What are the Privileged and Non-Privileged instructions in Operating System?

In operating systems, instructions are classified into two main categories based on their access level and potential impact on system security and stability. Privileged instructions can only be executed in kernel mode, while non-privileged instructions can be executed in user mode. This classification ensures system protection and controlled access to critical resources.

Privileged Instructions

Privileged instructions are machine-level instructions that can only be executed when the processor is operating in kernel mode (also called privileged mode). These instructions have direct access to system resources and can potentially compromise system security if misused.

Examples of Privileged Instructions

• System shutdown commands

• Modifying control registers or status registers

• Direct I/O device access and control

• Memory management operations (page table modifications)

• Timer interrupt configuration

• Switching between user and kernel modes

Characteristics of Privileged Instructions

• Mode restriction − Any attempt to execute a privileged instruction in user mode results in a hardware trap or exception, preventing execution.

• Timer control − Instructions that modify timer settings are privileged to ensure the OS maintains control over process scheduling.

• System protection − These instructions are essential for the OS to maintain system integrity and prevent unauthorized access to critical resources.

• Hardware enforcement − The processor hardware automatically checks the current mode before executing any instruction.

Non-Privileged Instructions

Non-privileged instructions, also called user-mode instructions or safe instructions, can be executed in both user mode and kernel mode. These instructions perform basic computational tasks and do not directly access system resources.

Examples of Non-Privileged Instructions

• Arithmetic operations (ADD, SUB, MUL, DIV)

• Logical operations (AND, OR, NOT, XOR)

• Data movement instructions (LOAD, STORE)

• Branching and jumping within user program space

• Register-to-register operations

Operating System Modes

Mode Switching Mechanism

The operating system provides mechanisms to switch between user and kernel modes:

• User ? Kernel − Triggered by system calls, interrupts, or exceptions (hardware-controlled)

• Kernel ? User − Executed via privileged instructions that change the processor mode

Comparison

Conclusion

The distinction between privileged and non-privileged instructions is fundamental to operating system security and stability. This two-level protection mechanism ensures that only the OS kernel can execute critical system operations, while user programs are restricted to safe instructions that cannot compromise system integrity.