Protected Mode in Computing

Protected mode, also known as protected virtual address mode, is an operational mode of x86-compatible central processing units (CPUs). It plays a crucial role in modern computer architecture, particularly in the context of operating systems like Windows and Linux. Protected mode is a mode of the Intel 80286 microprocessor, which enhances the capabilities of the CPU by allowing advanced memory management and multitasking.

Introduction to Protected Mode

Protected mode provides a set of features that enhance the functionality and security of a computer system. It enables the operating system to prevent applications from interfering with each other's memory, which is vital for system stability and security. By supporting virtual memory, paging, and segmentation, protected mode allows multiple tasks or processes to run simultaneously without interference.

Features of Protected Mode

Memory Protection

In protected mode, the CPU uses various hardware features to protect memory. Each process running on the system is allocated a separate memory space, and the operating system controls access to these memory regions using segmentation and paging. This prevents one process from accidentally or maliciously altering the memory of another process, which is a significant improvement over the previous real mode used by earlier processors.

Virtual Memory

Protected mode supports virtual memory, enabling the operating system to use disk space as an extension of RAM. This allows systems to run larger applications or more applications than would be possible with physical memory alone. The operating system manages memory through paging, which breaks up memory into blocks and maps them to physical memory as needed.

Multitasking

Protected mode supports multitasking, allowing multiple applications to run concurrently. The operating system can switch between tasks to give the illusion that they are running simultaneously. This is achieved through context switching, where the state of a task is saved and another task is loaded into the CPU's context, allowing efficient use of resources.

Enhanced Instruction Set

The switch to protected mode introduces an expanded instruction set, which includes instructions specifically designed for operating system-level tasks. These instructions help in managing memory protection, multitasking, and hardware interaction.

Historical Context

Protected mode was introduced with the Intel 80286 processor, although its full potential was not realized until the advent of the Intel 80386. The 80386 introduced advanced features such as virtual 8086 mode, enabling backward compatibility with real mode applications while maintaining the benefits of protected mode.

DOS Protected Mode Interface (DPMI)

The DOS Protected Mode Interface (DPMI) is a specification that allows DOS programs to run in protected mode. Introduced in 1989, DPMI provides a standardized way for DOS applications to access the extended memory capabilities offered by protected mode. This was especially important in the transition period from DOS to more advanced operating systems.

Compatibility and Use

Various operating systems, including early versions of Microsoft Windows and OS/2, utilized protected mode to improve system performance and reliability. Although modern operating systems have transitioned to more advanced architectures, the principles of protected mode remain foundational to current computing technologies.

Protected Mode