Question

Modern CPU architectures allow for two modes of operation. Explain what the goal of having these two modes is/are, and explain how two-mode operation is used to achieve this goal(s).

Answer 1

There are two modes of Operation:

1) Kernel Mode
2) User Mode

First, Let us understand what are these modes and what do they do.

Kernel Mode

Kernel is a bridge between Applications and Hardware.

When the system is switched on, It boots first in the Kernel Mode and then shifts to User Mode.

Mode bit for Kernel Mode is 0 and Mode bit for User Mode is 1. So, when changing the operating modes, actually Mode bit changed from 0 to 1 or from 1 to 0.

There are certain privileged operations which can only be performed in the kernel mode. Like Interrupts, I/O calls.

In Kernel mode, executing code can have completely unrestricted access to all the memory addresses and  Hardware.

Kernel Mode is almost always kept and used for trusted functions.

Because, there is so much control and power given kernel mode, and kind of crash and damage and impact the system to the roots and can be disastrous.

User Mode

The Operating System starts in the user mode. It frequently need help of the Kernel Mode to perform various operations.

In User mode, executing code has no direct access to Memory addresses and Hardware.

It must take help of the kernel mode with system APIs to have the access to address and hardware.

Because, User mode does not have so much control and power, crashes in this mode are mostly recoverable.   


Goals

Having these two modes is important because this isolation protects the system from severe damage.

If we do not have two modes, then giving all the access  in any 1 mode can be fatal to the system. Entire Operating System can be wiped out.

Multiple processes may write at the same time in the same system, with catastrophic results.

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Now, Let's see how the system protects itself when privileged access is wanted in user mode.



Here, we can see that a user process executes in user mode until a system call comes.
When that happens, A TRAP is generated and mode bit changed from 1 to 0.

That system call is executed in Kernel Mode and when that completes,
A return is generated and mode bit is changed from 0 to 1.

and the execution resumes in user mode.

This way, it is made sure that privileged instruction are always performed in kernel mode and not in user mode.


Please comment for any further assistance.