In computer architecture and operating systems, what does the processor's Supervisor (privileged) state signify, and who is permitted to execute instructions in this mode?

Introduction / Context:
Modern CPUs implement protection rings or privilege levels to separate trusted system code from untrusted application code. The Supervisor state (also called kernel mode, system mode, or privileged mode) is central to this design, ensuring that only the operating system (OS) can execute sensitive instructions that manage hardware and memory protection.

Given Data / Assumptions:

• Applications run in user mode with restricted privileges.
• Operating system services run in supervisor/kernel mode.
• Privileged instructions include operations like setting page tables, enabling/disabling interrupts, and direct device control.

Concept / Approach:
Supervisor state is a CPU execution mode that grants access to privileged instructions and protected resources. Entering this state typically occurs via controlled transitions (system calls, interrupts, exceptions). Allowing arbitrary programs to execute in supervisor state would defeat isolation and risk system stability and security.

Step-by-Step Solution:

Verification / Alternative check:
Attempting privileged instructions (for example, writing control registers) in user mode triggers exceptions, proving that only supervisor mode is permitted to execute them.

Why Other Options Are Wrong:

• Required to perform any I/O: Not all I/O is privileged at the API level; OS mediates I/O while apps remain in user mode.
• Entered by programs when they enter the processor: Programs do not automatically run in supervisor state.
• Never used: It is used constantly by the OS.
• None of the above: Incorrect because supervisor state is OS-only.

Common Pitfalls:
Confusing high-level I/O calls with privileged instructions, and assuming libraries imply supervisor access. The OS enforces boundaries regardless of language runtime.

Final Answer:
Only allowed to the operating system