• 1. List of Modes

Processor Modes

x86 has a variety of operating modes. On startup, the processor begins execution in Real Mode as if it were a simple 16 bit processor from 30 plus years ago. It is up to the user (generally the operating system developer) to determine if the processor supports certain modes, and then force the processor into one of them.

List of Modes

There exist three main modes of processor operation:

• Real Mode (16 bit)
• Protected Mode (32 bit) with option for Virtual-8086 Mode (16 bit) on individual threads
• Long Mode (64 bit) with option for Compatibility Mode (32 bit) on individual threads

In addition to those three, there exist two other modes that are generally unused by user facing software:

• Unreal Mode - a variant of Real Mode, but with access to a 4 GiB address space
• System Management Mode - intended for BIOSes and other very low level system initialization routines