition), published in 1998.

It's been a very interesting book so far, but I have come to the part dealing with process control, and it's using UNIX System V as one of its examples of an operating system that executes within user processes.

This concept has struck me as a little strange.

First of all, does this mean that OS instructions and data are stored in each user of the processes? Probably not, because that would be an absurdly redundant scheme. But if not, then what does it mean to "execute within" a user process?

Do any modern operating systems use this approach? It seems much more logical to have the operating system execute as its own process, or even independently of all processes, if you're short on memory. All the inter-accessiblilty of process data required for this layout seems to greatly complicate things. (But maybe that's just because I don't quite get the concept ;D)

Here is what the book says: "Execution within User Processes: An alternative that is common with operation systems on smaller machines is to execute virtually all operating system software in the context of a user process. ... "