Clarification on diagram about OS and user processes

Question

I was looking at different relationships that can exist between an operating system and a user process. I came across the figure below. It shows an architecture where the OS functions execute with user processes.

1) I wanted clarification on the part of the diagram labeled OS function. Is this part exactly the same for all user processes? In other words, is this shared code that each process accesses?

2) Is the only difference between the process switching functions (grey base in diagram) and the OS functions that the OS functions run as user processes whereas the process switching functions operate as OS processes? Why does there have to be such a difference?

Accompanying text to provide context to the diagram:

"An alternative that is common with operating systems on smaller computers (PCs, workstations) is to execute virtually all OS software in the context of a user process.The view is that the OS is primarily a collection of routines that the user calls to perform various functions, executed within the environment of the user’s process.... At any given point, the OS is managing n process images......... When an interrupt, trap, or supervisor call occurs, the processor is placed in kernel mode and control is passed to the OS. To pass control from a user program to the OS, the mode context is saved and a mode switch takes place to an operating system routine. However, execution continues within the current user process. Thus, a process switch is not performed, just a mode switch within the same process."

(P.136 Operating Systems by Stallings)

Answer 1

There are many perspectives to consider when explaining the user process and OS/kernel relationship. There's the memory layout and address space, data transfer, processor and execution levels/privileges, and various schemes for scheduling.

A diagram that has been pulled out of context is rather difficult to accurately explain. Where did this diagram come from? What was the accompanying text for this diagram? This could represent something about code organization just as well as something about process scheduling.

Addendum

The quoted paragraph only helps a little. There's probably more text that is supposed to explain that diagram. The quoted paragraph explains a scheme when "a process switch is not performed". Where's the explanation of when and why a process switch is performed?

The OS maintains state information about each process: the value of registers, the stack and virtual memory; collectively this is called the context of the "process". Just as important is the processor security/privilege level, the "mode".

Stallings mentions that the OS can switch either the "process" or the "mode" of the processor. The paragraph you quoted utilizes these processor states as an alternative method for (efficiently) handling system calls, traps and interrupts. That is, the OS could simply perform just a "mode" switch (from user mode to kernel mode) within the current "process" to service an interrupt/trap/syscall. Every process now has the burden of allocating enough stack space to accommodate system interrupts/traps as well as its own needs. This should partially answer your #2.

For question #1, there's (usually) only one copy of the kernel code in physical memory. So "OS functions" would be shared code. Of course the process has to be in kernel mode in order to have permission to execute kernel code.