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Concept
Long mode
Summary
In the x86-64 computer architecture, long mode is the mode where a 64-bit operating system can access 64-bit instructions and registers. 64-bit programs are run in a sub-mode called 64-bit mode, while 32-bit programs and 16-bit protected mode programs are executed in a sub-mode called compatibility mode. Real mode or virtual 8086 mode programs cannot be natively run in long mode.
An x86-64 processor acts identically to an IA-32 processor when running in real mode or protected mode, which are supported 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 to allow programs to detect if they are running under IA-32 emulation.
With a computer running legacy BIOS, the BIOS and the boot loader run in real mode. After execution passes to an operating system kernel which supports x86-64, the kernel verifies CPU support for long mode and then executes the instructions to enter it. With a computer running UEFI, the UEFI firmware (except CSM and legacy Option ROM), any UEFI boot loader, and the operating system kernel all run in Long mode.
x86-64 virtual address space
While register sizes have increased to 64 bits from the previous x86 architecture, memory addressing has not yet been increased to the full 64 bits. For the time being, it is impractical to equip computers with sufficient memory to require a full 64 bits. As long as that remains the case, load/store unit(s), cache tags, MMUs and TLBs can be simplified without any loss of usable memory. Despite this limitation, software is programmed using full 64-bit pointers, and will therefore be able to use progressively larger address spaces as they become supported by future processors and operating systems.
The first CPUs implementing the x86-64 architecture, namely the AMD Athlon 64 / Opteron (K8) CPUs, had 48-bit virtual and 40-bit physical addressing.
Official source
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