Privileged and Non-Privileged Instructions in Operating System

In any Operating System, it is necessary to have a Dual Mode Operation to ensure the protection and security of the System from unauthorized or errant users. This Dual Mode separates the User Mode from the System Mode or Kernel Mode. 

In an operating system, instructions are divided into two categories: privileged and non-privileged instructions.

Privileged instructions are those that can only be executed by the operating system kernel or a privileged process, such as a device driver. These instructions typically perform operations that require direct access to hardware or other privileged resources, such as setting up memory mappings or accessing I/O devices. Privileged instructions are executed in kernel mode, which provides unrestricted access to the system resources.

Non-privileged instructions are those that can be executed by any process, including user-level processes. These instructions are typically used for performing computations, accessing user-level resources such as files and memory, and managing process control. Non-privileged instructions are executed in user mode, which provides limited access to system resources and ensures that processes cannot interfere with one another.

The role of the operating system in managing privileged instructions are:

• Access control: The operating system employs access control mechanisms to limit access to privileged instructions. These mechanisms are restricted to authorized processes or users with elevated privileges. This guarantees that privileged instructions can only be executed by trusted processes and thwarts unauthorized access of malicious programs to system resources.
• Memory protection: By deploying memory protection techniques, the operating system restricts processes from accessing any memory location that belongs to it or other processes. This aids in preventing tampering with the operating system or other processes by ensuring that processes cannot do so. Furthermore, it hinders malevolent programs from resulting in system crashes or risking the system’s safety.
• Interrupt handling: The execution of privileged instructions like system calls or exceptions is handled by the operating system through interrupt handling to ensure safety and accuracy, so when an interrupt occurs during the execution of a process it’s necessary to save its state before transferring control to a suitable handler and then restoring it on completion.
• Virtualization: Using virtualization techniques allows the operating system to create a simulated environment where processes can execute privileged instructions without having direct access to the underlying hardware thus creating a more secure and isolated execution environment for privileged instructions by limiting process access to authorized hardware resources only.

Here are some key differences between privileged and non-privileged instructions:

1. Access to resources: Privileged instructions have direct access to system resources, while non-privileged instructions have limited access.
2. Execution mode: Privileged instructions are executed in kernel mode, while non-privileged instructions are executed in user mode.
3. Execution permissions: Privileged instructions require special permissions to execute, while non-privileged instructions do not.
4. Purpose: Privileged instructions are typically used for performing low-level system operations, while non-privileged instructions are used for general-purpose computing.
5. Risks: Because privileged instructions have access to system resources, they pose a higher risk of causing system crashes or security vulnerabilities if not used carefully. Non-privileged instructions are less risky in this regard.

In summary, privileged instructions are used by the operating system kernel and privileged processes to perform low-level system operations, while non-privileged instructions are used by user-level processes for general-purpose computing. The distinction between privileged and non-privileged instructions is an important mechanism for ensuring the security and stability of an operating system.

What are Privileged Instructions?

The Instructions that can run only in Kernel Mode are called Privileged Instructions .

 Privileged Instructions possess the following characteristics : 

(i) If any attempt is made to execute a Privileged Instruction in User Mode, then it will not be executed and treated as an illegal instruction. The Hardware traps it in the Operating System. 

(ii) Before transferring the control to any User Program, it is the responsibility of the Operating System to ensure that the Timer is set to interrupt. Thus, if the timer interrupts then the Operating System regains control. 

Thus, any instruction which can modify the contents of the Timer is Privileged Instruction. 

(iii) Privileged Instructions are used by the Operating System in order to achieve correct operation. 

(iv) Various examples of Privileged Instructions include:  

• I/O instructions and Halt instructions
• Turn off all Interrupts
• Set the Timer
• Context Switching
• Clear the Memory or Remove a process from the Memory
• Modify entries in the Device-status table

What are Non-Privileged Instructions?

The Instructions that can run only in User Mode are called Non-Privileged Instructions .

 Various examples of Non-Privileged Instructions include: 

• Reading the status of Processor
• Reading the System Time
• Generate any Trap Instruction
• Sending the final printout of Printer

Also, it is important to note that in order to change the mode from Privileged to Non-Privileged, we require a Non-privileged Instruction that does not generate any interrupt. 

Advantages:

1. Security: Privileged and non-privileged instructions provide a layer of security by preventing unauthorized access to system resources. Privileged instructions can only be executed by the operating system or other privileged processes, while non-privileged instructions can be executed by user programs.
2. Stability: Privileged and non-privileged instructions can help to ensure system stability by preventing user programs from interfering with critical system functions. Privileged instructions can only be executed by trusted processes, while non-privileged instructions are restricted in their access to system resources.
3. Flexibility: Privileged and non-privileged instructions allow operating systems to support a wide range of applications and hardware devices. By providing a well-defined interface between user programs and the operating system, it is easier to develop and deploy new applications and hardware.
4. Debugging: Privileged and non-privileged instructions make it easier to debug and diagnose problems with the operating system and applications. By providing a clear separation between trusted and untrusted processes, developers can identify and fix issues more quickly and easily.
5. Performance: By providing direct access to hardware resources, privileged instructions can help to improve system performance by minimizing overhead and reducing latency.
6. Modularity: The use of privileged and non-privileged instructions helps to promote modularity in operating system design by separating low-level system operations from higher-level computing tasks.

Disadvantages:

1. Overhead: Privileged and non-privileged instructions can introduce overhead and reduce system performance. Switching between privileged and non-privileged modes requires a context switch, which can be time-consuming and can impact system performance.
2. Complexity: Privileged and non-privileged instructions can increase system complexity and make it more difficult to develop and maintain operating systems. The need to support both privileged and non-privileged instructions can make it more challenging to design and implement system features and to ensure system stability.
3. Compatibility: Privileged and non-privileged instructions can introduce compatibility issues between different hardware platforms or operating systems. Differences in the way that privileged instructions are implemented can make it difficult to develop applications that work across different platforms.
4. Vulnerabilities: Privileged instructions have direct access to system resources, which can make them a target for attackers seeking to exploit vulnerabilities in the operating system.
5. Learning curve: Developers and system administrators may need to learn the specifics of how privileged and non-privileged instructions work in a given operating system, which can add a learning curve and increase the time needed to develop and maintain software.