What happens when OS is running without the kernel?

If the kernel is removed,you will have applications remaining,but you wouldn’t be able to use them. So to conclude, The kernel is part of the operating system and closer to the hardware it provides low level services like: device driver.

Why does Linux need an MMU?

An MMU is commonly deployed to support “process model”. This is a way to organize tasks in an operating system and is the one used by most “heavyweight” operating systems. If you want to use Linux, you need to have an MMU. Most real-time operating systems [RTOSes] do not use process model.

Why does the OS kernel need to be in main memory at the same time that a user application is running?

It is the part of the operating system that loads first, and it remains in main memory. Because it stays in memory, it is important for the kernel to be as small as possible while still providing all the essential services required by other parts of the operating system and applications.

How does the operating system is switching between kernel and user mode state the reason for this switching?

The system is in user mode when the operating system is running a user application such as handling a text editor. The transition from user mode to kernel mode occurs when the application requests the help of operating system or an interrupt or a system call occurs. The mode bit is set to 1 in the user mode.

What is the purpose of MMU?

The MMU provides per process address translation of linear (virtual) address to physical addresses. Protection. The MMU entries provide privilege checking and read/write protection of memory. Privilege checking ensures that the processor has the correct privilege level to access a particular memory region.

Does kernel use MMU?

Linux may run on hardware with no MMU, so kernel have to know, how to make translation, but on x86, I believe, it just use MMU for it.