Long mode

Source: Wikipedia, the free encyclopedia.

Part of a series on
Microprocessor modes for the x86 architecture
First supported platform shown in parentheses

In the

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.

Overview

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

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.

Memory limitations

See also:
x86-64 virtual address space

While register sizes have increased to 64 bits from the previous

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.

Current limits

The first CPUs implementing the x86-64 architecture, namely the AMD

virtual[1]: 129–130  and 40-bit physical addressing.[1]
: 4 

The virtual address space of these processors is divided into two 47-bit regions, one starting at the lowest possible address, the other extending down from the largest. Attempting to use addresses falling outside this range will cause a general protection fault.

The limit of physical addressing constrains how much installed

TB of RAM.[2]

When there is need, the microarchitecture can be expanded step by step without side-effects from software and simultaneously save cost with its implementation. For future expansion, the architecture supports expanding virtual address space to 64 bits, and physical memory addressing to 52 bits (limited by the

petabytes
) of physical address space.

See also

  • x86-64
  • 64-bit compatibility mode

References

  1. ^ a b "AMD64 Architecture Programmer's Manual Volume 2: System Programming" (PDF). 2016. Retrieved 2015-04-09.
  2. ^ "BIOS and Kernel Developer's Guide (BKDG) For AMD Family 10h Processors" (PDF). p. 30. Retrieved 2015-04-09. Physical address space increased to 48 bits.
  3. ^ AMD 2016, p. 24: "The AMD64 architecture enhances this support to allow translation of 64-bit virtual addresses into 52-bit physical addresses, although processor implementations can support smaller virtual-address and physical-address spaces."

External links

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