x86-64 (also known as x64, x86_64, AMD64, and Intel 64) is a
64-bit
In computer architecture, 64-bit integers, memory addresses, or other data units are those that are 64 bits wide. Also, 64-bit central processing units (CPU) and arithmetic logic units (ALU) are those that are based on processor registers, a ...
extension of the
x86
x86 (also known as 80x86 or the 8086 family) is a family of complex instruction set computer (CISC) instruction set architectures initially developed by Intel, based on the 8086 microprocessor and its 8-bit-external-bus variant, the 8088. Th ...
instruction set. It was announced in 1999 and first available in the
AMD Opteron family in 2003. It introduces two new operating modes: 64-bit mode and compatibility mode, along with a new four-level
paging
In computer operating systems, memory paging is a memory management scheme that allows the physical Computer memory, memory used by a program to be non-contiguous. This also helps avoid the problem of memory fragmentation and requiring compact ...
mechanism.
In 64-bit mode, x86-64 supports significantly larger amounts of
virtual memory
In computing, virtual memory, or virtual storage, is a memory management technique that provides an "idealized abstraction of the storage resources that are actually available on a given machine" which "creates the illusion to users of a ver ...
and
physical memory
Computer data storage or digital data storage is a technology consisting of computer components and recording media that are used to retain digital data. It is a core function and fundamental component of computers.
The central processin ...
compared to its
32-bit
In computer architecture, 32-bit computing refers to computer systems with a processor, memory, and other major system components that operate on data in a maximum of 32- bit units. Compared to smaller bit widths, 32-bit computers can perform la ...
predecessors, allowing programs to utilize more memory for data storage. The architecture expands the number of
general-purpose register
A processor register is a quickly accessible location available to a computer's processor. Registers usually consist of a small amount of fast storage, although some registers have specific hardware functions, and may be read-only or write-onl ...
s from 8 to 16, all fully general-purpose, and extends their width to 64 bits.
Floating-point arithmetic
In computing, floating-point arithmetic (FP) is arithmetic on subsets of real numbers formed by a ''significand'' (a Sign (mathematics), signed sequence of a fixed number of digits in some Radix, base) multiplied by an integer power of that ba ...
is supported through mandatory
SSE2
SSE2 (Streaming SIMD Extensions 2) is one of the Intel SIMD (Single Instruction, Multiple Data) processor supplementary instruction sets introduced by Intel with the initial version of the Pentium 4 in 2000. SSE2 instructions allow the use of ...
instructions in 64-bit mode. While the older
x87 FPU and MMX registers are still available, they are generally superseded by a set of sixteen 128-bit
vector registers (XMM registers). Each of these vector registers can store one or two
double-precision floating-point numbers, up to four
single-precision floating-point numbers, or various integer formats.
In 64-bit mode, instructions are modified to support 64-bit
operands
In mathematics, an operand is the object of a mathematical operation, i.e., it is the object or quantity that is operated on.
Unknown operands in equalities of expressions can be found by equation solving.
Example
The following arithmetic expres ...
and 64-bit
addressing mode
Addressing modes are an aspect of the instruction set architecture in most central processing unit (CPU) designs. The various addressing modes that are defined in a given instruction set architecture define how the machine language instructions ...
.
The x86-64 architecture defines a compatibility mode that allows 16-bit and 32-bit
user applications to run unmodified alongside 64-bit applications, provided the 64-bit operating system supports them.
Since the full x86-32 instruction sets remain implemented in hardware without the need for emulation, these older
executable
In computer science, executable code, an executable file, or an executable program, sometimes simply referred to as an executable or binary, causes a computer "to perform indicated tasks according to encoded instruction (computer science), in ...
s can run with little or no performance penalty,
while newer or modified applications can take advantage of new features of the processor design to achieve performance improvements. Also, processors supporting x86-64 still power on in
real mode
Real mode, also called real address mode, is an operating mode of all x86-compatible CPUs. The mode gets its name from the fact that addresses in real mode always correspond to real locations in memory. Real mode is characterized by a 20- bit s ...
to maintain
backward compatibility
In telecommunications and computing, backward compatibility (or backwards compatibility) is a property of an operating system, software, real-world product, or technology that allows for interoperability with an older legacy system, or with Input ...
with the original
8086 processor, as has been the case with x86 processors since the introduction of
protected mode
In computing, protected mode, also called protected virtual address mode, is an operational mode of x86-compatible central processing units (CPUs). It allows system software to use features such as Memory_segmentation, segmentation, virtual mem ...
with the
80286.
The original specification, created by
AMD
Advanced Micro Devices, Inc. (AMD) is an American multinational corporation and technology company headquartered in Santa Clara, California and maintains significant operations in Austin, Texas. AMD is a hardware and fabless company that de ...
and released in 2000, has been implemented by AMD,
Intel
Intel Corporation is an American multinational corporation and technology company headquartered in Santa Clara, California, and Delaware General Corporation Law, incorporated in Delaware. Intel designs, manufactures, and sells computer compo ...
, and
VIA. The
AMD K8 microarchitecture
In electronics, computer science and computer engineering, microarchitecture, also called computer organization and sometimes abbreviated as μarch or uarch, is the way a given instruction set architecture (ISA) is implemented in a particular ...
, in the
Opteron
Opteron is AMD's x86 former server and workstation Microprocessor, processor line, and was the first processor which supported the AMD64 instruction set architecture (known generically as x86-64). It was released on April 22, 2003, with the ''Sl ...
and
Athlon 64
The Athlon 64 is a ninth-generation, AMD64-architecture microprocessor produced by Advanced Micro Devices (AMD), released on September 23, 2003. It is the third processor to bear the name ''Athlon'', and the immediate successor to the Athlon XP. ...
processors, was the first to implement it. This was the first significant addition to the
x86
x86 (also known as 80x86 or the 8086 family) is a family of complex instruction set computer (CISC) instruction set architectures initially developed by Intel, based on the 8086 microprocessor and its 8-bit-external-bus variant, the 8088. Th ...
architecture designed by a company other than Intel. Intel was forced to follow suit and introduced a modified
NetBurst
The NetBurst microarchitecture, called P68 inside Intel, was the successor to the P6 microarchitecture in the x86 family of central processing units (CPUs) made by Intel. The first CPU to use this architecture was the Willamette-core Pentium ...
family which was software-compatible with AMD's specification.
VIA Technologies
VIA Technologies, Inc. () is a Taiwanese manufacturer of integrated circuits, mainly motherboard chipsets, CPUs, and memory. It was once the world's largest independent manufacturer of motherboard chipsets. As a fabless semiconductor company, ...
introduced x86-64 in their VIA Isaiah architecture, with the
VIA Nano.
The x86-64 architecture was quickly adopted for desktop and laptop personal computers and servers which were commonly configured for 16 GiB (
gibibyte
The byte is a unit of digital information that most commonly consists of eight bits. Historically, the byte was the number of bits used to encode a single character of text in a computer and for this reason it is the smallest addressable un ...
s) of memory or more. It has effectively replaced the discontinued Intel
Itanium
Itanium (; ) is a discontinued family of 64-bit computing, 64-bit Intel microprocessors that implement the Intel Itanium architecture (formerly called IA-64). The Itanium architecture originated at Hewlett-Packard (HP), and was later jointly dev ...
architecture (formerly
IA-64
IA-64 (Intel Itanium architecture) is the instruction set architecture (ISA) of the discontinued Itanium family of 64-bit Intel microprocessors. The basic ISA specification originated at Hewlett-Packard (HP), and was subsequently implemented by ...
), which was originally intended to replace the x86 architecture. x86-64 and Itanium are not compatible on the native instruction set level, and operating systems and applications compiled for one architecture cannot be run on the other natively.
AMD64
History
AMD64 (also variously referred to by
AMD
Advanced Micro Devices, Inc. (AMD) is an American multinational corporation and technology company headquartered in Santa Clara, California and maintains significant operations in Austin, Texas. AMD is a hardware and fabless company that de ...
in their literature and documentation as "AMD 64-bit Technology" and "AMD x86-64 Architecture") was created as an alternative to the radically different
IA-64
IA-64 (Intel Itanium architecture) is the instruction set architecture (ISA) of the discontinued Itanium family of 64-bit Intel microprocessors. The basic ISA specification originated at Hewlett-Packard (HP), and was subsequently implemented by ...
architecture designed by
Intel
Intel Corporation is an American multinational corporation and technology company headquartered in Santa Clara, California, and Delaware General Corporation Law, incorporated in Delaware. Intel designs, manufactures, and sells computer compo ...
and
Hewlett-Packard
The Hewlett-Packard Company, commonly shortened to Hewlett-Packard ( ) or HP, was an American multinational information technology company. It was founded by Bill Hewlett and David Packard in 1939 in a one-car garage in Palo Alto, California ...
, which was
backward-incompatible with
IA-32
IA-32 (short for "Intel Architecture, 32-bit", commonly called ''i386'') is the 32-bit version of the x86 instruction set architecture, designed by Intel and first implemented in the i386, 80386 microprocessor in 1985. IA-32 is the first incarn ...
, the 32-bit version of the
x86
x86 (also known as 80x86 or the 8086 family) is a family of complex instruction set computer (CISC) instruction set architectures initially developed by Intel, based on the 8086 microprocessor and its 8-bit-external-bus variant, the 8088. Th ...
architecture. AMD originally announced AMD64 in 1999 with a full specification available in August 2000. As AMD was never invited to be a contributing party for the IA-64 architecture and any kind of licensing seemed unlikely, the AMD64 architecture was positioned by AMD from the beginning as an evolutionary way to add
64-bit computing
In computer architecture, 64-bit integers, memory addresses, or other data units are those that are 64 bits wide. Also, 64-bit central processing units (CPU) and arithmetic logic units (ALU) are those that are based on processor registers, ...
capabilities to the existing x86 architecture while supporting legacy 32-bit x86
code
In communications and information processing, code is a system of rules to convert information—such as a letter, word, sound, image, or gesture—into another form, sometimes shortened or secret, for communication through a communicati ...
, as opposed to Intel's approach of creating an entirely new, completely x86-incompatible 64-bit architecture with IA-64.
The first AMD64-based processor, the
Opteron
Opteron is AMD's x86 former server and workstation Microprocessor, processor line, and was the first processor which supported the AMD64 instruction set architecture (known generically as x86-64). It was released on April 22, 2003, with the ''Sl ...
, was released in April 2003.
Implementations
AMD's processors implementing the AMD64 architecture include
Opteron
Opteron is AMD's x86 former server and workstation Microprocessor, processor line, and was the first processor which supported the AMD64 instruction set architecture (known generically as x86-64). It was released on April 22, 2003, with the ''Sl ...
,
Athlon 64
The Athlon 64 is a ninth-generation, AMD64-architecture microprocessor produced by Advanced Micro Devices (AMD), released on September 23, 2003. It is the third processor to bear the name ''Athlon'', and the immediate successor to the Athlon XP. ...
,
Athlon 64 X2,
Athlon 64 FX,
Athlon II (followed by "X2", "X3", or "X4" to indicate the number of cores, and XLT models),
Turion 64
AMD Turion is the brand name AMD applies to its x86-64 low-power consumption mobile processors codenamed ''K8L''. The Turion 64 and Turion 64 X2/Ultra processors compete with Intel's mobile processors, initially the '' Pentium M'' and the In ...
,
Turion 64 X2
AMD Turion is the brand name AMD applies to its x86-64 low-power consumption mobile processors codenamed ''K8L''. The Turion 64 and Turion 64 X2/Ultra processors compete with Intel Corporation, Intel's mobile processors, initially the ''Pentium ...
,
Sempron
Sempron has been the marketing name used by AMD for several different budget desktop CPUs, using several different technologies and CPU socket formats. The Sempron replaced the AMD Duron processor and competed against Intel's Celeron#Celeron D (Pr ...
("Palermo" E6 stepping and all "Manila" models),
Phenom (followed by "X3" or "X4" to indicate the number of cores),
Phenom II
Phenom II is a family of AMD's multi-core 45 nm central processing unit, processors using the AMD K10 microarchitecture, succeeding the original AMD Phenom, Phenom. Advanced Micro Devices released the Socket AM2+ version of Phenom II in Dece ...
(followed by "X2", "X3", "X4" or "X6" to indicate the number of cores),
FX,
Fusion/APU and
Ryzen
Ryzen ( ) is a brand of multi-core x86-64 microprocessors, designed and marketed by AMD for desktop, mobile, server, and embedded platforms, based on the Zen microarchitecture. It consists of central processing units (CPUs) marketed for mai ...
/
Epyc
Epyc (stylized as EPYC) is a brand of multi-core x86-64 microprocessors designed and sold by AMD, based on the company's Zen microarchitecture. Introduced in June 2017, they are specifically targeted for the server and embedded system market ...
.
Architectural features
The primary defining characteristic of AMD64 is the availability of 64-bit general-purpose
processor register
A processor register is a quickly accessible location available to a computer's processor. Registers usually consist of a small amount of fast storage, although some registers have specific hardware functions, and may be read-only or write-onl ...
s (for example, ), 64-bit
integer
An integer is the number zero (0), a positive natural number (1, 2, 3, ...), or the negation of a positive natural number (−1, −2, −3, ...). The negations or additive inverses of the positive natural numbers are referred to as negative in ...
arithmetic and logical operations, and 64-bit
virtual addresses. The designers took the opportunity to make other improvements as well.
Notable changes in the 64-bit extensions include:
; 64-bit integer capability
: All
general-purpose register
A processor register is a quickly accessible location available to a computer's processor. Registers usually consist of a small amount of fast storage, although some registers have specific hardware functions, and may be read-only or write-onl ...
s (GPRs) are expanded from 32
bit
The bit is the most basic unit of information in computing and digital communication. The name is a portmanteau of binary digit. The bit represents a logical state with one of two possible values. These values are most commonly represented as ...
s to 64 bits, and all arithmetic and logical operations, memory-to-register and register-to-memory operations, etc., can operate directly on 64-bit integers.
Pushes and pops on the
stack
Stack may refer to:
Places
* Stack Island, an island game reserve in Bass Strait, south-eastern Australia, in Tasmania’s Hunter Island Group
* Blue Stack Mountains, in Co. Donegal, Ireland
People
* Stack (surname) (including a list of people ...
default to 8-byte strides, and
pointers
Pointer may refer to:
People with the name
* Pointer (surname), a surname (including a list of people with the name)
* Pointer Williams (born 1974), American former basketball player
Arts, entertainment, and media
* ''Pointer'' (journal), the ...
are 8 bytes wide.
; Additional registers
: In addition to increasing the size of the general-purpose registers, the number of named general-purpose registers is increased from eight (i.e. , , , , , , , ) in x86 to 16 (i.e. , , , , , , , , , , , , , , , ). It is therefore possible to keep more local variables in registers rather than on the stack, and to let registers hold frequently accessed constants; arguments for small and fast subroutines may also be passed in registers to a greater extent.
: AMD64 still has fewer registers than many
RISC
In electronics and computer science, a reduced instruction set computer (RISC) is a computer architecture designed to simplify the individual instructions given to the computer to accomplish tasks. Compared to the instructions given to a comp ...
instruction set
In computer science, an instruction set architecture (ISA) is an abstract model that generally defines how software controls the CPU in a computer or a family of computers. A device or program that executes instructions described by that ISA, s ...
s (e.g.
Power ISA
Power ISA is a reduced instruction set computer (RISC) instruction set architecture (ISA) currently developed by the OpenPOWER Foundation, led by IBM. It was originally developed by IBM and the now-defunct Power.org industry group. Power IS ...
has 32 GPRs;
64-bit ARM,
RISC-V
RISC-V (pronounced "risk-five") is an open standard instruction set architecture (ISA) based on established reduced instruction set computer (RISC) principles. The project commenced in 2010 at the University of California, Berkeley. It transfer ...
I,
SPARC,
Alpha
Alpha (uppercase , lowercase ) is the first letter of the Greek alphabet. In the system of Greek numerals, it has a value of one. Alpha is derived from the Phoenician letter ''aleph'' , whose name comes from the West Semitic word for ' ...
,
MIPS, and
PA-RISC
Precision Architecture reduced instruction set computer, RISC (PA-RISC) or Hewlett Packard Precision Architecture (HP/PA or simply HPPA), is a computer, general purpose computer instruction set architecture (ISA) developed by Hewlett-Packard f ...
have 31) or
VLIW
Very long instruction word (VLIW) refers to instruction set architectures that are designed to exploit instruction-level parallelism (ILP). A VLIW processor allows programs to explicitly specify instructions to execute in parallel computing, para ...
-like machines such as the
IA-64
IA-64 (Intel Itanium architecture) is the instruction set architecture (ISA) of the discontinued Itanium family of 64-bit Intel microprocessors. The basic ISA specification originated at Hewlett-Packard (HP), and was subsequently implemented by ...
(which has 128 registers). However, an AMD64 implementation may have far more internal registers than the number of architectural registers exposed by the instruction set (see
register renaming
In computer architecture, register renaming is a technique that abstracts logical processor register, registers from physical registers.
Every logical register has a set of physical registers associated with it.
When a machine language instructio ...
). (For example, AMD Zen cores have 168 64-bit integer and 160 128-bit vector floating-point physical internal registers.)
; Additional XMM (SSE) registers
: Similarly, the number of 128-bit XMM registers (used for
Streaming SIMD instructions) is also increased from 8 to 16.
: The traditional x87 FPU register stack is not included in the register file size extension in 64-bit mode, compared with the XMM registers used by SSE2, which did get extended. The
x87 register stack is not a simple register file although it does allow direct access to individual registers by low cost exchange operations.
; Larger virtual address space
: The AMD64 architecture defines a 64-bit virtual address format, of which the low-order 48 bits are used in current implementations.
This allows up to 256
TiB (2
48 byte
The byte is a unit of digital information that most commonly consists of eight bits. Historically, the byte was the number of bits used to encode a single character of text in a computer and for this reason it is the smallest addressable un ...
s) of virtual address space. The architecture definition allows this limit to be raised in future implementations to the full 64 bits,
extending the virtual address space to 16
EiB (2
64 bytes). This is compared to just 4
GiB (2
32 bytes) for the x86.
: This means that very large files can be operated on by
mapping the entire file into the process's address space (which is often much faster than working with file read/write calls), rather than having to map regions of the file into and out of the address space.
; Larger physical address space
: The original implementation of the AMD64 architecture implemented 40-bit
physical address
In computing, a physical address (also real address, or binary address), is a memory address that is represented in the form of a binary number on the address bus circuitry in order to enable the data bus to access a ''particular'' storage cell o ...
es and so could address up to 1 TiB (2
40 bytes) of RAM.
Current implementations of the AMD64 architecture (starting from
AMD 10h microarchitecture) extend this to 48-bit physical addresses
and therefore can address up to 256 TiB (2
48 bytes) of RAM. The architecture permits extending this to 52 bits in the future
(limited by the page table entry format);
this would allow addressing of up to 4 PiB of RAM. For comparison, 32-bit x86 processors are limited to 64 GiB of RAM in
Physical Address Extension
In computing, Physical Address Extension (PAE), sometimes referred to as Page Address Extension,
is a memory management feature for the x86 architecture. PAE was first introduced by Intel in the Pentium Pro, and later by AMD in the Athlon process ...
(PAE) mode,
or 4 GiB of RAM without PAE mode.
; Larger physical address space in legacy mode
: When operating in
legacy mode the AMD64 architecture supports
Physical Address Extension
In computing, Physical Address Extension (PAE), sometimes referred to as Page Address Extension,
is a memory management feature for the x86 architecture. PAE was first introduced by Intel in the Pentium Pro, and later by AMD in the Athlon process ...
(PAE) mode, as do most current x86 processors, but AMD64 extends PAE from 36 bits to an architectural limit of 52 bits of physical address. Any implementation, therefore, allows the same physical address limit as under
long mode
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 mod ...
.
; Instruction pointer relative data access
: Instructions can now reference data relative to the instruction pointer (RIP register). This makes
position-independent code
In computing, position-independent code (PIC) or position-independent executable (PIE) is a body of machine code that executes properly regardless of its memory address. PIC is commonly used for shared libraries, so that the same library code c ...
, as is often used in
shared libraries
In computing, a library is a collection of resources that can be leveraged during software development to implement a computer program. Commonly, a library consists of executable code such as compiled functions and classes, or a library can ...
and code loaded at run time, more efficient.
; SSE instructions
: The original AMD64 architecture adopted Intel's
SSE and
SSE2
SSE2 (Streaming SIMD Extensions 2) is one of the Intel SIMD (Single Instruction, Multiple Data) processor supplementary instruction sets introduced by Intel with the initial version of the Pentium 4 in 2000. SSE2 instructions allow the use of ...
as core instructions. These instruction sets provide a vector supplement to the scalar
x87 FPU, for the single-precision and double-precision data types. SSE2 also offers integer vector operations, for data types ranging from 8bit to 64bit precision. This makes the vector capabilities of the architecture on par with those of the most advanced x86 processors of its time. These instructions can also be used in 32-bit mode. The proliferation of 64-bit processors has made these vector capabilities ubiquitous in home computers, allowing the improvement of the standards of 32-bit applications. The 32-bit edition of Windows 8, for example, requires the presence of SSE2 instructions.
SSE3
SSE3, Streaming SIMD Extensions 3, also known by its Intel code name Prescott New Instructions (PNI), is the third iteration of the SSE instruction set for the IA-32 (x86) architecture. Intel introduced SSE3 in early 2004 with the Prescott revis ...
instructions and later
Streaming SIMD Extensions
In computing, Streaming SIMD Extensions (SSE) is a single instruction, multiple data ( SIMD) instruction set extension to the x86 architecture, designed by Intel and introduced in 1999 in its Pentium III series of central processing units (CPU ...
instruction sets are not standard features of the architecture.
; No-Execute bit
: The No-Execute bit or
NX bit
The NX bit (no-execute bit) is a processor feature that separates areas of a virtual address space (the memory layout a program uses) into sections for storing data or program instructions. An operating system supporting the NX bit can mark certai ...
(bit 63 of the page table entry) allows the operating system to specify which pages of virtual address space can contain executable code and which cannot. An attempt to execute code from a page tagged "no execute" will result in a memory access violation, similar to an attempt to write to a read-only page. This should make it more difficult for malicious code to take control of the system via "
buffer overrun" or "unchecked buffer" attacks. A similar feature has been available on x86 processors since the
80286 as an attribute of
segment descriptors; however, this works only on an entire segment at a time.
:
Segmented addressing has long been considered an obsolete mode of operation, and all current PC operating systems in effect bypass it, setting all segments to a base address of zero and (in their 32-bit implementation) a size of 4 GiB. AMD was the first x86-family vendor to implement no-execute in linear addressing mode. The feature is also available in legacy mode on AMD64 processors, and recent Intel x86 processors, when PAE is used.
; Removal of older features
: A few "system programming" features of the x86 architecture were either unused or underused in modern operating systems and are either not available on AMD64 in long (64-bit and compatibility) mode, or exist only in limited form. These include segmented addressing (although the FS and GS segments are retained in vestigial form for use as extra-base pointers to operating system structures),
the
task state switch mechanism, and
virtual 8086 mode
In the 80386 microprocessor and later, virtual 8086 mode (also called virtual real mode, V86-mode, or VM86) allows the execution of real mode applications that are incapable of running directly in protected mode while the processor is running ...
. These features remain fully implemented in "legacy mode", allowing these processors to run 32-bit and 16-bit operating systems without modifications. Some instructions that proved to be rarely useful are not supported in 64-bit mode, including saving/restoring of segment registers on the stack, saving/restoring of all registers (PUSHA/POPA), decimal arithmetic, BOUND and INTO instructions, and "far" jumps and calls with immediate operands.
Virtual address space details
Canonical form addresses
Although virtual addresses are 64 bits wide in 64-bit mode, current implementations (and all chips that are known to be in the planning stages) do not allow the entire virtual address space of 2
64 bytes (16
EiB) to be used. This would be approximately four billion times the size of the virtual address space on 32-bit machines. Most operating systems and applications will not need such a large address space for the foreseeable future, 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
A page table is a data structure used by a virtual memory system in a computer to store mappings between virtual addresses and physical addresses. Virtual addresses are used by the program executed by the accessing process, while physical addr ...
lookup).
In addition, the AMD specification requires that the most significant 16 bits of any virtual address, bits 48 through 63, must be copies of bit 47 (in a manner akin to
sign extension). If this requirement is not met, the processor will raise an exception.
Addresses complying with this rule are referred to as "canonical form."
Canonical form addresses run from 0 through 00007FFF'FFFFFFFF, and from FFFF8000'00000000 through FFFFFFFF'FFFFFFFF, for a total of 256
TiB of usable virtual address space. This is still 65,536 times larger than the virtual 4 GiB address space of 32-bit machines.
This feature eases later scalability to true 64-bit addressing. Many operating systems (including, but not limited to, the
Windows NT
Windows NT is a Proprietary software, proprietary Graphical user interface, graphical operating system produced by Microsoft as part of its Windows product line, the first version of which, Windows NT 3.1, was released on July 27, 1993. Original ...
family) take the higher-addressed half of the address space (named
kernel space
A modern computer operating system usually uses virtual memory to provide separate address spaces or regions of a single address space, called user space and kernel space. This separation primarily provides memory protection and hardware prote ...
) for themselves and leave the lower-addressed half (
user space
A modern computer operating system usually uses virtual memory to provide separate address spaces or regions of a single address space, called user space and kernel space. This separation primarily provides memory protection and hardware prote ...
) 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, enforcing the "canonical form" of addresses by checking the unused address bits prevents their use by the operating system in
tagged pointers as flags, privilege markers, etc., as such use could become problematic when the architecture is extended to implement more virtual address bits.
The first versions of Windows for x64 did not even use the full 256 TiB; they were restricted to just 8 TiB of user space and 8 TiB of kernel space.
Windows did not support the entire 48-bit address space until
Windows 8.1, which was released in October 2013.
Page table structure

The 64-bit addressing mode ("
long mode
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 mod ...
") is a superset of
Physical Address Extension
In computing, Physical Address Extension (PAE), sometimes referred to as Page Address Extension,
is a memory management feature for the x86 architecture. PAE was first introduced by Intel in the Pentium Pro, and later by AMD in the Athlon process ...
s (PAE); because of this,
page
Page most commonly refers to:
* Page (paper), one side of a leaf of paper, as in a book
Page, PAGE, pages, or paging may also refer to:
Roles
* Page (assistance occupation), a professional occupation
* Page (servant), traditionally a young m ...
sizes may be 4
KiB (2
12 bytes) or 2
MiB
The byte is a unit of digital information that most commonly consists of eight bits. Historically, the byte was the number of bits used to encode a single character of text in a computer and for this reason it is the smallest addressable un ...
(2
21 bytes).
Long mode also supports page sizes of 1
GiB (2
30 bytes).
Rather than the three-level
page table
A page table is a data structure used by a virtual memory system in a computer to store mappings between virtual addresses and physical addresses. Virtual addresses are used by the program executed by the accessing process, while physical addr ...
system used by systems in PAE mode, systems running in
long mode
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 mod ...
use four levels of page table: PAE's ''Page-Directory Pointer Table'' is extended from four entries to 512, and an additional ''Page-Map Level 4 (PML4) Table'' is added, containing 512 entries in 48-bit implementations.
A full mapping hierarchy of 4 KiB pages for the whole 48-bit space would take a bit more than 512
GiB of memory (about 0.195% of the 256 TiB virtual space).
:
Intel has implemented a scheme with a
5-level page table, which allows Intel 64 processors to support 57-bit addresses, and in turn, a 128
PiB virtual address space. Further extensions may allow full 64-bit virtual address space and physical memory with 12-bit page table descriptors and 16- or 21-bit memory offsets for 64 KiB and 2 MiB page allocation sizes; the page table entry would be expanded to 128 bits to support additional hardware flags for page size and virtual address space size.
Operating system limits
The 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 details
Current AMD64 processors support a physical address space of up to 2
48 bytes of RAM, or 256
TiB.
However, , there were no known x86-64
motherboard
A motherboard, also called a mainboard, a system board, a logic board, and informally a mobo (see #Nomenclature, "Nomenclature" section), is the main printed circuit board (PCB) in general-purpose computers and other expandable systems. It ho ...
s that support 256 TiB of RAM. 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
The architecture has two primary modes of operation: long mode and legacy mode.
Long mode
Long mode is the 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 those modes are emulated in software.
However, such programs may be started from an operating system running in long mode on processors supporting
VT-x
x86 virtualization is the use of hardware-assisted virtualization capabilities on an x86/x86-64 CPU.
In the late 1990s x86 virtualization was achieved by complex software techniques, necessary to compensate for the processor's lack of hardware-a ...
or
AMD-V
x86 virtualization is the use of hardware-assisted virtualization capabilities on an x86/x86-64 CPU.
In the late 1990s x86 virtualization was achieved by complex software techniques, necessary to compensate for the processor's lack of hardware- ...
by creating a virtual processor running in the desired mode.
Since the basic
instruction set
In computer science, an instruction set architecture (ISA) is an abstract model that generally defines how software controls the CPU in a computer or a family of computers. A device or program that executes instructions described by that ISA, s ...
is the same, there is almost no performance penalty for executing protected mode x86 code. This is unlike Intel's
IA-64
IA-64 (Intel Itanium architecture) is the instruction set architecture (ISA) of the discontinued Itanium family of 64-bit Intel microprocessors. The basic ISA specification originated at Hewlett-Packard (HP), and was subsequently implemented by ...
, where differences in the underlying instruction set mean that running 32-bit code must be done either in emulation of x86 (making the process slower) or with a dedicated x86 coprocessor. However, on the x86-64 platform, many x86 applications could benefit from a 64-bit
recompile
In computing, a compiler is a computer program that translates computer code written in one programming language (the ''source'' language) into another language (the ''target'' language). The name "compiler" is primarily used for programs tha ...
, due to the additional registers in 64-bit code and guaranteed SSE2-based FPU support, which a
compiler
In computing, a compiler is a computer program that Translator (computing), translates computer code written in one programming language (the ''source'' language) into another language (the ''target'' language). The name "compiler" is primaril ...
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 mode
Legacy mode is the mode that the processor is in when it is not in long mode.
In this mode, the processor acts like an older 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 GiB.
64-bit programs cannot be run from legacy mode.
Protected mode
Protected mode
In computing, protected mode, also called protected virtual address mode, is an operational mode of x86-compatible central processing units (CPUs). It allows system software to use features such as Memory_segmentation, segmentation, virtual mem ...
is made into a submode of legacy mode.
It is the submode that 32-bit operating systems and 16-bit protected mode operating systems operate in when running on an x86-64 CPU.
Real mode
Real mode
Real mode, also called real address mode, is an operating mode of all x86-compatible CPUs. The mode gets its name from the fact that addresses in real mode always correspond to real locations in memory. Real mode is characterized by a 20- bit s ...
is the initial mode of operation when the processor is initialized, and is a submode of legacy mode. It is backwards compatible with the original
Intel 8086
The 8086 (also called iAPX 86) is a 16-bit computing, 16-bit microprocessor chip designed by Intel between early 1976 and June 8, 1978, when it was released. The Intel 8088, released July 1, 1979, is a slightly modified chip with an external 8-b ...
and
Intel 8088
The Intel 8088 ("''eighty-eighty-eight''", also called iAPX 88) microprocessor is a variant of the Intel 8086. Introduced on June 1, 1979, the 8088 has an eight-bit external data bus instead of the 16-bit bus of the 8086. The 16-bit registers ...
processors. Real mode is primarily used today by operating system bootloaders, which are required by the architecture to configure
virtual memory details before transitioning to higher modes. This mode is also used by any operating system that needs to communicate with the system firmware with a traditional
BIOS
In computing, BIOS (, ; Basic Input/Output System, also known as the System BIOS, ROM BIOS, BIOS ROM or PC BIOS) is a type of firmware used to provide runtime services for operating systems and programs and to perform hardware initialization d ...
-style interface.
Intel 64
Intel 64 is Intel's implementation of x86-64, used and implemented in various processors made by Intel.
History
Historically, AMD has developed and produced processors with instruction sets patterned after Intel's original designs, but with x86-64, roles were reversed: Intel found itself in the position of adopting the
ISA that AMD created as an extension to Intel's own x86 processor line.
Intel's project was originally
codename
A code name, codename, call sign, or cryptonym is a code word or name used, sometimes clandestinely, to refer to another name, word, project, or person. Code names are often used for military purposes, or in espionage. They may also be used in ...
d ''Yamhill'' (after the
Yamhill River
The Yamhill River is an tributary of the Willamette River, in the U.S. state of Oregon. Formed by the confluence of the South Yamhill River and the North Yamhill River about east of McMinnville, it drains part of the Northern Oregon Coast R ...
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.
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
IA-32 (short for "Intel Architecture, 32-bit", commonly called ''i386'') is the 32-bit version of the x86 instruction set architecture, designed by Intel and first implemented in the i386, 80386 microprocessor in 1985. IA-32 is the first incarn ...
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.
The first processor to implement Intel 64 was the multi-socket processor
Xeon
Xeon (; ) is a brand of x86 microprocessors designed, manufactured, and marketed by Intel, targeted at the non-consumer workstation, server, and embedded markets. It was introduced in June 1998. Xeon processors are based on the same archite ...
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
The NX bit (no-execute bit) is a processor feature that separates areas of a virtual address space (the memory layout a program uses) into sections for storing data or program instructions. An operating system supporting the NX bit can mark certai ...
) 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.
The first Intel
mobile processor implementing Intel 64 is the
Merom version of the
Core 2
Intel Core 2 is a processor family encompassing a range of Intel's mainstream 64-bit x86-64 single-, dual-, and quad-core microprocessors based on the Core microarchitecture. The single- and dual-core models are single- die, whereas the quad-c ...
processor, which was released on July 27, 2006. None of Intel's earlier notebook CPUs (
Core Duo,
Pentium M
The Pentium M is a family of mobile 32-bit single-core x86 microprocessors (with the modified Intel P6 (microarchitecture), P6 microarchitecture) introduced in March 2003 and forming a part of the Intel Centrino#Carmel platform (2003), Carmel no ...
,
Celeron M
Celeron is a series of IA-32 and x86-64 computer microprocessors targeted at low-cost personal computers, manufactured by Intel from 1998 until 2023.
The first Celeron-branded CPU was introduced on April 15, 1998, and was based on the Pentium ...
,
Mobile Pentium 4) implement Intel 64.
Implementations
Intel's processors implementing the Intel64 architecture include the
Pentium 4
Pentium 4 is a series of single-core central processing unit, CPUs for Desktop computer, desktops, laptops and entry-level Server (computing), servers manufactured by Intel. The processors were shipped from November 20, 2000 until August 8, 20 ...
F-series/5x1 series, 506, and 516,
Celeron D models 3x1, 3x6, 355, 347, 352, 360, and 365 and all later
Celeron
Celeron is a series of IA-32 and x86-64 computer microprocessor, microprocessors targeted at low-cost Personal computer, personal computers, manufactured by Intel from 1998 until 2023.
The first Celeron-branded CPU was introduced on April 15, ...
s, all models of
Xeon
Xeon (; ) is a brand of x86 microprocessors designed, manufactured, and marketed by Intel, targeted at the non-consumer workstation, server, and embedded markets. It was introduced in June 1998. Xeon processors are based on the same archite ...
since "
Nocona", all models of
Pentium Dual-Core
The Pentium Dual-Core brand was used for mainstream x86-architecture microprocessors from Intel from 2006 to 2009, when it was renamed to Pentium. The processors are based on either the 32-bit '' Yonah'' or (with quite different microarchitectu ...
processors since "
Merom-2M", the
Atom
Atoms are the basic particles of the chemical elements. An atom consists of a atomic nucleus, nucleus of protons and generally neutrons, surrounded by an electromagnetically bound swarm of electrons. The chemical elements are distinguished fr ...
230, 330, D410, D425, D510, D525, N450, N455, N470, N475, N550, N570, N2600 and N2800, all versions of the
Pentium D
Pentium D is a range of desktop 64-bit x86-64 processors based on the NetBurst microarchitecture, which is the Multi-core processor, dual-core variant of the Pentium 4 manufactured by Intel. Each CPU comprised two cores. The brand's first process ...
,
Pentium Extreme Edition
Pentium D is a range of desktop 64-bit x86-64 processors based on the NetBurst microarchitecture, which is the dual-core variant of the Pentium 4 manufactured by Intel. Each CPU comprised two cores. The brand's first processor, codenamed Smithfi ...
,
Core 2,
Core i9,
Core i7,
Core i5, and
Core i3 processors, and the
Xeon Phi 7200 series processors.
X86S
X86S was a simplification of x86-64 first proposed by Intel in May 2023. The new architecture would have removed support for 16-bit and 32-bit operating systems, although 32-bit programs would still run under a 64-bit OS. A compliant CPU would have no longer had
legacy mode, and started directly in 64-bit
long mode
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 mod ...
. There would have been a way to switch to
5-level paging without going through the unpaged mode. Specific removed features included:
* Segmentation gates
* 32-bit ring 0
** VT-x will no longer emulate this feature
*
Rings 1 and 2
* Ring 3 I/O port (/) access; see
port-mapped I/O
* String port I/O (/)
*
Real mode
Real mode, also called real address mode, is an operating mode of all x86-compatible CPUs. The mode gets its name from the fact that addresses in real mode always correspond to real locations in memory. Real mode is characterized by a 20- bit s ...
(including
huge real mode), 16-bit protected mode, VM86
* 16-bit addressing mode
** VT-x will no longer provide unrestricted mode
*
8259 support; the only
APIC supported would be X2APIC
* Some unused operating system mode bits
* 16-bit and 32-bit Startup
IPI (SIPI)
The draft specification received multiple updates, reaching version 1.2 by June 2024. It was eventually abandoned as of December 2024, following the formation of the
x86 Ecosystem Advisory Group by Intel and AMD.
Advanced Performance Extensions
Advanced Performance Extensions is a 2023 Intel proposal for new instructions and an additional 16 general-purpose registers.
VIA's x86-64 implementation
VIA Technologies
VIA Technologies, Inc. () is a Taiwanese manufacturer of integrated circuits, mainly motherboard chipsets, CPUs, and memory. It was once the world's largest independent manufacturer of motherboard chipsets. As a fabless semiconductor company, ...
introduced their first implementation of the x86-64 architecture in 2008 after five years of development by its CPU division,
Centaur Technology.
Codenamed "Isaiah", the 64-bit architecture was unveiled on January 24, 2008, and launched on May 29 under the
VIA Nano brand name.
The processor supports a number of VIA-specific x86 extensions designed to boost efficiency in low-power appliances.
It is expected that the Isaiah architecture will be twice as fast in integer performance and four times as fast in
floating-point
In computing, floating-point arithmetic (FP) is arithmetic on subsets of real numbers formed by a ''significand'' (a Sign (mathematics), signed sequence of a fixed number of digits in some Radix, base) multiplied by an integer power of that ba ...
performance as the previous-generation
VIA Esther at an equivalent
clock speed
Clock rate or clock speed in computing typically refers to the frequency at which the clock generator of a processor can generate pulses used to synchronize the operations of its components. It is used as an indicator of the processor's ...
. 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.
Being a completely new design, the Isaiah architecture was built with support for features like the x86-64 instruction set and
x86 virtualization
x86 virtualization is the use of hardware-assisted virtualization capabilities on an x86/x86-64 CPU.
In the late 1990s x86 virtualization was achieved by complex software techniques, necessary to compensate for the processor's lack of hardware ...
which were unavailable on its predecessors, the
VIA C7 line, while retaining their encryption extensions.
Microarchitecture levels
In 2020, through a collaboration between AMD, Intel,
Red Hat
Red Hat, Inc. (formerly Red Hat Software, Inc.) is an American software company that provides open source software products to enterprises and is a subsidiary of IBM. Founded in 1993, Red Hat has its corporate headquarters in Raleigh, North ...
, and
SUSE, three microarchitecture levels (or feature levels) on top of the x86-64 baseline were defined: x86-64-v2, x86-64-v3, and x86-64-v4. These levels define specific features that can be targeted by programmers to provide compile-time optimizations. The features exposed by each level are as follows:
The x86-64 microarchitecture feature levels can also be found as AMD64-v1, AMD64-v2 .. or AMD64_v1 .. in settings where the "AMD64" nomenclature is used. These are used as synonyms with the x86-64-vX nomenclature and are thus functionally identical. Examples of this include the Go language documentation and the Fedora Linux distribution.
All levels include features found in the previous levels. Instruction set extensions not concerned with general-purpose computation, including
AES-NI
An Advanced Encryption Standard instruction set (AES instruction set) is a set of instructions that are specifically designed to perform AES encryption and decryption operations efficiently. These instructions are typically found in modern proces ...
and
RDRAND
RDRAND (for "read random") is an instruction for returning random numbers from an Intel on-chip hardware random number generator which has been seeded by an on-chip entropy source. It is also known as Intel Secure Key Technology, codenamed Bull M ...
, are excluded from the level requirements.
On most recent x86_64 Linux distributions, all x86_64 feature levels supported by a CPU can be verified using command:
/lib64/ld-linux-x86-64.so.2 --help
(available since
glibc
The GNU C Library, commonly known as glibc, is the GNU Project implementation of the C standard library. It provides a wrapper around the system calls of the Linux kernel and other kernels for application use. Despite its name, it now also dir ...
2.33). The result will be visible at the end of command's output:
Subdirectories of glibc-hwcaps directories, in priority order:
x86-64-v4
x86-64-v3 (supported, searched)
x86-64-v2 (supported, searched)
Here x86-64-v4 feature level is not supported by CPU, but x86-64-v3 and x86-64-v2 are, which means this CPU does not support AVX512 required at v4 level.
Differences between AMD64 and Intel 64
Although nearly identical, there are some differences between the two instruction sets in the semantics of a few seldom used machine instructions (or situations), which are mainly used for
system programming
Systems programming, or system programming, is the activity of programming computer system software. The primary distinguishing characteristic of systems programming when compared to application programming is that application programming aims to ...
.
Unless instructed to otherwise via
-march
settings, compilers generally produce
executable
In computer science, executable code, an executable file, or an executable program, sometimes simply referred to as an executable or binary, causes a computer "to perform indicated tasks according to encoded instruction (computer science), in ...
s (i.e.
machine code
In computer programming, machine code is computer code consisting of machine language instructions, which are used to control a computer's central processing unit (CPU). For conventional binary computers, machine code is the binaryOn nonb ...
) 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
* Intel 64 allows
SYSCALL
/
SYSRET
only in 64-bit mode (not in compatibility mode), and allows
SYSENTER
/
SYSEXIT
in both modes. AMD64 lacks
SYSENTER
/
SYSEXIT
in both sub-modes of
long mode
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 mod ...
.
* When returning to a non-canonical address using
SYSRET
, AMD64 processors execute the general protection fault handler in privilege level 3, while on Intel 64 processors it is executed in privilege level 0.
* The
SYSRET
instruction will load a set of fixed values into the hidden part of the
SS
segment register (base-address, limit, attributes) on Intel 64 but leave the hidden part of
SS
unchanged on AMD64.
* AMD64 requires a different microcode update format and control MSRs (
model-specific registers), while Intel 64 implements
microcode
In processor design, microcode serves as an intermediary layer situated between the central processing unit (CPU) hardware and the programmer-visible instruction set architecture of a computer. It consists of a set of hardware-level instructions ...
update unchanged from their 32-bit only processors.
* Intel 64 lacks some MSRs that are considered architectural in AMD64. These include
SYSCFG
,
TOP_MEM
, and
TOP_MEM2
.
* Intel 64 lacks the ability to save and restore a reduced (and thus faster) version of the
floating-point
In computing, floating-point arithmetic (FP) is arithmetic on subsets of real numbers formed by a ''significand'' (a Sign (mathematics), signed sequence of a fixed number of digits in some Radix, base) multiplied by an integer power of that ba ...
state (involving the
FXSAVE
and
FXRSTOR
instructions).
* In 64-bit mode, near branches with the 66H (operand size override) prefix behave differently. Intel 64 ignores this prefix: the instruction has a 32-bit sign extended offset, and instruction pointer is not truncated. AMD64 uses a 16-bit offset field in the instruction, and clears the top 48 bits of instruction pointer.
* On Intel 64 but not AMD64, the
REX.W
prefix can be used with the far-pointer instructions (
LFS
,
LGS
,
LSS
, , ) to increase the size of their
far pointer argument to 80 bits (64-bit offset + 16-bit segment).
* When the
MOVSXD
instruction is executed with a memory source operand and an operand-size of 16 bits, the memory operand will be accessed with a 16-bit read on Intel 64, but a 32-bit read on AMD64.
* When the
PUSH
instruction is used with a
segment register and an operand-size of 32 bits in legacy/compatibility mode, AMD64 will zero-extend the register from 2 to 4 bytes and push that 4-byte value onto the stack. Intel 64 will also decrement the stack pointer by 4 but will just write 2 bytes, leaving a 2-byte hole that's not written.
* When the
POP
instruction is used with a segment register and an operand-size of 32 or 64 bits, AMD64 will perform a memory read from the stack that is as wide as the operand-size, while Intel 64 will perform a 16-bit memory read (but still increment the stack-pointer according to the operand size.)
* The
FCOMI
/
FCOMIP
/
FUCOMI
/
FUCOMIP
(x87 floating-point compare) instructions will clear the OF, SF and AF bits of
EFLAGS on Intel 64, but leave these flag bits unmodified on AMD64.
* For the
VMASKMOVPS
/
VMASKMOVPD
/
VPMASKMOVD
/
VPMASKMOVQ
(AVX/AVX2 masked move to/from memory) instructions, Intel 64 architecturally guarantees that the instructions will not cause memory faults (e.g. page-faults and segmentation-faults) for any zero-masked lanes, while AMD64 does not provide such a guarantee.
* If the
RDRAND
instruction fails to obtain a random number (as indicated by
EFLAGS.CF=0), the destination register is architecturally guaranteed to be set to 0 on Intel 64 but not AMD64.
* For the
VPINSRD
and
VPEXTRD
(AVX vector lane insert/extract) instructions outside 64-bit mode, AMD64 requires the instructions to be encoded with
VEX.W=0, while Intel 64 also accepts encodings with VEX.W=1. (In 64-bit mode, both AMD64 and Intel 64 require VEX.W=0.)
* When alignment checking is enabled (
EFLAGS.AC=1), AVX instructions with misaligned 128-bit or 256-bit memory operands and the SSE4.2
PCMP*STR*
instructions with misaligned 128-bit memory operands will cause #AC (alignment check) exceptions on AMD64 but not Intel 64.
* The
0F 0D /r
opcode with the
ModR/M
The ModR/M byte is an important part of instruction encoding for the x86 instruction set.
Description
Opcodes in x86 are generally one-byte, though two-byte instructions and prefixes exist. ModR/M is a byte that, if required, follows the opcode a ...
byte's Mod field set to
11b
is a Reserved-
NOP on Intel 64 but will cause #UD (invalid-opcode exception) on AMD64.
* The ordering guarantees provided by some
memory ordering instructions such as
LFENCE
and
MFENCE
differ between Intel 64 and AMD64:
**
LFENCE
is dispatch-serializing (enabling it to be used as a
speculation
In finance, speculation is the purchase of an asset (a commodity, good (economics), goods, or real estate) with the hope that it will become more valuable in a brief amount of time. It can also refer to short sales in which the speculator hope ...
fence) on Intel 64 but is not architecturally guaranteed to be dispatch-serializing on AMD64.
**
MFENCE
is a fully serializing instruction (including instruction fetch serialization) on AMD64 but not Intel 64.
** The
MOV
to CR8 and
INVPCID
instructions are serializing on AMD64 but not Intel 64.
** The
LMSW
instruction is serializing on Intel 64 but not AMD64.
**
WRMSR
to the
x2APIC ICR (Interrupt Command Register; MSR
830h
) is commonly used to produce an IPI (
Inter-processor interrupt) — on Intel 64 but not AMD64 CPUs, such an IPI can be reordered before an older memory store.
** On recent AMD64 processors (
Zen 4 and later),
WRMSR
to the
FS_BASE
,
GS_BASE
and
KernelGSBase
MSRs is non-serializing.
On Intel 64 processors as well as older AMD64 processors,
WRMSR
to these MSRs is serializing.
Older implementations
* The AMD64 processors prior to Revision F (distinguished by the switch from
DDR to
DDR2 memory and new sockets
AM2 AM2 can refer to:
* Socket AM2, a CPU socket for AMD desktop processors
* Sega AM2, a research and development team for the video game company Sega
* Arp-Madore 2, an open star cluster
* a fictional element from The Sten Chronicles
* Animusic
* ...
,
F and
S1) of 2006 lacked the
CMPXCHG16B
instruction, which is an extension of the
CMPXCHG8B
instruction present on most post-
80486 processors. Similar to
CMPXCHG8B
,
CMPXCHG16B
allows for
atomic operation
In concurrent programming, an operation (or set of operations) is linearizable if it consists of an ordered list of Execution (computing), invocation and response Event (computing), events, that may be extended by adding response events such tha ...
s on octa-words (128-bit values). This is useful for parallel algorithms that use
compare and swap In computer science, compare-and-swap (CAS) is an atomic instruction used in multithreading to achieve synchronization. It compares the contents of a memory location with a given (the previous) value and, only if they are the same, modifies the ...
on data larger than the size of a pointer, common in
lock-free and wait-free algorithms
In computer science, an algorithm is called non-blocking if failure or suspension of any thread cannot cause failure or suspension of another thread; for some operations, these algorithms provide a useful alternative to traditional blocking i ...
. Without
CMPXCHG16B
one must use workarounds, such as a
critical section or alternative lock-free approaches. Its absence also prevents 64-bit
Windows
Windows is a Product lining, product line of Proprietary software, proprietary graphical user interface, graphical operating systems developed and marketed by Microsoft. It is grouped into families and subfamilies that cater to particular sec ...
prior to Windows 8.1 from having a
user-mode
In computer science, hierarchical protection domains, often called protection rings, are mechanisms to protect data and functionality from faults (by improving fault tolerance) and malicious behavior (by providing computer security).
Computer ...
address space larger than 8
TiB. The 64-bit version of
Windows 8.1 requires the instruction.
* Early AMD64 and Intel 64 CPUs lacked
LAHF
and
SAHF
instructions in 64-bit mode. AMD introduced these instructions (also in 64-bit mode) with their
90 nm (revision D) processors, starting with Athlon 64 in October 2004. Intel introduced the instructions in October 2005 with the 0F47h and later revisions of
NetBurst
The NetBurst microarchitecture, called P68 inside Intel, was the successor to the P6 microarchitecture in the x86 family of central processing units (CPUs) made by Intel. The first CPU to use this architecture was the Willamette-core Pentium ...
. The 64-bit version of Windows 8.1 requires this feature.
* Early Intel CPUs with Intel 64 also lack the
NX bit
The NX bit (no-execute bit) is a processor feature that separates areas of a virtual address space (the memory layout a program uses) into sections for storing data or program instructions. An operating system supporting the NX bit can mark certai ...
of the AMD64 architecture. It was added in the stepping E0 (0F41h) Pentium 4 in October 2004. This feature is required by all versions of Windows 8.
* Early Intel 64 implementations had a 36-bit (64 GiB) physical addressing of memory while original AMD64 implementations had a 40-bit (1
TiB) physical addressing. Intel used the 40-bit physical addressing first on Xeon MP (
Potomac), launched on 29 March 2005. The difference is not a difference of the user-visible ISAs. In 2007
AMD 10h-based Opteron was the first to provide a 48-bit (256 TiB) physical address space. Intel 64's physical addressing was extended to 44 bits (16 TiB) in Nehalem-EX in 2010 and to 46 bits (64 TiB) in Sandy Bridge E in 2011. With the Ice Lake 3rd gen Xeon Scalable processors, Intel increased the virtual addressing to 57 bits (128
PiB) and physical to 52 bits (4 PiB) in 2021, necessitating a
5-level paging. The following year AMD64 added the same in 4th generation
EPYC
Epyc (stylized as EPYC) is a brand of multi-core x86-64 microprocessors designed and sold by AMD, based on the company's Zen microarchitecture. Introduced in June 2017, they are specifically targeted for the server and embedded system market ...
(Genoa). Non-server CPUs retain smaller address spaces for longer.
* On all AMD64 processors, the
BSF
and
BSR
instructions will, when given a source value of 0, leave their destination register unmodified. This is mostly the case on Intel 64 processors as well, except that on some older Intel 64 CPUs, executing these instructions with an operand size of 32 bits will clear the top 32 bits of their destination register even with a source value of 0 (with the low 32 bits kept unchanged.)
* AMD64 processors since
Opteron
Opteron is AMD's x86 former server and workstation Microprocessor, processor line, and was the first processor which supported the AMD64 instruction set architecture (known generically as x86-64). It was released on April 22, 2003, with the ''Sl ...
Rev. E and
Athlon 64
The Athlon 64 is a ninth-generation, AMD64-architecture microprocessor produced by Advanced Micro Devices (AMD), released on September 23, 2003. It is the third processor to bear the name ''Athlon'', and the immediate successor to the Athlon XP. ...
Rev. D reintroduced limited support for segmentation, via the Long Mode Segment Limit Enable (LMSLE) bit, to ease
virtualization
In computing, virtualization (abbreviated v12n) is a series of technologies that allows dividing of physical computing resources into a series of virtual machines, operating systems, processes or containers.
Virtualization began in the 1960s wit ...
of 64-bit guests. LMLSE support was removed in the Zen 3 processor.
* On all Intel 64 processors,
CLFLUSH
is ordered with respect to
SFENCE
- this is also the case on newer AMD64 processors (Zen 1 and later). On older AMD64 processors, imposing ordering on the
CLFLUSH
instruction instead required
MFENCE
.
Adoption

In
supercomputer
A supercomputer is a type of computer with a high level of performance as compared to a general-purpose computer. The performance of a supercomputer is commonly measured in floating-point operations per second (FLOPS) instead of million instruc ...
s tracked by
TOP500
The TOP500 project ranks and details the 500 most powerful non-distributed computing, distributed computer systems in the world. The project was started in 1993 and publishes an updated list of the supercomputers twice a year. The first of these ...
, the appearance of 64-bit extensions for the x86 architecture enabled 64-bit x86 processors by AMD and Intel to replace most RISC processor architectures previously used in such systems (including
PA-RISC
Precision Architecture reduced instruction set computer, RISC (PA-RISC) or Hewlett Packard Precision Architecture (HP/PA or simply HPPA), is a computer, general purpose computer instruction set architecture (ISA) developed by Hewlett-Packard f ...
,
SPARC,
Alpha
Alpha (uppercase , lowercase ) is the first letter of the Greek alphabet. In the system of Greek numerals, it has a value of one. Alpha is derived from the Phoenician letter ''aleph'' , whose name comes from the West Semitic word for ' ...
and others), as well as 32-bit x86, even though Intel itself initially tried unsuccessfully to replace x86 with a new incompatible 64-bit architecture in the
Itanium
Itanium (; ) is a discontinued family of 64-bit computing, 64-bit Intel microprocessors that implement the Intel Itanium architecture (formerly called IA-64). The Itanium architecture originated at Hewlett-Packard (HP), and was later jointly dev ...
processor.
, a
HPE EPYC
Epyc (stylized as EPYC) is a brand of multi-core x86-64 microprocessors designed and sold by AMD, based on the company's Zen microarchitecture. Introduced in June 2017, they are specifically targeted for the server and embedded system market ...
-based supercomputer called
Frontier
A frontier is a political and geographical term referring to areas near or beyond a boundary.
Australia
The term "frontier" was frequently used in colonial Australia in the meaning of country that borders the unknown or uncivilised, th ...
is number one. The first ARM-based supercomputer appeared on the list in 2018 and, in recent years, non-CPU architecture co-processors (
GPGPU
General-purpose computing on graphics processing units (GPGPU, or less often GPGP) is the use of a graphics processing unit (GPU), which typically handles computation only for computer graphics, to perform computation in applications traditiona ...
) have also played a big role in performance. Intel's
Xeon Phi "Knights Corner" coprocessors, which implement a subset of x86-64 with some vector extensions, are also used, along with x86-64 processors, in the
Tianhe-2
Tianhe-2 or TH-2 (, i.e. 'Milky Way 2') is a 33.86- petaflop supercomputer located in the National Supercomputer Center in Guangzhou, China. It was developed by a team of 1,300 scientists and engineers.
It was the world's fastest supercomputer ...
supercomputer.
Operating system compatibility and characteristics
The following operating systems and releases support the x86-64 architecture in
long mode
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 mod ...
.
BSD
DragonFly BSD
Preliminary infrastructure work was started in February 2004 for a x86-64 port.
This development later stalled. Development started again during July 2007
and continued during
Google Summer of Code
The Google Summer of Code, often abbreviated to GSoC, is an international annual program in which Google awards stipends to contributors who successfully complete a free and open-source software coding project during the summer. , the program is ...
2008 and SoC 2009.
The first official release to contain x86-64 support was version 2.4.
FreeBSD
FreeBSD
FreeBSD is a free-software Unix-like operating system descended from the Berkeley Software Distribution (BSD). The first version was released in 1993 developed from 386BSD, one of the first fully functional and free Unix clones on affordable ...
first added x86-64 support under the name "amd64" as an experimental architecture in 5.1-RELEASE in June 2003. It was included as a standard distribution architecture as of 5.2-RELEASE in January 2004. Since then, FreeBSD has designated it as a Tier 1 platform. The 6.0-RELEASE version cleaned up some quirks with running x86 executables under amd64, and most drivers work just as they do on the x86 architecture. Work is currently being done to integrate more fully the x86
application binary interface
An application binary interface (ABI) is an interface exposed by software that is defined for in-process machine code access. Often, the exposing software is a library, and the consumer is a program.
An ABI is at a relatively low-level of a ...
(ABI), in the same manner as the Linux 32-bit ABI compatibility currently works.
NetBSD
x86-64 architecture support was first committed to the
NetBSD
NetBSD is a free and open-source Unix-like operating system based on the Berkeley Software Distribution (BSD). It was the first open-source BSD descendant officially released after 386BSD was fork (software development), forked. It continues to ...
source tree on June 19, 2001. As of NetBSD 2.0, released on December 9, 2004, ''NetBSD/amd64'' is a fully integrated and supported port.
32-bit code is still supported in 64-bit mode, with a netbsd-32 kernel compatibility layer for 32-bit syscalls. The NX bit is used to provide non-executable stack and heap with per-page granularity (segment granularity being used on 32-bit x86).
OpenBSD
OpenBSD
OpenBSD is a security-focused operating system, security-focused, free software, Unix-like operating system based on the Berkeley Software Distribution (BSD). Theo de Raadt created OpenBSD in 1995 by fork (software development), forking NetBSD ...
has supported AMD64 since OpenBSD 3.5, released on May 1, 2004. Complete in-tree implementation of AMD64 support was achieved prior to the hardware's initial release because AMD had loaned several machines for the project's
hackathon
A hackathon (also known as a hack day, hackfest, datathon or codefest; a portmanteau of '' hacking'' and ''marathon'') is an event where people engage in rapid and collaborative engineering over a relatively short period of time such as 24 or 48 h ...
that year. OpenBSD developers have taken to the platform because of its support for the
NX bit
The NX bit (no-execute bit) is a processor feature that separates areas of a virtual address space (the memory layout a program uses) into sections for storing data or program instructions. An operating system supporting the NX bit can mark certai ...
, which allowed for an easy implementation of the
W^X feature.
The code for the AMD64 port of OpenBSD also runs on Intel 64 processors which contains cloned use of the AMD64 extensions, but since Intel left out the page table NX bit in early Intel 64 processors, there is no W^X capability on those Intel CPUs; later Intel 64 processors added the NX bit under the name "XD bit".
Symmetric multiprocessing
Symmetric multiprocessing or shared-memory multiprocessing (SMP) involves a multiprocessor computer hardware and software architecture where two or more identical processors are connected to a single, shared main memory, have full access to all ...
(SMP) works on OpenBSD's AMD64 port, starting with release 3.6 on November 1, 2004.
DOS
It is possible to enter
long mode
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 mod ...
under
DOS
DOS (, ) is a family of disk-based operating systems for IBM PC compatible computers. The DOS family primarily consists of IBM PC DOS and a rebranded version, Microsoft's MS-DOS, both of which were introduced in 1981. Later compatible syste ...
without a DOS extender, but the user must return to real mode in order to call BIOS or DOS interrupts.
It may also be possible to enter
long mode
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 mod ...
with a
DOS extender
A DOS extender is a computer software program running under DOS that enables software to run in a protected mode environment even though the host operating system is only capable of operating in real mode.
DOS extenders were initially developed ...
similar to
DOS/4GW, but more complex since x86-64 lacks
virtual 8086 mode
In the 80386 microprocessor and later, virtual 8086 mode (also called virtual real mode, V86-mode, or VM86) allows the execution of real mode applications that are incapable of running directly in protected mode while the processor is running ...
. DOS itself is not aware of that, and no benefits should be expected unless running DOS in an emulation with an adequate virtualization driver backend, for example: the mass storage interface.
Linux
Linux
Linux ( ) is a family of open source Unix-like operating systems based on the Linux kernel, an kernel (operating system), operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically package manager, pac ...
was the first operating system kernel to run the x86-64 architecture in
long mode
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 mod ...
, starting with the 2.4 version in 2001 (preceding the hardware's availability). Linux also provides backward compatibility for running 32-bit executables. This permits programs to be recompiled into long mode while retaining the use of 32-bit programs. Current Linux distributions ship with x86-64-native kernels and
userlands. Some, such as
Arch Linux
Arch Linux () is an Open-source software, open source, rolling release Linux distribution. Arch Linux is kept up-to-date by regularly updating the individual pieces of software that it comprises. Arch Linux is intentionally minimal, and is meant ...
,
SUSE,
Mandriva
Mandriva S.A. was a Public company, public software company specializing in Linux and open-source software. Its corporate headquarters was in Paris, and it had development centers in Metz, France and Curitiba, Brazil. Mandriva, S.A. was the deve ...
, and
Debian
Debian () is a free and open-source software, free and open source Linux distribution, developed by the Debian Project, which was established by Ian Murdock in August 1993. Debian is one of the oldest operating systems based on the Linux kerne ...
, allow users to install a set of 32-bit components and libraries when installing off a 64-bit distribution medium, thus allowing most existing 32-bit applications to run alongside the 64-bit OS.
x32 ABI (Application Binary Interface), introduced in Linux 3.4, allows programs compiled for the x32 ABI to run in the 64-bit mode of x86-64 while only using 32-bit pointers and data fields.
Though this limits the program to a virtual address space of 4 GiB, it also decreases the memory footprint of the program and in some cases can allow it to run faster.
64-bit Linux allows up to 128
TiB of virtual address space for individual processes, and can address approximately 64 TiB of physical memory, subject to processor and system limitations,
or up to 128 PiB (virtual) and 4 PiB (physical) with 5-level paging enabled.
[
]
macOS
Mac OS X 10.4.7 and higher versions of
Mac OS X 10.4 run 64-bit command-line tools using the POSIX and math libraries on 64-bit Intel-based machines, just as all versions of Mac OS X 10.4 and 10.5 run them on 64-bit PowerPC machines. No other libraries or frameworks work with 64-bit applications in Mac OS X 10.4.
The kernel, and all kernel extensions, are 32-bit only.
Mac OS X 10.5 supports 64-bit GUI applications using
Cocoa,
Quartz
Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The Atom, atoms are linked in a continuous framework of SiO4 silicon–oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tet ...
,
OpenGL
OpenGL (Open Graphics Library) is a Language-independent specification, cross-language, cross-platform application programming interface (API) for rendering 2D computer graphics, 2D and 3D computer graphics, 3D vector graphics. The API is typic ...
, and
X11
The X Window System (X11, or simply X) is a windowing system for bitmap displays, common on Unix-like operating systems.
X originated as part of Project Athena at Massachusetts Institute of Technology (MIT) in 1984. The X protocol has been at ...
on 64-bit Intel-based machines, as well as on 64-bit
PowerPC
PowerPC (with the backronym Performance Optimization With Enhanced RISC – Performance Computing, sometimes abbreviated as PPC) is a reduced instruction set computer (RISC) instruction set architecture (ISA) created by the 1991 Apple Inc., App ...
machines.
All non-GUI libraries and frameworks also support 64-bit applications on those platforms. The kernel, and all kernel extensions, are 32-bit only.
Mac OS X 10.6 is the first version of
macOS
macOS, previously OS X and originally Mac OS X, is a Unix, Unix-based operating system developed and marketed by Apple Inc., Apple since 2001. It is the current operating system for Apple's Mac (computer), Mac computers. With ...
that supports a 64-bit
kernel. However, not all 64-bit computers can run the 64-bit kernel, and not all 64-bit computers that can run the 64-bit kernel will do so by default.
The 64-bit kernel, like the 32-bit kernel, supports 32-bit applications; both kernels also support 64-bit applications. 32-bit applications have a virtual address space limit of 4 GiB under either kernel.
The 64-bit kernel does not support 32-bit
kernel extensions, and the 32-bit kernel does not support 64-bit kernel extensions.
OS X 10.8 includes only the 64-bit kernel, but continues to support 32-bit applications; it does not support 32-bit kernel extensions, however.
macOS 10.15
macOS Catalina (version 10.15) is the sixteenth major release of macOS, Apple Inc.'s desktop operating system for Macintosh computers. It is the successor to macOS Mojave and was announced at WWDC 2019 on June 3, 2019 and released to the pub ...
includes only the 64-bit kernel and no longer supports 32-bit applications. This removal of support has presented a problem for
Wine
Wine is an alcoholic drink made from Fermentation in winemaking, fermented fruit. Yeast in winemaking, Yeast consumes the sugar in the fruit and converts it to ethanol and carbon dioxide, releasing heat in the process. Wine is most often made f ...
(and the commercial version
CrossOver), as it needs to still be able to run 32-bit Windows applications. The solution, termed ''wine32on64'', was to add
thunk
In computer programming
Computer programming or coding is the composition of sequences of instructions, called computer program, programs, that computers can follow to perform tasks. It involves designing and implementing algorithms, step-by- ...
s that bring the CPU in and out of 32-bit compatibility mode in the nominally 64-bit application.
macOS uses the
universal binary format to package 32- and 64-bit versions of application and library code into a single file; the most appropriate version is automatically selected at load time. In Mac OS X 10.6, the universal binary format is also used for the kernel and for those kernel extensions that support both 32-bit and 64-bit kernels.
Solaris
Solaris
Solaris is the Latin word for sun.
It may refer to:
Arts and entertainment Literature, television and film
* ''Solaris'' (novel), a 1961 science fiction novel by Stanisław Lem
** ''Solaris'' (1968 film), directed by Boris Nirenburg
** ''Sol ...
10 and later releases support the x86-64 architecture.
For Solaris 10, just as with the
SPARC architecture, there is only one operating system image, which contains a 32-bit kernel and a 64-bit kernel; this is labeled as the "x64/x86" DVD-ROM image. The default behavior is to boot a 64-bit kernel, allowing both 64-bit and existing or new 32-bit executables to be run. A 32-bit kernel can also be manually selected, in which case only 32-bit executables will run. The
isainfo
command can be used to determine if a system is running a 64-bit kernel.
For Solaris 11, only the 64-bit kernel is provided. However, the 64-bit kernel supports both 32- and 64-bit executables, libraries, and system calls.
Windows
x64 editions of Microsoft Windows client and server—
Windows XP Professional x64 Edition
Windows XP Professional x64 Edition is an edition of Microsoft's Windows XP operating system that supports the x86-64 architecture. It was released on April 25, 2005, alongside the x86-64 versions of Windows Server 2003.
Windows XP Profession ...
and
Windows Server 2003
Windows Server 2003, codenamed "Whistler Server", is the sixth major version of the Windows NT operating system produced by Microsoft and the first server version to be released under the Windows Server brand name. It is part of the Windows NT ...
x64 Edition—were released in March 2005. Internally they are actually the same build (5.2.3790.1830 SP1), as they share the same source base and operating system binaries, so even system updates are released in unified packages, much in the manner as Windows 2000 Professional and Server editions for x86.
Windows Vista
Windows Vista is a major release of the Windows NT operating system developed by Microsoft. It was the direct successor to Windows XP, released five years earlier, which was then the longest time span between successive releases of Microsoft W ...
, which also has many different editions, was released in January 2007.
Windows 7 was released in July 2009.
Windows Server 2008 R2
Windows Server 2008 R2, codenamed "Windows Server 7" or "Windows Server 2008 Release 2", is the eighth major version of the Windows NT operating system produced by Microsoft to be released under the Windows Server brand name. It was release ...
was sold in only x64 and Itanium editions; later versions of Windows Server only offer an x64 edition.
Versions of Windows for x64 prior to Windows 8.1 and Windows Server 2012 R2 offer the following:
* 8 TiB of virtual address space per process, accessible from both user mode and kernel mode, referred to as the user mode address space. An x64 program can use all of this, subject to backing store limits on the system, and provided it is linked with the "large address aware" option, which is present by default.
This is a 4096-fold increase over the default 2 GiB user-mode virtual address space offered by 32-bit Windows.
* 8 TiB of kernel mode virtual address space for the operating system.
As with the user mode address space, this is a 4096-fold increase over 32-bit Windows versions. The increased space primarily benefits the file system cache and kernel mode "heaps" (non-paged pool and paged pool). Windows only uses a total of 16 TiB out of the 256 TiB implemented by the processors because early AMD64 processors lacked a
CMPXCHG16B
instruction.
Under Windows 8.1 and Windows Server 2012 R2, both user mode and kernel mode virtual address spaces have been extended to 128 TiB.
These versions of Windows will not install on processors that lack the
CMPXCHG16B
instruction.
The following additional characteristics apply to all x64 versions of Windows:
* Ability to run existing 32-bit applications (
.exe
programs) and dynamic link libraries (
.dll
s) using
WoW64 if WoW64 is supported on that version. Furthermore, a 32-bit program, if it was linked with the "large address aware" option,
can use up to 4 GiB of virtual address space in 64-bit Windows, instead of the default 2 GiB (optional 3 GiB with
/3GB
boot option and "large address aware" link option) offered by 32-bit Windows.
Unlike the use of the
/3GB
boot option on x86, this does not reduce the kernel mode virtual address space available to the operating system. 32-bit applications can, therefore, benefit from running on x64 Windows even if they are not recompiled for x86-64.
* Both 32- and 64-bit applications, if not linked with "large address aware", are limited to 2 GiB of virtual address space.
* Ability to use up to 128 GiB (Windows XP/Vista), 192 GiB (Windows 7), 512 GiB (Windows 8), 1 TiB (Windows Server 2003), 2 TiB (Windows Server 2008/Windows 10), 4 TiB (Windows Server 2012), or 24 TiB (Windows Server 2016/2019) of physical random access memory (RAM).
*
LLP64 data model: in C/C++, "int" and "long" types are 32 bits wide, "long long" is 64 bits, while pointers and types derived from pointers are 64 bits wide.
* Kernel mode device drivers must be 64-bit versions; there is no way to run 32-bit kernel mode executables within the 64-bit operating system. User mode device drivers can be either 32-bit or 64-bit.
* 16-bit Windows (Win16) and DOS applications will not run on x86-64 versions of Windows due to the removal of the
virtual DOS machine subsystem (NTVDM) which relied upon the ability to use virtual 8086 mode. Virtual 8086 mode cannot be entered while running in long mode.
* Full implementation of the
NX (No Execute) page protection feature. This is also implemented on recent 32-bit versions of Windows when they are started in PAE mode.
* Instead of FS segment descriptor on x86 versions of the
Windows NT
Windows NT is a Proprietary software, proprietary Graphical user interface, graphical operating system produced by Microsoft as part of its Windows product line, the first version of which, Windows NT 3.1, was released on July 27, 1993. Original ...
family, GS segment descriptor is used to point to two operating system defined structures: Thread Information Block (NT_TIB) in user mode and Processor Control Region (KPCR) in kernel mode. Thus, for example, in user mode
GS:0
is the address of the first member of the Thread Information Block. Maintaining this convention made the x86-64 port easier, but required AMD to retain the function of the FS and GS segments in long mode – even though segmented addressing ''per se'' is not really used by any modern operating system.
* Early reports claimed that the operating system scheduler would not save and restore the
x87 FPU machine state across thread context switches. Observed behavior shows that this is not the case: the x87 state is saved and restored, except for kernel mode-only threads (a limitation that exists in the 32-bit version as well). The most recent documentation available from Microsoft states that the x87/
MMX/
3DNow!
3DNow! is a deprecated extension to the x86 instruction set developed by Advanced Micro Devices (AMD). It adds single instruction multiple data (SIMD) instructions to the base x86 instruction set, enabling it to perform vector processing of float ...
instructions may be used in long mode, but that they are deprecated and may cause compatibility problems in the future.
(3DNow! is no longer available on AMD processors, with the exception of the
PREFETCH
and
PREFETCHW
instructions, which are also supported on Intel processors as of
Broadwell.)
* Some components like
Jet Database Engine
The Access Database Engine (also Office Access Connectivity Engine or ACE and formerly Microsoft Jet Database Engine, ''Microsoft JET Engine'' or simply ''Jet'') is a database engine on which several Microsoft products have been built. The firs ...
and
Data Access Objects will not be ported to 64-bit architectures such as x86-64 and IA-64.
*
Microsoft Visual Studio
Visual Studio is an integrated development environment (IDE) developed by Microsoft. It is used to develop computer programs including websites, web apps, web services and mobile apps. Visual Studio uses Microsoft software development platforms ...
can compile
native applications to target either the x86-64 architecture, which can run only on 64-bit Microsoft Windows, or the
IA-32
IA-32 (short for "Intel Architecture, 32-bit", commonly called ''i386'') is the 32-bit version of the x86 instruction set architecture, designed by Intel and first implemented in the i386, 80386 microprocessor in 1985. IA-32 is the first incarn ...
architecture, which can run as a 32-bit application on 32-bit Microsoft Windows or 64-bit Microsoft Windows in
WoW64 emulation mode.
Managed applications can be compiled either in IA-32, x86-64 or AnyCPU modes. Software created in the first two modes behave like their IA-32 or x86-64 native code counterparts respectively; When using the AnyCPU mode, however, applications in 32-bit versions of Microsoft Windows run as 32-bit applications, while they run as a 64-bit application in 64-bit editions of Microsoft Windows.
Video game consoles
The
PlayStation 4
The PlayStation 4 (PS4) is a home video game console developed by Sony Interactive Entertainment. Announced as the successor to the PlayStation 3 in February 2013, it was launched on November 15, 2013, in North America, November 29, 2013, in ...
and
Xbox One
The Xbox One is a home video game console developed by Microsoft. Announced in May 2013, it is the successor to Xbox 360 and the third console in the Xbox#Consoles, Xbox series. It was first released in North America, parts of Europe, Austra ...
use AMD x86-64 processors based on the
Jaguar
The jaguar (''Panthera onca'') is a large felidae, cat species and the only extant taxon, living member of the genus ''Panthera'' that is native to the Americas. With a body length of up to and a weight of up to , it is the biggest cat spe ...
microarchitecture
In electronics, computer science and computer engineering, microarchitecture, also called computer organization and sometimes abbreviated as μarch or uarch, is the way a given instruction set architecture (ISA) is implemented in a particular ...
.
Firmware and games are written in x86-64 code; no legacy x86 code is involved. The
PlayStation 5
The PlayStation 5 (PS5) is a home video game console developed by Sony Interactive Entertainment. It was announced as the successor to the PlayStation 4 in April 2019, was launched on November 12, 2020, in Australia, Japan, New Zealand, North ...
and
Xbox Series X/S
The Xbox Series X and Xbox Series S are the fourth generation of consoles in the Xbox series, succeeding the previous generation's Xbox One. Released on November 10, 2020, the higher-end Xbox Series X and lower-end Xbox Series S are part o ...
use AMD x86-64 processors based on the
Zen 2 microarchitecture. The
Steam Deck uses a custom AMD x86-64
accelerated processing unit (APU) based on the Zen 2 microarchitecture.
Industry naming conventions
Since AMD64 and Intel 64 are substantially similar, many software and hardware products use one vendor-neutral term to indicate their compatibility with both implementations. AMD's original designation for this processor architecture, "x86-64", is still used for this purpose,
as is the variant "x86_64".
Other companies, such as
Microsoft
Microsoft Corporation is an American multinational corporation and technology company, technology conglomerate headquartered in Redmond, Washington. Founded in 1975, the company became influential in the History of personal computers#The ear ...
and
Sun Microsystems
Sun Microsystems, Inc., often known as Sun for short, was an American technology company that existed from 1982 to 2010 which developed and sold computers, computer components, software, and information technology services. Sun contributed sig ...
/
Oracle Corporation
Oracle Corporation is an American Multinational corporation, multinational computer technology company headquartered in Austin, Texas. Co-founded in 1977 in Santa Clara, California, by Larry Ellison, who remains executive chairman, Oracle was ...
,
use the contraction "x64" in marketing material.
The term
IA-64
IA-64 (Intel Itanium architecture) is the instruction set architecture (ISA) of the discontinued Itanium family of 64-bit Intel microprocessors. The basic ISA specification originated at Hewlett-Packard (HP), and was subsequently implemented by ...
refers to the
Itanium
Itanium (; ) is a discontinued family of 64-bit computing, 64-bit Intel microprocessors that implement the Intel Itanium architecture (formerly called IA-64). The Itanium architecture originated at Hewlett-Packard (HP), and was later jointly dev ...
processor, and should not be confused with x86-64, as it is a completely different instruction set.
Many operating systems and products, especially those that introduced x86-64 support prior to Intel's entry into the market, use the term "AMD64" or "amd64" to refer to both AMD64 and Intel 64.
* amd64
** Most
BSD
The Berkeley Software Distribution (BSD), also known as Berkeley Unix or BSD Unix, is a discontinued Unix operating system developed and distributed by the Computer Systems Research Group (CSRG) at the University of California, Berkeley, beginni ...
systems such as
FreeBSD
FreeBSD is a free-software Unix-like operating system descended from the Berkeley Software Distribution (BSD). The first version was released in 1993 developed from 386BSD, one of the first fully functional and free Unix clones on affordable ...
,
MidnightBSD,
NetBSD
NetBSD is a free and open-source Unix-like operating system based on the Berkeley Software Distribution (BSD). It was the first open-source BSD descendant officially released after 386BSD was fork (software development), forked. It continues to ...
and
OpenBSD
OpenBSD is a security-focused operating system, security-focused, free software, Unix-like operating system based on the Berkeley Software Distribution (BSD). Theo de Raadt created OpenBSD in 1995 by fork (software development), forking NetBSD ...
refer to both AMD64 and Intel 64 under the architecture name "amd64".
** Some
Linux distribution
A Linux distribution, often abbreviated as distro, is an operating system that includes the Linux kernel for its kernel functionality. Although the name does not imply product distribution per se, a distro—if distributed on its own—is oft ...
s such as
Debian
Debian () is a free and open-source software, free and open source Linux distribution, developed by the Debian Project, which was established by Ian Murdock in August 1993. Debian is one of the oldest operating systems based on the Linux kerne ...
,
Ubuntu
Ubuntu ( ) is a Linux distribution based on Debian and composed primarily of free and open-source software. Developed by the British company Canonical (company), Canonical and a community of contributors under a Meritocracy, meritocratic gover ...
,
Gentoo Linux
Gentoo Linux (pronounced ) is a Linux distribution built using the Portage package management system. Unlike a binary software distribution, the source code is compiled locally according to the user's preferences and is often optimized for ...
refer to both AMD64 and Intel 64 under the architecture name "amd64".
**
Microsoft Windows
Windows is a Product lining, product line of Proprietary software, proprietary graphical user interface, graphical operating systems developed and marketed by Microsoft. It is grouped into families and subfamilies that cater to particular sec ...
's x64 versions use the AMD64 moniker internally to designate various components which use or are compatible with this architecture. For example, the
environment variable
An environment variable is a user-definable value that can affect the way running processes will behave on a computer. Environment variables are part of the environment in which a process runs. For example, a running process can query the va ...
PROCESSOR_ARCHITECTURE is assigned the value "AMD64" as opposed to "x86" in 32-bit versions, and the system directory on a Windows x64 Edition installation CD-ROM is named "AMD64", in contrast to "i386" in 32-bit versions.
** Sun's
Solaris
Solaris is the Latin word for sun.
It may refer to:
Arts and entertainment Literature, television and film
* ''Solaris'' (novel), a 1961 science fiction novel by Stanisław Lem
** ''Solaris'' (1968 film), directed by Boris Nirenburg
** ''Sol ...
's ''isalist'' command identifies both AMD64- and Intel 64-based systems as "amd64".
**
Java Development Kit
The Java Development Kit (JDK) is a distribution of Java technology by Oracle Corporation. It implements the Java Language Specification (JLS) and the Java Virtual Machine Specification (JVMS) and provides the Standard Edition (SE) of the Java ...
(JDK): the name "amd64" is used in directory names containing x86-64 files.
* x86_64
** The
Linux kernel
The Linux kernel is a Free and open-source software, free and open source Unix-like kernel (operating system), kernel that is used in many computer systems worldwide. The kernel was created by Linus Torvalds in 1991 and was soon adopted as the k ...
and the
GNU Compiler Collection
The GNU Compiler Collection (GCC) is a collection of compilers from the GNU Project that support various programming languages, Computer architecture, hardware architectures, and operating systems. The Free Software Foundation (FSF) distributes ...
refers to 64-bit architecture as "x86_64".
** Some Linux distributions, such as
Fedora
A fedora () is a hat with a soft brim and indented crown.Kilgour, Ruth Edwards (1958). ''A Pageant of Hats Ancient and Modern''. R. M. McBride Company. It is typically creased lengthwise down the crown and "pinched" near the front on both sides ...
,
openSUSE
openSUSE () is a free and open-source software, free and open-source Linux distribution developed by the openSUSE project. It is offered in two main variations: ''Tumbleweed'', an upstream rolling release distribution, and ''Leap'', a stable r ...
,
Arch Linux
Arch Linux () is an Open-source software, open source, rolling release Linux distribution. Arch Linux is kept up-to-date by regularly updating the individual pieces of software that it comprises. Arch Linux is intentionally minimal, and is meant ...
,
Gentoo Linux
Gentoo Linux (pronounced ) is a Linux distribution built using the Portage package management system. Unlike a binary software distribution, the source code is compiled locally according to the user's preferences and is often optimized for ...
refer to this 64-bit architecture as "x86_64".
** Apple
macOS
macOS, previously OS X and originally Mac OS X, is a Unix, Unix-based operating system developed and marketed by Apple Inc., Apple since 2001. It is the current operating system for Apple's Mac (computer), Mac computers. With ...
refers to 64-bit architecture as "x86-64" or "x86_64", as seen in the Terminal command
arch
and in their developer documentation.
** Breaking with most other BSD systems,
DragonFly BSD
DragonFly BSD is a free and open-source Unix-like operating system forked from FreeBSD 4.8. Matthew Dillon, an Amiga developer in the late 1980s and early 1990s and FreeBSD developer between 1994 and 2003, began working on DragonFly BSD in ...
refers to 64-bit architecture as "x86_64".
**
Haiku
is a type of short form poetry that originated in Japan. Traditional Japanese haiku consist of three phrases composed of 17 Mora (linguistics), morae (called ''On (Japanese prosody), on'' in Japanese) in a 5, 7, 5 pattern; that include a ''kire ...
refers to 64-bit architecture as "x86_64".
Licensing
x86-64/AMD64 was solely developed by AMD. Until April 2021 when the relevant patents expired, AMD held patents on techniques used in AMD64; those patents had to be licensed from AMD in order to implement AMD64. Intel entered into a cross-licensing agreement with AMD, licensing to AMD their patents on existing x86 techniques, and licensing from AMD their patents on techniques used in x86-64. In 2009, AMD and Intel settled several lawsuits and cross-licensing disagreements, extending their cross-licensing agreements.
See also
*
AGESA
AMD Generic Encapsulated Software Architecture (AGESA) is a procedure library developed by Advanced Micro Devices (AMD), used to perform the Platform Initialization (PI) on mainboards using their AMD64 architecture. As part of the BIOS of such ma ...
(AMD Generic Encapsulated Software Architecture)
*
Transient execution CPU vulnerability
Notes
References
External links
AMD Developer Guides, Manuals & ISA Documents– technical talk by the architect of AMD64
video archive, an
video archive
Intel tweaks EM64T for full AMD64 compatibilityAnalyst: Intel Reverse-Engineered AMD64Early report of differences between Intel IA32e and AMD64Porting to 64-bit GNU/Linux Systems by Andreas Jaeger from
GCC Summit 2003. An excellent paper explaining almost all practical aspects for a transition from 32-bit to 64-bit.
Intel 64 ArchitectureIntel Software Network: "64 bits"TurboIRC.COM tutorials, including examples of how to of enter protected and long mode the raw way from DOSSeven Steps of Migrating a Program to a 64-bit SystemMemory Limits for Windows Releases
{{AMD technology
Computer-related introductions in 2003
X86 architecture
64-bit computers
AMD technologies