Execution mode

The modes in which x86 code can be executed in are:

  • Real mode (16-bit)

    • Computers that use BIOS start up in this mode.
    • 20-bit segmented memory address space (meaning that only 1 MB of memory can be addressed— actually since 80286 a little more through HMA)
    • Direct software access to peripheral hardware
    • No concept of memory protection or multitasking at the hardware level.

  • Protected mode (16-bit and 32-bit)

    • Expands addressable physical memory to 16 MB and addressable virtual memory to 1 GB.
    • Provides privilege levels and protected memory, which prevents programs from corrupting one another.
    • 16-bit protected mode (used during the end of the DOS era) used a complex, multi-segmented memory model.
    • 32-bit protected mode uses a simple, flat memory model.

  • Long mode (64-bit)

    • Mostly an extension of the 32-bit (protected mode) instruction set, but unlike the 16–to–32-bit transition, many instructions were dropped in the 64-bit mode. Pioneered by AMD.

  • Virtual 8086 mode (16-bit)

    • A special hybrid operating mode that allows real mode programs and operating systems to run while under the control of a protected mode supervisor operating system

  • System Management Mode (16-bit)

    • Handles system-wide functions like power management, system hardware control, and proprietary OEM designed code.
    • It is intended for use only by system firmware.
    • All normal execution, including the operating system, is suspended.
    • An alternate software system (which usually resides in the computer's firmware, or a hardware-assisted debugger) is then executed with high privileges.