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2015-09-20 09:53:13
x86-64 is an extension of the x86 instruction set. It supports vastly larger virtual and physical address spaces than are possible on x86, thereby allowing programmers to conveniently work with much larger data sets. x86-64 also provides 64-bit general purpose registers and numerous other enhancements. The original specification was created by AMD, and has been implemented by AMD, Intel, VIA, and others. It is fully backwards compatible with Intel x86 16-bit and 32-bit code.[1](p13-14) Because the full x86 16-bit and 32-bit instruction sets remains implemented in hardware without any intervening emulation, existing x86 executables run with no compatibility or performance penalties,[2] although existing applications that are recoded to take advantage of new features of the processor design may see performance increases.
AMD's method of extending Intel's x86 32-bit instruction set to be a subset of its x86-64 instruction set is the same technique Intel employed to extend its 16-bit x86 instruction set to 32-bits.
Prior to launch, "x86-64" and "x86_64" were used to refer to the instruction set. Upon release, AMD named it AMD64[3] Intel initially used the names IA-32e and EM64T before finally settling on Intel 64 for their implementation. x86-64 is still used by many in the industry, while others, notably Sun Microsystems[4] (now Oracle Corporation) and Microsoft,[5] use x64 while the BSD family of OSs use AMD64.
The AMD K8 core was the first to implement the architecture; this was the first significant addition to the x86 architecture designed by a company other than Intel. Intel was forced to follow suit and introduced a modified NetBurst family which was fully software-compatible with AMD's design and specification. VIA Technologies introduced x86-64 in their VIA Isaiah architecture, with the VIA Nano.
The x86-64 specification is distinct from the Intel Itanium (formerly IA-64) architecture, which is not compatible on the native instruction set level with the x86 architecture.
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The AMD64 instruction set is implemented in AMD's Athlon 64, Athlon 64 FX, Athlon 64 X2, Athlon II, Athlon X2, Opteron, Phenom, Phenom II, Turion 64, Turion 64 X2, and later Sempron processors.
History of AMD64AMD64 was created as an alternative to the radically different IA-64 architecture, which was designed by Intel and Hewlett Packard. Originally announced in 1999[6] with a full specification in August 2000,[7] the AMD64 architecture was positioned by AMD from the beginning as an evolutionary way to add 64-bit computing capabilities to the existing x86 architecture, as opposed to Intel's approach of creating an entirely new 64-bit architecture with IA-64.
The first AMD64-based processor, the Opteron, was released in April 2003.
Architectural featuresThe primary defining characteristic of AMD64 is the availability of 64-bit general-purpose processor registers, e.g. rax, rbx etc., 64-bit integer arithmetic and logical operations, and 64-bit virtual addresses. The designers took the opportunity to make other improvements as well. The most significant changes include:
Although virtual addresses are 64 bits wide in 64-bit mode, current implementations (and all chips known to be in the planning stages) do not allow the entire virtual address space of 264 bytes (16 EB) to be used. Most operating systems and applications will not need such a large address space for the foreseeable future (for example, Windows implementations for AMD64 are only populating 16 TB, or 44 bits' worth), so implementing such wide virtual addresses would simply increase the complexity and cost of address translation with no real benefit. AMD therefore decided that, in the first implementations of the architecture, only the least significant 48 bits of a virtual address would actually be used in address translation (page table lookup).[1](p120) Further, bits 48 through 63 of any virtual address must be copies of bit 47 (in a manner akin to sign extension), or the processor will raise an exception.[1](p131) Addresses complying with this rule are referred to as "canonical form."[1](p130) Canonical form addresses run from 0 through 00007FFF'FFFFFFFF, and from FFFF8000'00000000 through FFFFFFFF'FFFFFFFF, for a total of 256 TB of usable virtual address space.
This "quirk" allows an important feature for later scalability to true 64-bit addressing: many operating systems (including, but not limited to, the Windows NT family) take the higher-addressed half of the address space (named kernel space) for themselves and leave the lower-addressed half (user space) for application code, user mode stacks, heaps, and other data regions. The "canonical address" design ensures that every AMD64 compliant implementation has, in effect, two memory halves: the lower half starts at 00000000'00000000 and "grows upwards" as more virtual address bits become available, while the higher half is "docked" to the top of the address space and grows downwards. Also, fixing the contents of the unused address bits prevents their use by operating system as flags, privilege markers, etc., as such use could become problematic when the architecture is extended to implement more bits of virtual addresses.
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Current 48-bit implementation
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56-bit implementation
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Full 64-bit implementation
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The 64-bit addressing mode ("long mode") is a superset of Physical Address Extensions (PAE); because of this, page sizes may be 4 KB (212 bytes) or 2 MB (221 bytes).[1](p120) Long mode also supports page sizes of 1 GB (230 bytes).[1](p120) Rather than the three-level page table system used by systems in PAE mode, systems running in long mode use four levels of page table: PAE's Page-Directory Pointer Table is extended from 4 entries to 512, and an additional Page-Map Level 4 (PML4) Table is added, containing 512 entries in 48-bit implementations.[1](p131) In implementations providing larger virtual addresses, this latter table would either grow to accommodate sufficient entries to describe the entire address range, up to a theoretical maximum of 33,554,432 entries for a 64-bit implementation, or be over ranked by a new mapping level, such as a PML5. A full mapping hierarchy of 4 KB pages for the whole 48-bit space would take a bit more than 512 GB of RAM (about 0.196% of the 256 TB virtual space).
Operating system limitsThe operating system can also limit the virtual address space. Details, where applicable, are given in the "Operating system compatibility and characteristics" section.
Physical address space detailsCurrent AMD64 implementations support a physical address space of up to 248 bytes of RAM, or 256 TB,.[9] A larger amount of installed RAM allows the operating system to keep more of the workload's pageable data and code in RAM, which can improve performance,[12] though various workloads will have different points of diminishing returns.[13][14]
The upper limit on RAM that can be used in a given x86-64 system depends on a variety of factors and can be far less than that implemented by the processor. For example, as of June 2010, there are no known motherboards for x86-64 processors that support 256 TB of RAM.[15][16][17][18] The operating system may place additional limits on the amount of RAM that is usable or supported. Details on this point are given in the "Operating system compatibility and characteristics" section of this article.
Operating modes| Operating mode | Operating system required | Compiled-application rebuild required | Default address size | Default operand size | Register extensions | Typical GPR width | |
|---|---|---|---|---|---|---|---|
| Long mode | 64-bit mode | OS with 64-bit support, or bootloader for 64-bit OS | Yes | 64 | 32 | Yes | 64 |
| Compatibility mode | No | 32 | 32 | No | 32 | ||
| 16 | 16 | 16 | |||||
| Legacy mode | Protected mode | Legacy 16-bit or 32-bit OS; or bootloader for 16, 32, or 64-bit OS | No | 32 | 32 | No | 32 |
| 16 | 16 | 16 | |||||
| Virtual 8086 mode | Legacy 16-bit or 32-bit OS | 16 | 16 | 16 | |||
| Real mode | Legacy 16-bit OS; or bootloader for 16, 32, or 64 bit OS | ||||||
The architecture has two primary modes of operation:
Long modeThe architecture's intended primary mode of operation; it is a combination of the processor's native 64-bit mode and a combined 32-bit and 16-bit compatibility mode. It is used by 64-bit operating systems. Under a 64-bit operating system, 64-bit programs run under 64-bit mode, and 32-bit and 16-bit protected mode applications (that do not need to use either real mode or virtual 8086 mode in order to execute at any time) run under compatibility mode. Real-mode programs and programs that use virtual 8086 mode at any time cannot be run in long mode unless they are emulated in software.
Since the basic instruction set is the same, there is almost no performance penalty for executing protected mode x86 code. This is unlike Intel's IA-64, where differences in the underlying ISA means that running 32-bit code must be done either in emulation of x86 (making the process slower) or with a dedicated x86 core. However, on the x86-64 platform, many x86 applications could benefit from a 64-bit recompile, due to the additional registers in 64-bit code and guaranteed SSE2-based FPU support, which a compiler can use for optimization. However, applications that regularly handle integers wider than 32 bits, such as cryptographic algorithms, will need a rewrite of the code handling the huge integers in order to take advantage of the 64-bit registers.
Legacy modeThe mode used by 16-bit ('protected mode' or 'real mode') and 32-bit operating systems. In this mode, the processor acts like a 32-bit x86 processor, and only 16-bit and 32-bit code can be executed. Legacy mode allows for a maximum of 32 bit virtual addressing which limits the virtual address space to 4 GB.[1](p14)(p24)(p118) 64-bit programs cannot be run from legacy mode.
AMD64 implementationsThe following processors implement the AMD64 architecture:
Intel 64 is Intel's implementation of x86-64. It is used in newer versions of Pentium 4, Celeron D, Xeon and Pentium Dual-Core processors, the Atom D510, N450, N550, N2600 and N2800 and in all versions of the Pentium Extreme Edition, Core 2, Core i7, Core i5, and Core i3 processors.
History of Intel 64Historically, AMD has developed and produced processors patterned after Intel's original designs, but with x86-64, roles were reversed: Intel found itself in the position of adopting the architecture which AMD had created as an extension to Intel's own x86 processor line.
Intel's project was originally codenamed Yamhill (after the Yamhill River in Oregon's Willamette Valley). After several years of denying its existence, Intel announced at the February 2004 IDF that the project was indeed underway. Intel's chairman at the time, Craig Barrett, admitted that this was one of their worst kept secrets.[19][20]
Intel's name for this instruction set has changed several times. The name used at the IDF was CT (presumably for Clackamas Technology, another codename from an Oregon river); within weeks they began referring to it as IA-32e (for IA-32 extensions) and in March 2004 unveiled the "official" name EM64T (Extended Memory 64 Technology). In late 2006 Intel began instead using the name Intel 64 for its implementation, paralleling AMD's use of the name AMD64.[21]
Intel 64 implementationsThe first processor to implement Intel 64 was the multi-socket processor Xeon code-named Nocona in June 2004. In contrast, the initial Prescott chips (February 2004) did not enable this feature. Intel subsequently began selling Intel 64-enabled Pentium 4s using the E0 revision of the Prescott core, being sold on the OEM market as the Pentium 4, model F. The E0 revision also adds eXecute Disable (XD) (Intel's name for the NX bit) to Intel 64, and has been included in then current Xeon code-named Irwindale. Intel's official launch of Intel 64 (under the name EM64T at that time) in mainstream desktop processors was the N0 Stepping Prescott-2M. All 9xx, 8xx, 6xx, 5x9, 5x6, 5x1, 3x6, and 3x1 series CPUs have Intel 64 enabled, as do the Core 2 CPUs, as will future Intel CPUs for workstations or servers. Intel 64 is also present in the last members of the Celeron D line.
The first Intel mobile processor implementing Intel 64 is the Merom version of the Core 2 processor, which was released on 27 July 2006. None of Intel's earlier notebook CPUs (Core Duo, Pentium M, Celeron M, Mobile Pentium 4) implements Intel 64.
The following processors implement the Intel 64 architecture:
The VIA Nano (formerly code named VIA Isaiah) is a 64-bit CPU for personal computers. The VIA Nano was released by VIA Technologies in 2008 after five years of development[22] by its CPU division, Centaur Technology. This new Isaiah 64-bit architecture was designed from scratch, unveiled on 24 January 2008,[23][24][25][26] and launched on May 29, including low-voltage variants and the Nano brand name.[27] The processor supports a number of VIA-specific x86 extensions designed to boost efficiency in low-power appliances. It is expected that the VIA Isaiah will be twice as fast in integer performance and four times as fast in floating-point performance as the previous-generation VIA Esther at an equivalent clock speed. Power consumption is also expected to be on par with the previous-generation VIA CPUs, with thermal design power ranging from 5 W to 25 W.[28] Being a completely new design, the Isaiah architecture was built with support for features like the x86-64 instruction set and x86 virtualization which were unavailable on its predecessors, the VIA C7 line, while retaining their encryption extensions.
Differences between AMD64 and Intel 64Although nearly identical, there are some differences between the two instruction sets in the semantics of a few seldom used machine instructions (and/or situations), which are mainly used for system programming.[29] Compilers generally produce executables (i.e. machine code) that avoid any differences, at least for ordinary application programs. This is therefore of interest mainly to developers of compilers, operating systems and similar, which must deal with individual and special system instructions.
Recent implementations