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Table of contents
1.
Introduction
2.
What is dual mode operation in OS(Operating System)?
3.
Types of Dual Mode in Operating System
3.1.
User Mode
3.2.
Kernel Mode
4.
User mode and kernel mode Switching 
5.
Need for Dual mode in OS
6.
Difference between User Mode and Kernel Mode
7.
Frequently Asked Questions 
7.1.
What is dual mode operation in OS?
7.2.
What are the two modes of operation?
7.3.
What is the purpose of dual mode operation? 
7.4.
What is the timer operation in operating system?
8.
Conclusion
Last Updated: Mar 27, 2024
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Dual Mode Operations in OS

Author Alisha Chhabra
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Introduction

An operating system is a program that operates as a bridge between the user and the computer hardware. Now there are certain situations where some processes do not need to interact with the hardware. Also, there are some processes that need to interact with the hardware. To allow so and make such processes separate from each other so that there must be no conflict in the system failure, dual-mode in the OS is there. 

In this article, we will be looking at the Dual Mode of the Operating System. We will define these modes first, and then examine how the Operating System acts in each of these modes.

Dual Mode operations in OS

What is dual mode operation in OS(Operating System)?

Dual mode operation in OS (operating system) is the ability of the system to switch between two different modes of operation for computation processes and to avoid system crashes. The Dual Mode Operations in the OS guard it against illegal users. There are two types of operations: the user and kernel modes. 

Also See, FCFS Scheduling Algorithm, Multiprogramming vs Multitasking

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Types of Dual Mode in Operating System

I/O protection, memory protection, and CPU protection all rely on dual-mode operation. There are two distinct modes in dual-mode: 

Modes of Operating System

kernel-mode (also known as system mode and monitor mode) and user mode. The CPU can use all instructions and access all memory areas while in kernel mode. The CPU is limited to unprivileged instructions and a specific area of memory in user mode. The OS design ensures that user code is always executed in user mode. OS code is executed when responding to system calls, other traps/exceptions, and interrupts. When an interrupt or trap occurs, the CPU automatically switches to kernel mode. As a result, the OS code is executed in kernel mode. 

FCFS Scheduling

Let us now understand each mode in detail:

User Mode

When the computer system runs user applications, such as creating a text document or using an application program, it is in user mode. The mode bit is added to the computer's hardware to indicate the current mode. Bit 1 is set to represent the user mode. 

When an application requests a service from the operating system, or an interrupt or system call occurs, the system switches from user to kernel mode to fulfil the request.

The representation given below depicts what happens when an interrupt occurs: 

User Mode

Kernel Mode

Certain processes operate in kernel mode while the system is running because they require access to operating system calls. This provides security by limiting which processes have access to kernel mode operations. As illustrated in the diagram above, the system will allow certain user-mode processes to execute system calls by allowing the process to temporarily run in kernel mode. While in kernel mode, the process has direct access to all hardware and memory in the system (also called privileged mode). Bit 0 is set to represent the Kernel mode. 

If a user process attempts to run privileged instructions in user mode, it will treat the instruction as illegal and report it to the operating system. Some of the privileged instructions are as follows:

  • Handling system Interrupts
  • The transition from user to kernel mode.
  • I/O device management.
     

Must Read Process Management in OS

User mode and kernel mode Switching 

A process runs in both user and kernel mode during its lifetime. The user mode is the default mode, in which the process has limited access. While the kernel mode is the privileged mode in which the process has complete access to system resources such as hardware, memory, and so on. By accessing kernel data in kernel mode, a process can gain access to services such as hardware I/O. Anything involving process management, I/O hardware management, and memory management necessitates that the process run in Kernel mode.

The kernel provides a System Call Interface (SCI), which are entry points into kernel mode for user processes. System calls are the only way for a process to transition from user mode to kernel mode. 

The diagram below explains the transition from user mode to kernel mode in detail:

User mode and kernel mode Switching

When in user mode, the application process calls Glibc, a library used by software developers. This call informs the operating system kernel that the application wishes to perform an action that only the kernel is authorised to perform. The operating system/kernel will verify that this application process has the necessary permissions to perform the requested action. If it has the required permission, the operating system allows the operation to continue; otherwise, an error message is displayed to the user.

You can also read about layered structure of operating system, Open Source Operating System.

Need for Dual mode in OS

You must be wondering why operating system designers go to such lengths to create dual modes. Why not just run everything in kernel mode and avoid the overhead of all this switching?

There are two main reasons for this:

  • If everything runs in a single mode, we get the problem that Microsoft had in earlier versions of Windows. If a process was able to exploit a vulnerability, that process gained control of the system.
  • A trap, also known as an exception or a system fault, is typically caused by an exceptional condition such as division by zero, invalid memory access, and so on. If the process is running in kernel mode, a trap situation like this can cause the entire operating system to crash. When a user mode process encounters a trap situation, only the user mode process crashes. As a result, the overhead of switching is acceptable in order to ensure a more stable and secure system.


There are some other factors that may indicate the need for Dual mode in OS:

  • An error in one program can have an adverse impact on many processes, it might modify data in another program, and even affect the operating system. For example, if a process becomes stuck in an infinite loop, the correct operation of other processes may be put at risk. So, in order to ensure that the operating system is properly executed, there are two modes of operation in OS. 
  • Certain processes, as well as tasks that do not require any type of hardware support, are to be hidden from the user. These tasks can be handled separately by using the dual mode in OS.
  • Furthermore, the operating system must operate in dual mode because Kernel Level programs perform all of the bottom-level functions such as process managementmemory management, and so on. If the user modifies these, the system as a whole may fail. As a result, Dual Mode is required for an operating system to specify the users' access to only the tasks of their use.

 

Must Read Multiprocessing Operating System

Difference between User Mode and Kernel Mode

Terms User Mode Kernel Mode
Definition User Mode is a restricted mode in which application programs execute and begin. Kernel Mode is the privileged mode that the computer enters when it accesses the hardware resources. 
Modes User Mode is also known as slave mode or restricted mode. Kernel mode is also known as system mode, master mode, or privileged mode.
Interruptions In User Mode, if an interrupt occurs, only that process fails. In Kernel Mode, if an interrupt occurs, the operating system might fail.
Address Space A process in User mode has its own address space. In Kernel Mode, processes get single address space.
Restrictions In user mode, there are restrictions to access kernel programs. They cannot be accessed directly. In kernel mode, both user programs and kernel programs can be accessed.

 Read about Batch Operating System here.

Frequently Asked Questions 

What is dual mode operation in OS?

Dual mode operation in OS (operating system) is the ability of the system to switch between two different modes of operation for computation processes and to avoid system crashes. There are two types of operations: the user and kernel modes. 

What are the two modes of operation?

There are two types of operations: the user and kernel modes: kernel mode (also known as system mode and monitor mode) and user mode. The CPU can use all instructions and access all memory areas while in kernel mode, whereas, it is limited to unprivileged instructions and a specific memory area in user mode.

What is the purpose of dual mode operation? 

A dual mode operation does the job of protecting the operating system from illicit users and illicit users from one another.

What is the timer operation in operating system?

The purpose of a timer is to interrupt the CPU after a specific period of time. This specific period of time is set by the operating system and the timer's value may very from 1 ms to 1 second.

Conclusion

To summarize the discussion, we looked at Dual mode in OS, which is the most fundamental concept for understanding the OS. The most important questions that every student must know are how mode switches work and why there are user and kernel modes.

Operating System is a very important subject to have a strong command of, whether it is a university-level exam, the Gate exam, or any other competitive exam.

 

Recommended Readings:


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