The x86 processor architecture, developed by Intel, has been the dominant architecture in personal computing for decades. Over the years, x86 processors have evolved, introducing various operating modes to accommodate the changing needs of computing environments. Understanding these operating modes is crucial for developers, system administrators, and anyone interested in computer architecture.

The most common different modes are:

1. Real Mode (16-bit)
2. Protected Mode (32-bit)
3. Long Mode (64-bit)

1 Real Mode (16-bit)

Real mode is the initial operating mode of x86 processors. When the processor is powered on or reset, it starts executing instructions in real mode. In this mode:

• It is the default operating mode.
• The processor operates as a 16-bit processor.
• In this mode we have a 20-bit address space, giving access to (2^20 bytes) 1 MB of memory.
• Memory addressing is done using 16-bit segmented addressing, allowing access to a maximum of 1 MB of memory.
• It can execute only 8086/8088-compatible instructions.

Real mode is primarily used during the boot process of an operating system. Boot loaders and the BIOS (Basic Input/Output System) operate in real mode, performing necessary hardware initialization and loading the operating system kernel.

Every intel processor operates in this mode after the startup Because real mode can execute instruction of the 8086 and 8088 processors (as these were 16-bit processors). So intel provides the backward compatibility.

Backward Compatibility

Backward compatibility refers to the ability of newer hardware or software to run older applications or support older hardware interfaces. In the context of x86 processors, backward compatibility ensures that software written for older generations of processors can still run on newer processors.

Why Backward Compatibility matters

1. Preservation of Legacy Software: Backward compatibility ensures that legacy software, which may have been written decades ago, can still be executed on modern hardware. This is particularly important for businesses and organizations that rely on proprietary or custom software that may not have been updated to run on newer systems.
2. Cost-Effectiveness: Backward compatibility reduces the need for organizations to update or rewrite their software to run on newer hardware. This saves both time and money, as they can continue to use their existing software without incurring the costs associated with upgrading or redeveloping it.
3. Smooth Transition to New Hardware: Backward compatibility allows for a smooth transition to new hardware. Organizations can upgrade their hardware gradually, without having to immediately replace all of their existing software. This minimizes disruption and allows for a more gradual transition.
4. Support for Older Hardware Interfaces: Backward compatibility also extends to hardware interfaces, such as peripheral devices and expansion cards. This ensures that older hardware can still be used with newer systems, preventing the need for costly upgrades or replacements.

Backward Compatibility in x86 Processors:

The x86 processor architecture is known for its strong backward compatibility features. Newer generations of x86 processors are designed to ensure that software written for older processors can still run seamlessly on newer hardware.

1. Real Mode Compatibility: Even the latest x86 processors are capable of running software written for the original 16-bit Intel 8086 processor in real mode. This ensures that even the oldest software can still be executed on modern hardware.
2. Protected Mode Compatibility: Protected mode processors are backward compatible with real mode. They can execute 16-bit real mode software without any issues. This ensures that legacy software continues to run on modern systems.
3. Long Mode Compatibility: Long mode processors are backward compatible with both protected mode and real mode. They can execute 32-bit and 16-bit software seamlessly, in addition to 64-bit applications.

CPU manufacturers must go to great lengths to keep their CPUs (i.e., their specific instruction set) compatible with earlier CPUs, ensuring that older software and operating systems can still run on the most modern CPUs. In the case of Intel and compatible CPUs, the solution implemented is to emulate the oldest CPU in the family: the Intel 8086.

Emulating the Intel 8086

The Intel 8086 had support for 16-bit instructions and no notion (concept) of memory protection. However, memory protection is crucial for the stability of modern operating systems, as it allows an operating system to restrict a user’s process from accessing sensitive areas of memory, such as kernel memory. This prevents processes from accidentally or intentionally circumventing security mechanisms or crashing the system.

For backward compatibility, modern CPUs boot initially in 16-bit real mode, emulating the behavior of the Intel 8086. This allows older operating systems to run on modern CPUs without any modifications. Modern operating systems are then required to explicitly switch up into the more advanced 32-bit or 64-bit protected mode to take advantage of features such as memory protection.

2 Protected Mode

Protected mode was introduced with the Intel 80286 processor and has been the primary operating mode for x86 processors since then. In protected mode:

• The processor can operate in 16-bit or 32-bit modes.
• It supports memory protection, allowing multiple programs to run concurrently without interfering with each other's memory space.
• It provides support for virtual memory, enabling the use of paging mechanisms.
• It offers access to the full 32-bit address space (4 GB).

Protected mode allows modern operating systems to provide features like multitasking, memory protection, and virtual memory, making it suitable for running multiple applications simultaneously.

3 Long Mode

Long mode is the latest operating mode introduced with 64-bit x86 processors (starting with the AMD Opteron and later Intel's EM64T architecture). In long mode:

• The processor operates in 64-bit mode.
• It supports a 64-bit address space, allowing access to up to 2^64 bytes (16 exabytes) of memory.
• It is backward compatible, allowing it to run both 32-bit and 16-bit applications.
• It provides additional registers and new instructions optimized for 64-bit operations.

Long mode allows modern operating systems and applications to take advantage of larger memory addressing, improved performance, and enhanced security features.

Transition Between Modes

The transition between operating modes typically occurs during the boot process:

1. Real Mode to Protected Mode Transition: The transition from real mode to protected mode is usually performed by the system's firmware (BIOS or UEFI) during the boot process. Once the system is initialized and the operating system kernel is loaded, the processor switches to protected mode.
2. Protected Mode to Long Mode Transition: The transition to long mode is usually done by the operating system kernel. It involves setting up the necessary data structures and transitioning the processor to 64-bit mode.

Understanding 16-Bit and 32-Bit CPUs

When we say that a CPU is 16-bit, it means that its instructions can work with a maximum of 16-bits at once. For example, a 16-bit CPU will have instructions that can add two 16-bit numbers together in one CPU cycle. If a process needs to add together two 32-bit numbers, it would take more cycles, making use of 16-bit addition multiple times.