PROCESSOR MODES: REAL MODE, PROTECTED MODE, VIRTUAL REAL MODE, 64-BIT EXTENSION MODE (AMD 64, EM 64)

Real Mode :

It is also called real address mode or compatibility mode, is an operating mode of 80286 and later x86-compatible CPUs. Real mode is characterized by a 20bit segmented memory address space (meaning that a maximum of 1 MB of memory can be addressed), direct software access to BIOS routines and peripheral hardware, and no concept of memory protection or multitasking at the hardware level. All x86 CPUs in the 80286 series and later start in real mode at power-on.

Protected mode :

In computing, the protected mode also called protected virtual address mode, is an operational mode of x86-compatible central processing units (CPU). It was first added to the x86 architecture with the release of Intel's 80286 (286) processor and later extended with the release of the 80386 (386). When a processor that supports X-86 protected mode or powered on it begins execution instructions in real mode in order to maintain base word complexity.

Protected mode allows system software to utilize features such as virtual memory, paging, safe multi-tasking, and other features designed to increase an operating system's control over application software.

When a processor that supports x86 protected mode is powered on, it begins executing instructions in real mode, in order to maintain backward compatibility with earlier x86 processors. The protected mode may only be entered after the system software sets up several descriptor tables and enables the Protection Enable (PE) bit in the Control Register 0 (CR0).

Due to the enhancements added by protected mode, it has become widely adopted and has become the foundation for all subsequent enhancements to the x86 architecture.

The 286 maintained backward compatibility with the previous 8086 by initially entering real mode on power up. Real mode functions identically to the 8086 and allowed older software to run unmodified on the newer 286. To access the extended the functionality of the 286, the operating system would set the processor into protected mode. This enabled 24-bit addressing which allowed the processor to access 2^24 bytes of memory, equivalent to 16 mebibytes.

The 386 has an address bus size of 32-bits, which allows for 232 bytes of memory accessing, equivalent to 4 gibibytes. The segment sizes were also increased to 32bits, meaning that the full address space of 4 gibibytes could be accessed without the need to switch between multiple segments. In addition to the increased size of the address bus and segment registers, many other new features were added with the intention of increasing operational security and stability.

Protected mode is now utilized in virtually all modern operating systems that will run on the x86 architecture, such as Microsoft Windows, Linux, and many others

With the release of the 386, the following additional features were added to the protected mode :

• Paging 
• 32-bit physical and virtual address space
• 32-bit segment offsets
• Ability to switch back to real mode 
• Virtual 8086 mode 

64-bit :

In computer architecture, 64-bit integers, memory addresses, or other data units are those that are at most 64 bits (8 octets) wide. Also, 64-bit CPU and ALU architectures are those that are based on registers, address buses, or data buses of that size.

A CPU that is 64-bit internally might have external data buses or address buses with a different size, either larger or smaller; the term "64-bit” is often used to describe the size of these buses as well. For instance, many current machines with 32-bit processors use 64-bit buses (e.g. the original Pentium and later CPUs), and may occasionally be referred to as "64-bit” for this reason.

"64-bit" computer architecture generally has integer registers that are 64-bits wide, which allows it to support (both internally and externally) 64-bit "chunks" of integer data.

32 vs 64-bit :

A change from a 32-bit to a 64-bit architecture is a fundamental alteration, as most operating systems must be extensively modified to take advantage of the new architecture. Other software must also be ported to use the new capabilities; older software is usually supported through either a hardware compatibility mode, in which the new processors support the older 32-bit version of the instruction set as well as the 64-bit version, through software emulation, or by the actual implementation of a 32-bit processor core within the 64-bit processor. The operating systems for those 64-bit architectures generally support both 32-bit and 64-bit applications.