Long mode

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In the AMD64 (x86-64) computer architecture, long mode is the mode where a 64-bit application (or operating system) can access the 64-bit instructions and registers, while 32-bit and 16-bit programs are executed in a compatibility sub-mode.

An x86-64 processor acts the same 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 processor can go to long mode, which allows a program to detect an x86-64 processor. This is similar to the CPUID attributes bit that Intel IA-64 processors use for allowing programs to detect they are running under IA-32 emulation.

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