In the x86-64 computer architecture, long mode is the mode where a 64-bit application (or operating system) can access 64-bit instructions and registers. 32-bit programs and 16-bit protected mode programs are executed in a compatibility sub-mode; real mode or virtual 8086 mode programs cannot run in this mode.
Overview
An x86-64 processor acts identically as an IA-32 processor when running in real mode or protected mode, which are supported sub-modes when the processor is not in long mode.
A bit in the CPUID extended attributes field informs programs in real or protected modes if the pro...