Protected mode

Protected Mode

Protected mode is a computer system operation mode that provides enhanced security and memory protection features. It is a mode of operation in x86-based processors that allows the operating system to have more control over the hardware and software running on the system. In this mode, the processor provides hardware support for memory protection, preventing one program from accessing the memory of another program without proper authorization.

Overview

Protected mode is a feature of x86 processors, starting from the 80286 processor introduced by Intel in 1982. It is the successor to the real mode, which was the default mode of operation in earlier x86 processors. In real mode, the processor operates in a simple and backward-compatible manner, allowing direct access to the entire memory space without any memory protection.

In contrast, protected mode provides a more advanced and secure environment for running operating systems and applications. It introduces several key features, including:

1. **Memory Protection**: Protected mode allows the operating system to allocate memory in a controlled manner and enforce access restrictions. Each program running in protected mode is assigned its own memory space, preventing unauthorized access to other programs' memory.

2. **Virtual Memory**: Protected mode supports virtual memory, which allows the operating system to provide each program with a virtual address space. This enables efficient memory management, as physical memory can be dynamically allocated and shared among multiple programs.

3. **Privilege Levels**: Protected mode introduces four privilege levels, known as rings, which provide different levels of access to system resources. Ring 0 is the most privileged level, reserved for the operating system kernel, while higher-numbered rings are used for user applications. This hierarchical structure helps prevent unauthorized access to critical system resources.

4. **Interrupt Handling**: Protected mode provides improved interrupt handling mechanisms, allowing the operating system to efficiently handle hardware interrupts and exceptions. This enables better error handling and system stability.

Usage

Protected mode is primarily used by modern operating systems, such as Windows, Linux, and macOS, to provide a secure and stable environment for running applications. It allows these operating systems to enforce memory protection, isolate processes, and manage system resources effectively.

Applications running in protected mode can take advantage of the enhanced security features provided by the operating system. They can access protected memory areas, utilize virtual memory, and make use of system services through system calls.

Impact

The introduction of protected mode has had a significant impact on computer security and system stability. By enforcing memory protection and privilege levels, protected mode helps prevent unauthorized access to sensitive data and system resources. It also isolates processes, reducing the risk of one program crashing or compromising the entire system.

Protected mode has enabled the development of more complex and secure operating systems, allowing for the execution of multiple applications simultaneously. It has also paved the way for advanced features such as multitasking, virtualization, and modern security mechanisms like address space layout randomization (ASLR) and data execution prevention (DEP).

See Also

• Real Mode - The predecessor to protected mode in x86 processors.
• Virtual Memory - A memory management technique used in protected mode.
• Operating System - The software that manages computer hardware and software resources.
• Memory Protection - Techniques used to prevent unauthorized access to memory.
• Privilege Level - The different levels of access provided by protected mode.

References

1. Intel Corporation. (1982). "Intel 80286 Programmer's Reference Manual." Retrieved from [1].

2. Tanenbaum, A. S., & Bos, H. (2014). "Modern Operating Systems." Pearson Education.

3. Stallings, W. (2018). "Operating Systems: Internals and Design Principles." Pearson Education.

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