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In
computer architecture In computer engineering, computer architecture is a set of rules and methods that describe the functionality, organization, and implementation of computer systems. The architecture of a system refers to its structure in terms of separately specifi ...
, 64-bit
integers An integer (from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of t ...
, memory addresses, or other
data Data (; ) are individual facts A fact is something that is truth, true. The usual test for a statement of fact is verifiability—that is whether it can be demonstrated to correspond to experience. Standard reference works are often used ...
units are those that are 64
bit The bit is a basic unit of information in computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithm of an algorithm (Euclid's algo ...
s (8 octets) wide. Also, 64-bit
central processing unit A central processing unit (CPU), also called a central processor, main processor or just processor, is the electronic circuit 200px, A circuit built on a printed circuit board (PCB). An electronic circuit is composed of individual electroni ...

central processing unit
(CPU) and
arithmetic logic unit In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both computer hardware , hardware and sof ...
(ALU) architectures are those that are based on
processor register Processor may refer to: Computing Hardware * Processor (computing) In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic p ...
s,
address bus In computer architecture In computer engineering Computer engineering (CoE or CpE) is a branch of engineering Engineering is the use of scientific method, scientific principles to design and build machines, structures, and other ...
es, or data buses of that size. 64-bit
microcomputer A microcomputer is a small, relatively inexpensive computer with a microprocessor as its central processing unit (CPU). It includes a microprocessor, Computer memory, memory and minimal input/output (I/O) circuitry mounted on a single printed ci ...
s are computers in which 64-bit
microprocessor A microprocessor is a computer processor where the data processing logic and control is included on a single integrated circuit An integrated circuit or monolithic integrated circuit (also referred to as an IC, a chip, or a microchip ...

microprocessor
s are the norm. From the software perspective, 64-bit computing means the use of
machine code In computer programming Computer programming is the process of designing and building an executable computer program to accomplish a specific computing result or to perform a particular task. Programming involves tasks such as analysis, ge ...
with 64-bit
virtual memory In computing, virtual memory, or virtual storage is a Memory management (operating systems), memory management technique that provides an "idealized abstraction of the storage resources that are actually available on a given machine" which "cre ...

virtual memory
addresses. However, not all 64-bit instruction sets support full 64-bit virtual memory addresses;
x86-64 x86-64 (also known as x64, x86_64, AMD64, and Intel 64) is a 64-bit version of the x86 instruction set, first released in 1999. It introduced two new modes of operation, 64-bit mode and compatibility mode, along with a new 4-level paging mod ...
and
ARMv8 ARM (stylised in lowercase as arm, previously an acronym for Advanced RISC Machines and originally Acorn RISC Machine) is a family of reduced instruction set computing In computer engineering Computer engineering (CoE or CpE) is a ...
, for example, support only 48 bits of virtual address, with the remaining 16 bits of the virtual address required to be all 0's or all 1's, and several 64-bit instruction sets support fewer than 64 bits of physical memory address. The term ''64-bit'' describes a generation of computers in which 64-bit processors are the norm. 64 bits is a
word In linguistics Linguistics is the scientific study of language A language is a structured system of communication used by humans, including speech (spoken language), gestures (Signed language, sign language) and writing. Most lang ...
size that defines certain classes of computer architecture, buses, memory, and CPUs and, by extension, the software that runs on them. 64-bit CPUs have been used in
supercomputer upright=1.5, Computing power of the top 1 supercomputer each year, measured in FLOPS A supercomputer is a computer with a high level of performance as compared to a general-purpose computer. The performance of a supercomputer is commonly mea ...

supercomputer
s since the 1970s (
Cray-1 The Cray-1 was a supercomputer A supercomputer is a 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) ...

Cray-1
, 1975) and in
reduced instruction set computer In computer engineering, a reduced instruction set computer (RISC) is a computer designed to simplify the individual instructions given to the computer in order to realise a task. Unlike the instructions given to a complex instruction set comp ...
s (RISC) based
workstation A workstation is a special computer designed for technical or computational science, scientific applications. Intended primarily to be used by one person at a time, they are commonly connected to a local area network and run multi-user operating ...

workstation
s and
server Server may refer to: Computing *Server (computing) In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and dev ...
s since the early 1990s, notably the MIPS
R4000 The R4000 is a microprocessor A microprocessor is a computer processor where the data processing logic and control is included on a single integrated circuit, or a small number of integrated circuits. The microprocessor is a multipurpose, c ...

R4000
,
R8000The R8000 is a microprocessor A microprocessor is a computer processor where the data processing logic and control is included on a single integrated circuit, or a small number of integrated circuits. The microprocessor is a multipurpose, clo ...

R8000
, and
R10000 The R10000, code-named "T5", is a RISC microprocessor implementation of the MIPS IV instruction set architecture In computer science Computer science deals with the theoretical foundations of information, algorithms and the architectures ...
, the
Digital Equipment Corporation Digital Equipment Corporation (DEC ), using the trademark A trademark (also written trade mark or trade-mark) is a type of intellectual property Intellectual property (IP) is a category of property Property is a system of rights ...
(DEC)
Alpha Alpha (uppercase , lowercase ; grc, ἄλφα, ''álpha'', modern pronunciation ''álfa'') is the first letter Letter, letters, or literature may refer to: Characters typeface * Letter (alphabet) A letter is a segmental symbol A s ...
, the
Sun Microsystems Sun Microsystems, Inc. (Sun for short) was an American technology company that sold computer A computer is a machine that can be programmed to Execution (computing), carry out sequences of arithmetic or logical operations automatically. M ...
UltraSPARC The UltraSPARC is a microprocessor A microprocessor is a computer processor where the data processing logic and control is included on a single integrated circuit, or a small number of integrated circuits. The microprocessor is a multipurpo ...
, and the
IBM International Business Machines Corporation (IBM) is an American multinational technology company headquartered in Armonk, New York, with operations in over 170 countries. The company began in 1911, founded in Endicott, New York, as the C ...

IBM
RS64 The IBM RS64 is a family of microprocessor A microprocessor is a computer processor where the data processing logic and control is included on a single integrated circuit, or a small number of integrated circuits. The microprocessor contains ...
and
POWER3 The POWER3 is a , designed and exclusively manufactured by , that implemented the 64-bit version of the (ISA), including all of the optional instructions of the ISA (at the time) such as instructions present in the version of the but not ...

POWER3
and later
POWER Power typically refers to: * Power (physics) In physics, power is the amount of energy transferred or converted per unit time. In the International System of Units, the unit of power is the watt, equal to one joule per second. In older works, p ...
microprocessor A microprocessor is a computer processor where the data processing logic and control is included on a single integrated circuit An integrated circuit or monolithic integrated circuit (also referred to as an IC, a chip, or a microchip ...

microprocessor
s. In 2003, 64-bit CPUs were introduced to the (formerly
32-bit 32-bit microcomputer A microcomputer is a small, relatively inexpensive computer with a microprocessor as its central processing unit (CPU). It includes a microprocessor, Computer memory, memory and minimal input/output (I/O) circuitry mount ...
) mainstream
personal computer A personal computer (PC) is a multi-purpose computer whose size, capabilities, and price make it feasible for individual use. Personal computers are intended to be operated directly by an end user, rather than by a computer expert or technician ...
market in the form of
x86-64 x86-64 (also known as x64, x86_64, AMD64, and Intel 64) is a 64-bit version of the x86 instruction set, first released in 1999. It introduced two new modes of operation, 64-bit mode and compatibility mode, along with a new 4-level paging mod ...
processors and the
PowerPC G5 The PowerPC 970, PowerPC 970FX, and PowerPC 970MP are 64-bit In computer architecture, 64-bit integers, memory addresses, or other data units are those that are 64 bits (8 octets) wide. Also, 64-bit central processing unit A centra ...
, and were introduced in 2012 into the
ARM architecture ARM (stylised in lowercase as arm, previously an acronym for Advanced RISC Machines and originally Acorn RISC Machine) is a family of reduced instruction set computing (RISC) instruction set, architectures for central processing unit, comp ...
targeting
smartphone A smartphone is a portable device A mobile device (or handheld computer) is a computer A computer is a machine that can be programmed to carry out sequences of arithmetic or logical operations automatically. Modern computers can per ...

smartphone
s and
tablet computer A tablet computer, commonly shortened to tablet, is a mobile device A mobile device (or handheld computer) is a computer A computer is a machine that can be programmed to carry out sequences of arithmetic or logical operations aut ...
s, first sold on September 20, 2013, in the
iPhone 5S The iPhone 5S (stylized and marketed as iPhone 5s) is a smartphone A smartphone is a portable device A mobile device (or handheld computer) is a computer A computer is a machine that can be programmed to carry out sequences of a ...

iPhone 5S
powered by the
ARMv8-A ARM (stylised in lowercase as arm, previously an acronym for Advanced RISC Machines and originally Acorn RISC Machine) is a family of reduced instruction set computing In computer engineering Computer engineering (CoE or CpE) is ...
Apple A7 The Apple A7 is a 64-bit In computer architecture, 64-bit Integer (computer science), integers, Memory address, memory addresses, or other Data#Uses%20of%20data%20in%20computing, data units are those that are 64 bits (8 Octet (computing), oc ...
system on a chip A system on a chip (SoC; or This article uses the convention that SoC is pronounced . Therefore, it uses the convention "an" for the indefinite article Indefinite may refer to: * the opposite of definite in grammar ** indefinite article ...
(SoC). A 64-bit register can hold any of 264 (over 18
quintillion This article lists and discusses the usage and derivation of names of large numbers, together with their possible extensions. The following table lists those names of large numbers that are found in many English dictionaries and thus have a clai ...
or 1.8×1019) different values. The range of
integer An integer (from the Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', meaning "to share" or "to ...
values that can be stored in 64 bits depends on the integer representation used. With the two most common representations, the range is 0 through 18,446,744,073,709,551,615 (264 − 1) for representation as an (
unsigned Unsigned can refer to: * An unsigned artist is a musical artist or group not attached or signed to a record label ** Unsigned Music Awards, ceremony noting achievements of unsigned artists ** Unsigned band web, online community * Similarly, the con ...
)
binary number In mathematics and digital electronics Digital electronics is a field of electronics The field of electronics is a branch of physics and electrical engineering that deals with the emission, behaviour and effects of electrons The electr ...
, and −9,223,372,036,854,775,808 (−263) through 9,223,372,036,854,775,807 (263 − 1) for representation as
two's complement Two's complement is a mathematical operation In mathematics, an operation is a Function (mathematics), function which takes zero or more input values (called ''operands'') to a well-defined output value. The number of operands is the arity of the ...
. Hence, a processor with 64-bit memory addresses can directly access 264 bytes (=16
exabyte The byte is a unit of digital information that most commonly consists of eight bit The bit is a basic unit of information in computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It ...
s) of byte-addressable memory. With no further qualification, a ''64-bit computer architecture'' generally has integer and addressing
processor register Processor may refer to: Computing Hardware * Processor (computing) In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic p ...
s that are 64 bits wide, allowing direct support for 64-bit data types and addresses. However, a CPU might have external data buses or
address bus In computer architecture In computer engineering Computer engineering (CoE or CpE) is a branch of engineering Engineering is the use of scientific method, scientific principles to design and build machines, structures, and other ...
es with different sizes from the registers, even larger (the 32-bit
Pentium Pentium is a brand used for a series of x86 architecture-compatible microprocessors produced by Intel. The Pentium (original), original Pentium was released in 1993. After that, the Pentium II and Pentium III were released. In their form , ...
had a 64-bit data bus, for instance). The term may also refer to the size of low-level data types, such as 64-bit
floating-point arithmetic In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both computer hardware , hardware and soft ...
numbers.


Architectural implications

Processor registers are typically divided into several groups: ''integer'', ''floating-point'', ''single-instruction-multiple-data'' (
SIMD Single instruction, multiple data (SIMD) is a type of parallel computer, parallel processing in Flynn's taxonomy. SIMD can be internal (part of the hardware design) and it can be directly accessible through an instruction set architecture (IS ...

SIMD
), ''control'', and often special registers for address arithmetic which may have various uses and names such as ''address'', ''index'', or ''base registers''. However, in modern designs, these functions are often performed by more general purpose ''integer'' registers. In most processors, only integer or address-registers can be used to address data in memory; the other types of registers cannot. The size of these registers therefore normally limits the amount of directly addressable memory, even if there are registers, such as floating-point registers, that are wider. Most high performance 32-bit and 64-bit processors (some notable exceptions are older or embedded
ARM architecture ARM (stylised in lowercase as arm, previously an acronym for Advanced RISC Machines and originally Acorn RISC Machine) is a family of reduced instruction set computing (RISC) instruction set, architectures for central processing unit, comp ...
(ARM) and 32-bit
MIPS architecture MIPS (Microprocessor without Interlocked Pipelined Stages) is a reduced instruction set computer A reduced instruction set computer, or RISC (), is a computer with a small, highly optimized set of instructions, rather than the more special ...
(MIPS) CPUs) have integrated floating point hardware, which is often, but not always, based on 64-bit units of data. For example, although the
x86 x86 is a family of instruction set architecture In computer science, an instruction set architecture (ISA), also called computer architecture, is an abstract model of a computer. A device that executes instructions described by that ISA, ...

x86
/
x87 x87 is a floating-point-related subset of the x86 architecture instruction set In computer science Computer science deals with the theoretical foundations of information, algorithms and the architectures of its computation as well as pra ...
architecture has instructions able to load and store 64-bit (and 32-bit) floating-point values in memory, the internal floating point data and register format is 80 bits wide, while the general-purpose registers are 32 bits wide. In contrast, the 64-bit
Alpha Alpha (uppercase , lowercase ; grc, ἄλφα, ''álpha'', modern pronunciation ''álfa'') is the first letter Letter, letters, or literature may refer to: Characters typeface * Letter (alphabet) A letter is a segmental symbol A s ...
family uses a 64-bit floating-point data and register format, and 64-bit integer registers.


History

Many computer
instruction set In computer science Computer science deals with the theoretical foundations of information, algorithms and the architectures of its computation as well as practical techniques for their application. Computer science is the study of co ...
s are designed so that a single integer register can store the
memory address In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both computer hardware , hardware and softwa ...
to any location in the computer's physical or
virtual memory In computing, virtual memory, or virtual storage is a Memory management (operating systems), memory management technique that provides an "idealized abstraction of the storage resources that are actually available on a given machine" which "cre ...

virtual memory
. Therefore, the total number of addresses to memory is often determined by the width of these registers. The
IBM International Business Machines Corporation (IBM) is an American multinational technology company headquartered in Armonk, New York, with operations in over 170 countries. The company began in 1911, founded in Endicott, New York, as the C ...

IBM
System/360 The IBM System/360 (S/360) is a family of mainframe computer A mainframe computer, informally called a mainframe or big iron, is a computer used primarily by large organizations for critical applications like bulk data processing for t ...
of the 1960s was an early 32-bit computer; it had 32-bit integer registers, although it only used the low order 24 bits of a word for addresses, resulting in a 16  MB [] address space. 32-bit superminicomputers, such as the Digital Equipment Corporation, DEC VAX, became common in the 1970s, and 32-bit microprocessors, such as the Motorola 68000 family and the 32-bit members of the x86 family starting with the
Intel 80386 The Intel 386, originally released as 80386 and later renamed i386, is a 32-bit microprocessor introduced in 1985. The first versions had 275,000 transistorsaddress register meant that 232 addresses, or 4  of
random-access memory Random-access memory (RAM; ) is a form of computer memory In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic proces ...
(RAM), could be referenced. When these architectures were devised, 4 GB of memory was so far beyond the typical amounts (4 MB) in installations, that this was considered to be enough ''headroom'' for addressing. 4.29 billion addresses were considered an appropriate size to work with for another important reason: 4.29 billion integers are enough to assign unique references to most entities in applications like
database In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both computer hardware , hardware and sof ...

database
s. Some
supercomputer upright=1.5, Computing power of the top 1 supercomputer each year, measured in FLOPS A supercomputer is a computer with a high level of performance as compared to a general-purpose computer. The performance of a supercomputer is commonly mea ...

supercomputer
architectures of the 1970s and 1980s, such as the
Cray-1 The Cray-1 was a supercomputer A supercomputer is a 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) ...

Cray-1
, used registers up to 64 bits wide, and supported 64-bit integer arithmetic, although they did not support 64-bit addressing. In the mid-1980s,
Intel i860 Intel Corporation is an American multinational corporation A multinational company (MNC) is a corporation, corporate organization that owns and controls the production of goods or services in at least one country other than its home country. ...
development began culminating in a (too late for Windows NT) 1989 release; the i860 had 32-bit integer registers and 32-bit addressing, so it was not a fully 64-bit processor, although its graphics unit supported 64-bit integer arithmetic. However, 32 bits remained the norm until the early 1990s, when the continual reductions in the cost of memory led to installations with amounts of RAM approaching 4 GB, and the use of virtual memory spaces exceeding the 4 GB ceiling became desirable for handling certain types of problems. In response, MIPS and DEC developed 64-bit microprocessor architectures, initially for high-end
workstation A workstation is a special computer designed for technical or computational science, scientific applications. Intended primarily to be used by one person at a time, they are commonly connected to a local area network and run multi-user operating ...

workstation
and
server Server may refer to: Computing *Server (computing) In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and dev ...
machines. By the mid-1990s,
HAL Computer Systems HAL Computer Systems, Inc was a Campbell, California-based computer manufacturer founded in 1990 by Andrew Heller, a principal designer of the original IBM POWER instruction set architecture, IBM POWER architecture. His idea was to build computers ...
,
Sun Microsystems Sun Microsystems, Inc. (Sun for short) was an American technology company that sold computer A computer is a machine that can be programmed to Execution (computing), carry out sequences of arithmetic or logical operations automatically. M ...
,
IBM International Business Machines Corporation (IBM) is an American multinational technology company headquartered in Armonk, New York, with operations in over 170 countries. The company began in 1911, founded in Endicott, New York, as the C ...

IBM
,
Silicon Graphics Silicon Graphics, Inc. (stylized as ''SiliconGraphics'' before 1999, later rebranded SGI, historically known as Silicon Graphics Computer Systems or SGCS) was an American high-performance computing A supercomputer is a computer with a hig ...
, and
Hewlett Packard The Hewlett-Packard Company, commonly shortened to Hewlett-Packard ( ) or HP, was an American multinational information technology company headquartered in Palo Alto, California. HP developed and provided a wide variety of hardware compone ...
had developed 64-bit architectures for their workstation and server systems. A notable exception to this trend were
mainframes A mainframe computer, informally called a mainframe or big iron, is a computer A computer is a machine that can be programmed to carry out sequences of arithmetic or logical operations automatically. Modern computers can perform generic ...
from IBM, which then used 32-bit data and 31-bit address sizes; the IBM mainframes did not include 64-bit processors until 2000. During the 1990s, several low-cost 64-bit microprocessors were used in consumer electronics and embedded applications. Notably, the
Nintendo 64 The (abbreviated as N64) is a home video game console developed by Nintendo. The successor to the Super Nintendo Entertainment System, it was released on June 23, 1996 in Japan, on September 29, 1996 in North America, and March 1, 1997 in Europ ...

Nintendo 64
and the
PlayStation 2 The PlayStation 2 (PS2) is a home video game console A home video game console is a video game console A video game console is an electronic device that output Output may refer to: * The information produced by a computer, see Input/ ...

PlayStation 2
had 64-bit microprocessors before their introduction in personal computers. High-end printers, network equipment, and industrial computers, also used 64-bit microprocessors, such as the Quantum Effect Devices
R5000 The R5000 is a 64-bit, little endian (mipsel) superscalar, in-order execution 2-issue design microprocessor, that implements the MIPS architecture, MIPS IV instruction set architecture (ISA) developed by Quantum Effect Design (QED) in 1996. The p ...
. 64-bit computing started to trickle down to the personal computer desktop from 2003 onward, when some models in
Apple An apple is an edible fruit In botany Botany, also called , plant biology or phytology, is the science of plant life and a branch of biology. A botanist, plant scientist or phytologist is a scientist who specialises in this fie ...
's Macintosh lines switched to
PowerPC 970 The PowerPC 970, PowerPC 970FX, and PowerPC 970MP are 64-bit In computer architecture, 64-bit integers, memory addresses, or other data units are those that are 64 bits (8 octets) wide. Also, 64-bit central processing unit A centra ...
processors (termed ''G5'' by Apple), and
Advanced Micro Devices Advanced Micro Devices, Inc. (AMD) is an American multinational company, multinational semiconductor industry, semiconductor company based in Santa Clara, California, Santa Clara, California, that develops computer processors and related techno ...
(AMD) released its first 64-bit
x86-64 x86-64 (also known as x64, x86_64, AMD64, and Intel 64) is a 64-bit version of the x86 instruction set, first released in 1999. It introduced two new modes of operation, 64-bit mode and compatibility mode, along with a new 4-level paging mod ...
processor.


Limits of processors

In principle, a 64-bit microprocessor can address 16  EBs (, or about 18.4 exabytes) of memory. However, not all instruction sets, and not all processors implementing those instruction sets, support a full 64-bit virtual or physical address space. The x86-64 architecture () allows 48 bits for virtual memory and, for any given processor, up to 52 bits for physical memory. These limits allow memory sizes of 256  TB () and 4  PB (), respectively. A PC cannot currently contain 4 petabytes of memory (due to the physical size of the memory chips), but AMD envisioned large servers, shared memory clusters, and other uses of physical address space that might approach this in the foreseeable future. Thus the 52-bit physical address provides ample room for expansion while not incurring the cost of implementing full 64-bit physical addresses. Similarly, the 48-bit virtual address space was designed to provide 65,536 (216) times the 32-bit limit of 4 GB (), allowing room for later expansion and incurring no overhead of translating full 64-bit addresses. The Power ISA v3.0 allows 64 bits for an effective address, mapped to a segmented address with between 65 and 78 bits allowed, for virtual memory, and, for any given processor, up to 60 bits for physical memory. The Oracle
SPARC SPARC (Scalable Processor Architecture) is a reduced instruction set computing (RISC) instruction set architecture originally developed by Sun Microsystems. Its design was strongly influenced by the experimental Berkeley RISC system developed i ...

SPARC
Architecture 2015 allows 64 bits for virtual memory and, for any given processor, between 40 and 56 bits for physical memory. The ARM AArch64 Virtual Memory System Architecture allows 48 bits for virtual memory and, for any given processor, from 32 to 48 bits for physical memory. The
DEC Alpha Alpha, originally known as Alpha AXP, is a 64-bit computing, 64-bit reduced instruction set computing (RISC) instruction set architecture (ISA) developed by Digital Equipment Corporation (DEC). Alpha was designed to replace 32-bit computing, 3 ...
specification requires minimum of 43 bits of virtual memory address space (8 TB) to be supported, and hardware need to check and trap if the remaining unsupported bits are zero (to support compatibility on future processors).
Alpha 21064 The Alpha 21064 is a developed and fabricated by that implemented the (introduced as the Alpha AXP) (ISA). It was introduced as the DECchip 21064 before it was renamed in 1994. The 21064 is also known by its code name, EV4. It was announced ...
supported 43 bits of virtual memory address space (8 TB) and 34 bits of physical memory address space (16 GB).
Alpha 21164 processor board. The Alpha 21164, also known by its code name, EV5, is a microprocessor A microprocessor is a computer processor where the data processing logic and control is included on a single integrated circuit, or a small number of in ...
supported 43 bits of virtual memory address space (8 TB) and 40 bits of physical memory address space (1 TB).
Alpha 21264 The Alpha 21264 is a Digital Equipment Corporation Digital Equipment Corporation (DEC ), using the trademark Digital, was a major American company in the computer industry from the 1960s to the 1990s. The company was co-founded by Ken Olsen and ...

Alpha 21264
supported user-configurable 43 or 48 bits of virtual memory address space (8 TB or 256 TB) and 44 bits of physical memory address space (16 TB).


64-bit data timeline

;1961: IBM delivers the
IBM 7030 Stretch The IBM 7030, also known as Stretch, was IBM International Business Machines Corporation (IBM) is an American multinational technology company headquartered in Armonk, New York, with operations in over 170 countries. The company began in 1 ...
supercomputer upright=1.5, Computing power of the top 1 supercomputer each year, measured in FLOPS A supercomputer is a computer with a high level of performance as compared to a general-purpose computer. The performance of a supercomputer is commonly mea ...

supercomputer
, which uses 64-bit data words and 32- or 64-bit instruction words. ;1974:
Control Data Corporation Control may refer to: Basic meanings Economics and business * Control (management) Control is a function of management which helps to check errors in order to take corrective actions. This is done to minimize deviation from standards and ensur ...
launches the
CDC Star-100 The CDC STAR-100 is a vector supercomputer by the largest supercomputer over time A supercomputer is a computer A computer is a machine that can be programmed to carry out sequences of arithmetic or logical operations automatically. Mo ...
vector supercomputer, which uses a 64-bit word architecture (prior CDC systems were based on a 60-bit architecture). :
International Computers Limited International Computers Limited (ICL) was a British computer hardware Computer hardware includes the physical parts of a computer, such as the computer case, case, central processing unit (CPU), Computer monitor, monitor, Computer mouse, mou ...
launches the
ICL 2900 Series The ICL 2900 Series was a range of mainframe A pair of IBM mainframes. On the left is the IBM z Systems z13. On the right is the IBM LinuxONE Rockhopper.">IBM_LinuxONE.html" ;"title="IBM z Systems z13. On the right is the IBM LinuxONE">IBM z ...
with 32-bit, 64-bit, and 128-bit
two's complement Two's complement is a mathematical operation In mathematics, an operation is a Function (mathematics), function which takes zero or more input values (called ''operands'') to a well-defined output value. The number of operands is the arity of the ...
integers; 64-bit and 128-bit floating point; 32-bit, 64-bit, and 128-bit packed decimal and a 128-bit accumulator register. The architecture has survived through a succession of ICL and Fujitsu machines. The latest is the Fujitsu Supernova, which emulates the original environment on 64-bit Intel processors. ;1976:
Cray Research Cray Inc., a subsidiary of Hewlett Packard Enterprise, is an American supercomputer manufacturer headquartered in Seattle, Washington. It also manufactures systems for data storage and analytics. Several Cray supercomputer systems are listed i ...
delivers the first
Cray-1 The Cray-1 was a supercomputer A supercomputer is a 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) ...

Cray-1
supercomputer, which is based on a 64-bit word architecture and will form the basis for later Cray vector supercomputers. ;1983:
Elxsi Elxsi Corporation was a minicomputer A minicomputer, or colloquially mini, is a class of smaller general purpose computer A computer is a machine that can be programmed to carry out sequences of arithmetic or logical operations automatic ...
launches the Elxsi 6400 parallel minisupercomputer. The Elxsi architecture has 64-bit data registers but a 32-bit address space. ;1989:
Intel Intel Corporation is an American multinational corporation A multinational company (MNC) is a corporate A corporation is an organization—usually a group of people or a company A company, abbreviated as co., is a Legal personalit ...

Intel
introduces the
Intel i860 Intel Corporation is an American multinational corporation A multinational company (MNC) is a corporation, corporate organization that owns and controls the production of goods or services in at least one country other than its home country. ...
reduced instruction set computer In computer engineering, a reduced instruction set computer (RISC) is a computer designed to simplify the individual instructions given to the computer in order to realise a task. Unlike the instructions given to a complex instruction set comp ...
(RISC) processor. Marketed as a "64-Bit Microprocessor", it had essentially a 32-bit architecture, enhanced with a 3D graphics unit capable of 64-bit integer operations. ;1993:
Atari Atari () is a brand name owned by several entities since its inception in 1972, currently by Atari Interactive Atari Interactive is a name used by several separate groups and corporations since the mid-1990s. In 1996, it was the name of Ata ...

Atari
introduces the
Atari Jaguar The Atari Jaguar is a home video game console A home video game console is a video game console that is designed to be connected to a display device, such as a television, and an external power source as to play video games. Home consoles ar ...
video game console A video game console is an electronic device that output Output may refer to: * The information produced by a computer, see Input/output In computing, input/output (I/O, or informally io or IO) is the communication between an information pro ...
, which includes some 64-bit wide data paths in its architecture.


64-bit address timeline

;1991:
MIPS Computer Systems MIPS Technologies, Inc., formerly MIPS Computer Systems, Inc., was an American American(s) may refer to: * American, something of, from, or related to the United States of America, commonly known as the United States The United States o ...
produces the first 64-bit microprocessor, the
R4000 The R4000 is a microprocessor A microprocessor is a computer processor where the data processing logic and control is included on a single integrated circuit, or a small number of integrated circuits. The microprocessor is a multipurpose, c ...

R4000
, which implements the
MIPS III MIPS (Microprocessor without Interlocked Pipelined Stages) is a reduced instruction set computer (RISC) instruction set architecture (ISA)Price, Charles (September 1995). ''MIPS IV Instruction Set'' (Revision 3.2), MIPS Technologies, Inc. develope ...
architecture, the third revision of its
MIPS architecture MIPS (Microprocessor without Interlocked Pipelined Stages) is a reduced instruction set computer A reduced instruction set computer, or RISC (), is a computer with a small, highly optimized set of instructions, rather than the more special ...
. The CPU is used in
SGI SGI may refer to: Companies *Saskatchewan Government Insurance Saskatchewan Government Insurance (SGI) is a Canada, Canadian insurance company and a Crown corporations, Crown corporation wholly owned by the Government of Saskatchewan. SGI's opera ...
graphics workstations starting with the IRIS Crimson. Kendall Square Research deliver their first KSR1 supercomputer, based on a proprietary 64-bit RISC processor architecture running Tru64 UNIX#OSF/1, OSF/1. ;1992:
Digital Equipment Corporation Digital Equipment Corporation (DEC ), using the trademark A trademark (also written trade mark or trade-mark) is a type of intellectual property Intellectual property (IP) is a category of property Property is a system of rights ...
(DEC) introduces the pure 64-bit
Alpha Alpha (uppercase , lowercase ; grc, ἄλφα, ''álpha'', modern pronunciation ''álfa'') is the first letter Letter, letters, or literature may refer to: Characters typeface * Letter (alphabet) A letter is a segmental symbol A s ...
architecture which was born from the DEC Prism, Prism project. ;1994:
Intel Intel Corporation is an American multinational corporation A multinational company (MNC) is a corporate A corporation is an organization—usually a group of people or a company A company, abbreviated as co., is a Legal personalit ...

Intel
announces plans for the 64-bit IA-64 architecture (jointly developed with Hewlett-Packard) as a successor to its 32-bit IA-32 processors. A 1998 to 1999 launch date was targeted. ;1995: Sun Microsystems, Sun launches a 64-bit
SPARC SPARC (Scalable Processor Architecture) is a reduced instruction set computing (RISC) instruction set architecture originally developed by Sun Microsystems. Its design was strongly influenced by the experimental Berkeley RISC system developed i ...

SPARC
processor, the
UltraSPARC The UltraSPARC is a microprocessor A microprocessor is a computer processor where the data processing logic and control is included on a single integrated circuit, or a small number of integrated circuits. The microprocessor is a multipurpo ...
. Fujitsu-owned
HAL Computer Systems HAL Computer Systems, Inc was a Campbell, California-based computer manufacturer founded in 1990 by Andrew Heller, a principal designer of the original IBM POWER instruction set architecture, IBM POWER architecture. His idea was to build computers ...
launches workstations based on a 64-bit CPU, HAL's independently designed first-generation HAL SPARC64, SPARC64. IBM releases the A10 and A30 microprocessors, the first 64-bit PowerPC AS processors. IBM also releases a 64-bit AS/400 system upgrade, which can convert the operating system, database and applications. ;1996: Nintendo introduces the
Nintendo 64 The (abbreviated as N64) is a home video game console developed by Nintendo. The successor to the Super Nintendo Entertainment System, it was released on June 23, 1996 in Japan, on September 29, 1996 in North America, and March 1, 1997 in Europ ...

Nintendo 64
video game console, built around a low-cost variant of the MIPS R4000. HP releases the first implementation of its 64-bit PA-RISC, PA-RISC 2.0 architecture, the PA-8000. ;1998: IBM releases the
POWER3 The POWER3 is a , designed and exclusively manufactured by , that implemented the 64-bit version of the (ISA), including all of the optional instructions of the ISA (at the time) such as instructions present in the version of the but not ...

POWER3
line of full-64-bit PowerPC/IBM POWER instruction set architecture, POWER processors. ;1999: Intel releases the
instruction set In computer science Computer science deals with the theoretical foundations of information, algorithms and the architectures of its computation as well as practical techniques for their application. Computer science is the study of co ...
for the IA-64 architecture. AMD publicly discloses its set of 64-bit extensions to IA-32, called
x86-64 x86-64 (also known as x64, x86_64, AMD64, and Intel 64) is a 64-bit version of the x86 instruction set, first released in 1999. It introduced two new modes of operation, 64-bit mode and compatibility mode, along with a new 4-level paging mod ...
(later branded AMD64). ;2000: IBM ships its first 64-bit z/Architecture Mainframe computer, mainframe, the zSeries z900. z/Architecture is a 64-bit version of the 32-bit ESA/390 architecture, a descendant of the 32-bit
System/360 The IBM System/360 (S/360) is a family of mainframe computer A mainframe computer, informally called a mainframe or big iron, is a computer used primarily by large organizations for critical applications like bulk data processing for t ...
architecture. ;2001: Intel ships its IA-64 processor line, after repeated delays in getting to market. Now branded Itanium and targeting high-end servers, sales fail to meet expectations. ;2003: AMD introduces its Opteron and Athlon 64 processor lines, based on its AMD64 architecture which is the first x86-based 64-bit processor architecture. Apple Computer, Apple also ships the 64-bit "G5"
PowerPC 970 The PowerPC 970, PowerPC 970FX, and PowerPC 970MP are 64-bit In computer architecture, 64-bit integers, memory addresses, or other data units are those that are 64 bits (8 octets) wide. Also, 64-bit central processing unit A centra ...
CPU produced by IBM. Intel maintains that its Itanium chips would remain its only 64-bit processors. ;2004: Intel, reacting to the market success of AMD, admits it has been developing a clone of the AMD64 extensions named IA-32e (later renamed EM64T, then yet again renamed to Intel 64). Intel ships updated versions of its Xeon and Pentium 4 processor families supporting the new 64-bit instruction set. :VIA Technologies announces the VIA Isaiah, Isaiah 64-bit processor. ;2006: Sony, IBM, and Toshiba begin manufacturing the 64-bit Cell (microprocessor), Cell processor for use in the PlayStation 3, servers, workstations, and other appliances. Intel released Intel Core (microarchitecture), Core 2 Duo as the first mainstream x86-64 processor for its mobile, desktop, and workstation line. Prior 64-bit extension processor lines were not widely available in the consumer retail market (most of 64-bit Pentium 4/D were OEM), 64-bit Pentium 4, Pentium D, and Celeron were not into mass production until late 2006 due to poor yield issue (most of good yield wafers were targeted at server and mainframe while mainstream still remain 130 nm 32-bit processor line until 2006) and soon became low end after Core 2 debuted. AMD released their first 64-bit mobile processor and manufactured in 90 nm. ;2011: ARM Holdings announces ARMv8-A, the first 64-bit version of the
ARM architecture ARM (stylised in lowercase as arm, previously an acronym for Advanced RISC Machines and originally Acorn RISC Machine) is a family of reduced instruction set computing (RISC) instruction set, architectures for central processing unit, comp ...
. ;2012: ARM Holdings announced their Cortex-A53 and Cortex-A57 cores, their first cores based on their 64-bit architecture, on 30 October 2012. ;2013:Apple announces the
iPhone 5S The iPhone 5S (stylized and marketed as iPhone 5s) is a smartphone A smartphone is a portable device A mobile device (or handheld computer) is a computer A computer is a machine that can be programmed to carry out sequences of a ...

iPhone 5S
, with the world's first 64-bit processor in a smartphone, which uses their Apple A7, A7 ARMv8-A-based system-on-a-chip. ;2014:Google announces the Nexus 9 tablet, the first Android device to run on the 64-bit Tegra K1 chip.


64-bit operating system timeline

;1985: Cray releases UNICOS, the first 64-bit implementation of the Unix operating system. ;1993: DEC releases the 64-bit Tru64 UNIX#OSF/1, DEC OSF/1 AXP Unix-like operating system (later renamed Tru64 UNIX) for its systems based on the
Alpha Alpha (uppercase , lowercase ; grc, ἄλφα, ''álpha'', modern pronunciation ''álfa'') is the first letter Letter, letters, or literature may refer to: Characters typeface * Letter (alphabet) A letter is a segmental symbol A s ...
architecture. ;1994: Support for the
R8000The R8000 is a microprocessor A microprocessor is a computer processor where the data processing logic and control is included on a single integrated circuit, or a small number of integrated circuits. The microprocessor is a multipurpose, clo ...

R8000
processor is added by
Silicon Graphics Silicon Graphics, Inc. (stylized as ''SiliconGraphics'' before 1999, later rebranded SGI, historically known as Silicon Graphics Computer Systems or SGCS) was an American high-performance computing A supercomputer is a computer with a hig ...
to the IRIX operating system in release 6.0. ;1995: DEC releases OpenVMS 7.0, the first full 64-bit version of OpenVMS for Alpha. First 64-bit Linux distribution for the Alpha architecture is released. ;1996: Support for the R4x00 processors in 64-bit mode is added by
Silicon Graphics Silicon Graphics, Inc. (stylized as ''SiliconGraphics'' before 1999, later rebranded SGI, historically known as Silicon Graphics Computer Systems or SGCS) was an American high-performance computing A supercomputer is a computer with a hig ...
to the IRIX operating system in release 6.2. ;1998: Sun releases Solaris (operating system), Solaris 7, with full 64-bit
UltraSPARC The UltraSPARC is a microprocessor A microprocessor is a computer processor where the data processing logic and control is included on a single integrated circuit, or a small number of integrated circuits. The microprocessor is a multipurpo ...
support. ;2000: IBM releases z/OS, a 64-bit operating system descended from MVS, for the new zSeries 64-bit mainframes; 64-bit Linux on z Systems follows the CPU release almost immediately. ;2001: Linux becomes the first OS kernel to fully support
x86-64 x86-64 (also known as x64, x86_64, AMD64, and Intel 64) is a 64-bit version of the x86 instruction set, first released in 1999. It introduced two new modes of operation, 64-bit mode and compatibility mode, along with a new 4-level paging mod ...
(on a simulator, as no x86-64 processors had been released yet). ;2001: Microsoft releases Windows XP 64-Bit Edition for the Itanium's IA-64 architecture; it could run 32-bit applications through an execution layer. ;2003: Apple releases its Mac OS X Panther, Mac OS X 10.3 "Panther" operating system which adds support for native 64-bit integer arithmetic on
PowerPC 970 The PowerPC 970, PowerPC 970FX, and PowerPC 970MP are 64-bit In computer architecture, 64-bit integers, memory addresses, or other data units are those that are 64 bits (8 octets) wide. Also, 64-bit central processing unit A centra ...
processors. Several Linux Linux distribution, distributions release with support for AMD64. FreeBSD releases with support for AMD64. ;2005: On January 4, Microsoft discontinues Windows XP 64-Bit Edition, as no PCs with IA-64 processors had been available since the previous September, and announces that it is developing x86-64 versions of Windows to replace it. On January 31, Sun releases Solaris (operating system), Solaris 10 with support for AMD64 and EM64T processors. On April 29, Apple releases Mac OS X Tiger, Mac OS X 10.4 "Tiger" which provides limited support for 64-bit command-line applications on machines with PowerPC 970 processors; later versions for Intel-based Macs supported 64-bit command-line applications on Macs with EM64T processors. On April 30, Microsoft releases Windows XP Professional x64 Edition and Windows Server 2003 x64 Edition for AMD64 and EM64T processors. ;2006: Microsoft releases Windows Vista, including a 64-bit version for AMD64/EM64T processors that retains 32-bit compatibility. In the 64-bit version, all Windows applications and components are 64-bit, although many also have their 32-bit versions included for compatibility with plug-in (computing), plug-ins. ;2007: Apple releases Mac OS X Leopard, Mac OS X 10.5 "Leopard", which fully supports 64-bit applications on machines with PowerPC 970 or EM64T processors. ;2009: Microsoft releases Windows 7, which, like Windows Vista, includes a full 64-bit version for AMD64/Intel 64 processors; most new computers are loaded by default with a 64-bit version. Microsoft also releases Windows Server 2008 R2, which is the first 64-bit only server operating system. Apple releases Mac OS X Snow Leopard, Mac OS X 10.6, "Snow Leopard", which ships with a 64-bit kernel for AMD64/Intel64 processors, although only certain recent models of Apple computers will run the 64-bit kernel by default. Most applications bundled with Mac OS X 10.6 are now also 64-bit. ;2011: Apple releases Mac OS X Lion, Mac OS X 10.7, "Lion", which runs the 64-bit kernel by default on supported machines. Older machines that are unable to run the 64-bit kernel run the 32-bit kernel, but, as with earlier releases, can still run 64-bit applications; Lion does not support machines with 32-bit processors. Nearly all applications bundled with Mac OS X 10.7 are now also 64-bit, including iTunes. ;2012: Microsoft releases Windows 8 which supports UEFI Class 3 (UEFI without CSM) and Secure Boot. ;2013: Apple releases iOS 7, which, on machines with AArch64 processors, has a 64-bit kernel that supports 64-bit applications. ;2014: Google releases Android Lollipop, the first version of the Android (operating system), Android operating system with support for 64-bit processors. ;2017: Apple releases iOS 11, supporting only machines with AArch64 processors. It has a 64-bit kernel that only supports 64-bit applications. 32-bit applications are no longer compatible. ;2019:Apple releases macOS Catalina, macOS 10.15 "Catalina", dropping support for 32-bit Intel applications.


64-bit applications


32-bit vs 64-bit

A change from a
32-bit 32-bit microcomputer A microcomputer is a small, relatively inexpensive computer with a microprocessor as its central processing unit (CPU). It includes a microprocessor, Computer memory, memory and minimal input/output (I/O) circuitry mount ...
to a 64-bit architecture is a fundamental alteration, as most operating systems must be extensively modified to take advantage of the new architecture, because that software has to manage the actual memory addressing hardware. Other software must also be Porting, ported to use the new abilities; older 32-bit software may be supported either by virtue of the 64-bit instruction set being a superset of the 32-bit instruction set, so that processors that support the 64-bit instruction set can also run code for the 32-bit instruction set, or through software emulator, emulation, or by the actual implementation of a 32-bit processor core within the 64-bit processor, as with some Itanium processors from Intel, which included an IA-32 processor core to run 32-bit
x86 x86 is a family of instruction set architecture In computer science, an instruction set architecture (ISA), also called computer architecture, is an abstract model of a computer. A device that executes instructions described by that ISA, ...

x86
applications. The operating systems for those 64-bit architectures generally support both 32-bit and 64-bit applications. One significant exception to this is the AS/400, software for which is compiled into a virtual instruction set architecture (ISA) called ''Technology Independent Machine Interface'' (TIMI); TIMI code is then translated to native machine code by low-level software before being executed. The translation software is all that must be rewritten to move the full OS and all software to a new platform, as when IBM transitioned the native instruction set for AS/400 from the older 32/48-bit ''IMPI'' to the newer 64-bit ''PowerPC-AS'', codenamed ''Amazon''. The IMPI instruction set was quite different from even 32-bit PowerPC, so this transition was even bigger than moving a given instruction set from 32 to 64 bits. On 64-bit hardware with
x86-64 x86-64 (also known as x64, x86_64, AMD64, and Intel 64) is a 64-bit version of the x86 instruction set, first released in 1999. It introduced two new modes of operation, 64-bit mode and compatibility mode, along with a new 4-level paging mod ...
architecture (AMD64), most 32-bit operating systems and applications can run with no compatibility issues. While the larger address space of 64-bit architectures makes working with large data sets in applications such as digital video, scientific computing, and large
database In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both computer hardware , hardware and sof ...

database
s easier, there has been considerable debate on whether they or their 32-bit compatibility modes will be faster than comparably priced 32-bit systems for other tasks. A compiled Java program can run on a 32- or 64-bit Java virtual machine with no modification. The lengths and precision of all the built-in types, such as char, short, int, long, float, and double, and the types that can be used as array indices, are specified by the standard and are not dependent on the underlying architecture. Java programs that run on a 64-bit Java virtual machine have access to a larger address space. Speed is not the only factor to consider in comparing 32-bit and 64-bit processors. Applications such as multi-tasking, stress testing, and clustering – for high-performance computing (HPC) – may be more suited to a 64-bit architecture when deployed appropriately. For this reason, 64-bit clusters have been widely deployed in large organizations, such as IBM, HP, and Microsoft. Summary: *A 64-bit processor performs best with 64-bit software. *A 64-bit processor may have backward compatibility, allowing it to run 32-bit application software for the 32-bit version of its instruction set, and may also support running 32-bit operating systems for the 32-bit version of its instruction set. *A 32-bit processor is incompatible with 64-bit software.


Pros and cons

A common misconception is that 64-bit architectures are no better than 32-bit architectures unless the computer has more than 4 GB of
random-access memory Random-access memory (RAM; ) is a form of computer memory In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic proces ...
. This is not entirely true: *Some operating systems and certain hardware configurations limit the physical memory space to 3 GB on IA-32 systems, due to much of the 3–4 GB region being reserved for hardware addressing; see 3 GB barrier; 64-bit architectures can address far more than 4 GB. However, IA-32 processors from the Pentium Pro onward allow a 36-bit ''physical'' memory address space, using Physical Address Extension (PAE), which gives a 64 GB physical address range, of which up to 62 GB may be used by main memory; operating systems that support PAE may not be limited to 4 GB of physical memory, even on IA-32 processors. However, drivers and other kernel mode software, more so older versions, may be incompatible with PAE; this has been cited as the reason for 32-bit versions of Microsoft Windows being limited to 4 GB of physical RAM (although the validity of this explanation has been disputed). *Some operating systems reserve portions of process (computing), process address space for OS use, effectively reducing the total address space available for mapping memory for user programs. For instance, 32-bit Windows reserves 1 or 2 GB (depending on the settings) of the total address space for the kernel, which leaves only 3 or 2 GB (respectively) of the address space available for user mode. This limit is much higher on 64-bit operating systems. *Memory-mapped files are becoming more difficult to implement in 32-bit architectures as files of over 4 GB become more common; such large files cannot be memory-mapped easily to 32-bit architectures, as only part of the file can be mapped into the address space at a time, and to access such a file by memory mapping, the parts mapped must be swapped into and out of the address space as needed. This is a problem, as memory mapping, if properly implemented by the OS, is one of the most efficient disk-to-memory methods. *Some 64-bit programs, such as encoders, decoders and encryption software, can benefit greatly from 64-bit registers, while the performance of other programs, such as 3D graphics-oriented ones, remains unaffected when switching from a 32-bit to a 64-bit environment. *Some 64-bit architectures, such as
x86-64 x86-64 (also known as x64, x86_64, AMD64, and Intel 64) is a 64-bit version of the x86 instruction set, first released in 1999. It introduced two new modes of operation, 64-bit mode and compatibility mode, along with a new 4-level paging mod ...
, support more general-purpose registers than their 32-bit counterparts (although this is not due specifically to the word length). This leads to a significant speed increase for tight loops since the processor does not have to fetch data from the cache or main memory if the data can fit in the available registers. :Example in C (programming language), C: int a, b, c, d, e; for (a = 0; a < 100; a++) :If a processor can keep only two or three values or variables in registers, it would need to move some values between memory and registers to be able to process variables d and e also; this is a process that takes many CPU cycles. A processor that can hold all values and variables in registers can loop through them with no need to move data between registers and memory for each iteration. This behavior can easily be compared with virtual memory, although any effects are contingent on the compiler. The main disadvantage of 64-bit architectures is that, relative to 32-bit architectures, the same data occupies more space in memory (due to longer pointers and possibly other types, and alignment padding). This increases the memory requirements of a given process and can have implications for efficient processor cache use. Maintaining a partial 32-bit model is one way to handle this, and is in general reasonably effective. For example, the z/OS operating system takes this approach, requiring program code to reside in 31-bit address spaces (the high order bit is not used in address calculation on the underlying hardware platform) while data objects can optionally reside in 64-bit regions. Not all such applications require a large address space or manipulate 64-bit data items, so these applications do not benefit from these features.


Software availability

x86-based 64-bit systems sometimes lack equivalents of software that is written for 32-bit architectures. The most severe problem in Microsoft Windows is incompatible device drivers for obsolete hardware. Most 32-bit application software can run on a 64-bit operating system in a compatibility mode, also termed an emulator, emulation mode, e.g., Microsoft WoW64 Technology for IA-64 and AMD64. The 64-bit Windows Native Mode driver environment runs atop 64-bit NTDLL.DLL, which cannot call 32-bit Win32 subsystem code (often devices whose actual hardware function is emulated in user mode software, like Winprinters). Because 64-bit drivers for most devices were unavailable until early 2007 (Vista x64), using a 64-bit version of Windows was considered a challenge. However, the trend has since moved toward 64-bit computing, more so as memory prices dropped and the use of more than 4 GB of RAM increased. Most manufacturers started to provide both 32-bit and 64-bit drivers for new devices, so unavailability of 64-bit drivers ceased to be a problem. 64-bit drivers were not provided for many older devices, which could consequently not be used in 64-bit systems. Driver compatibility was less of a problem with open-source drivers, as 32-bit ones could be modified for 64-bit use. Support for hardware made before early 2007, was problematic for open-source platforms, due to the relatively small number of users. 64-bit versions of Windows cannot run 16-bit#16-bit application, 16-bit software. However, most 32-bit applications will work well. 64-bit users are forced to install a virtual machine of a 16- or 32-bit operating system to run 16-bit applications. Mac OS X Tiger, Mac OS X 10.4 "Tiger" and Mac OS X Leopard, Mac OS X 10.5 "Leopard" had only a 32-bit kernel, but they can run 64-bit user-mode code on 64-bit processors. Mac OS X Snow Leopard, Mac OS X 10.6 "Snow Leopard" had both 32- and 64-bit kernels, and, on most Macs, used the 32-bit kernel even on 64-bit processors. This allowed those Macs to support 64-bit processes while still supporting 32-bit device drivers; although not 64-bit drivers and performance advantages that can come with them. Mac OS X Lion, Mac OS X 10.7 "Lion" ran with a 64-bit kernel on more Macs, and OS X Mountain Lion, OS X 10.8 "Mountain Lion" and later macOS releases only have a 64-bit kernel. On systems with 64-bit processors, both the 32- and 64-bit macOS kernels can run 32-bit user-mode code, and all versions of macOS include 32-bit versions of libraries that 32-bit applications would use, so 32-bit user-mode software for macOS will run on those systems. Linux and most other Unix-like operating systems, and the C (programming language), C and C++ toolchains for them, have supported 64-bit processors for many years. Many applications and libraries for those platforms are open-source software, written in C and C++, so that if they are 64-bit-safe, they can be compiled into 64-bit versions. This source-based distribution model, with an emphasis on frequent releases, makes availability of application software for those operating systems less of an issue.


64-bit data models

In 32-bit programs, pointer (computer programming), pointers and data types such as integers generally have the same length. This is not necessarily true on 64-bit machines. Mixing data types in programming languages such as C (programming language), C and its descendants such as C++ and Objective-C may thus work on 32-bit implementations but not on 64-bit implementations. In many programming environments for C and C-derived languages on 64-bit machines, int variables are still 32 bits wide, but long integers and pointers are 64 bits wide. These are described as having an ''LP64'' data model. Other models are the ''ILP64'' data model in which all three data types are 64 bits wide, and even the ''SILP64'' model where ''short'' integers are also 64 bits wide. However, in most cases the modifications required are relatively minor and straightforward, and many well-written programs can simply be recompiled for the new environment with no changes. Another alternative is the ''LLP64'' model, which maintains compatibility with 32-bit code by leaving both int and long as 32-bit. ''LL'' refers to the ''long long integer'' type, which is at least 64 bits on all platforms, including 32-bit environments. There are also systems with 64-bit processors using an ''ILP32'' data model, with the addition of 64-bit long long integers; this is also used on many platforms with 32-bit processors. This model reduces code size and the size of data structures containing pointers, at the cost of a much smaller address space, a good choice for some embedded systems. For instruction sets such as x86 and ARM in which the 64-bit version of the instruction set has more registers than does the 32-bit version, it provides access to the additional registers without the space penalty. It is common in 64-bit RISC machines, explored in x86 as x32 ABI, and has recently been used in the Apple Watch Series 4 and 5. Many 64-bit platforms today use an ''LP64'' model (including Solaris, AIX, HP-UX, Linux, macOS, BSD, and IBM z/OS). Microsoft Windows uses an ''LLP64'' model. The disadvantage of the LP64 model is that storing a long into an int may truncate. On the other hand, converting a pointer to a long will “work” in LP64. In the LLP64 model, the reverse is true. These are not problems which affect fully standard-compliant code, but code is often written with implicit assumptions about the widths of data types. C code should prefer (u)intptr_t instead of long when casting pointers into integer objects. A programming model is a choice made to suit a given compiler, and several can coexist on the same OS. However, the programming model chosen as the primary model for the OS application programming interface (API) typically dominates. Another consideration is the data model used for device drivers. Drivers make up the majority of the operating system code in most modern operating systems (although many may not be loaded when the operating system is running). Many drivers use pointers heavily to manipulate data, and in some cases have to load pointers of a certain size into the hardware they support for direct memory access (DMA). As an example, a driver for a 32-bit PCI device asking the device to DMA data into upper areas of a 64-bit machine's memory could not satisfy requests from the operating system to load data from the device to memory above the 4 gibibyte barrier, because the pointers for those addresses would not fit into the DMA registers of the device. This problem is solved by having the OS take the memory restrictions of the device into account when generating requests to drivers for DMA, or by using an input–output memory management unit (IOMMU).


Current 64-bit architectures

, 64-bit architectures for which processors are being manufactured include: *The 64-bit extension created by
Advanced Micro Devices Advanced Micro Devices, Inc. (AMD) is an American multinational company, multinational semiconductor industry, semiconductor company based in Santa Clara, California, Santa Clara, California, that develops computer processors and related techno ...
(AMD) to Intel's
x86 x86 is a family of instruction set architecture In computer science, an instruction set architecture (ISA), also called computer architecture, is an abstract model of a computer. A device that executes instructions described by that ISA, ...

x86
architecture (later licensed by Intel); commonly termed ''
x86-64 x86-64 (also known as x64, x86_64, AMD64, and Intel 64) is a 64-bit version of the x86 instruction set, first released in 1999. It introduced two new modes of operation, 64-bit mode and compatibility mode, along with a new 4-level paging mod ...
'', ''AMD64'', or ''x64'': **AMD's AMD64 extensions (used in Athlon 64, Opteron, Sempron, Turion 64, Phenom (processor), Phenom, Athlon II, Phenom II, AMD APU, APU, AMD FX, FX, Ryzen, and Epyc processors) **
Intel Intel Corporation is an American multinational corporation A multinational company (MNC) is a corporate A corporation is an organization—usually a group of people or a company A company, abbreviated as co., is a Legal personalit ...

Intel
's Intel 64 extensions, used in Intel Core 2/i3/i5/i7/i9, some Intel Atom, Atom, and newer Celeron,
Pentium Pentium is a brand used for a series of x86 architecture-compatible microprocessors produced by Intel. The Pentium (original), original Pentium was released in 1993. After that, the Pentium II and Pentium III were released. In their form , ...
, and Xeon processors ***
Intel Intel Corporation is an American multinational corporation A multinational company (MNC) is a corporate A corporation is an organization—usually a group of people or a company A company, abbreviated as co., is a Legal personalit ...

Intel
's K1OM architecture, a variant of ''Intel 64'' with no CMOV, MMX, and SSE instructions, used in first-generation Xeon Phi (Knights Corner) coprocessors, binary incompatible with x86-64 programs **VIA Technologies' 64-bit extensions, used in the VIA Nano processors *IBM's PowerPC/Power ISA: **IBM's POWER4, POWER5, POWER6, POWER7, POWER8, POWER9, and IBM A2 processors *
SPARC SPARC (Scalable Processor Architecture) is a reduced instruction set computing (RISC) instruction set architecture originally developed by Sun Microsystems. Its design was strongly influenced by the experimental Berkeley RISC system developed i ...

SPARC
V9 architecture: **Oracle Corporation, Oracle's M8 and S7 processors **Fujitsu's SPARC64 XII and SPARC64 V#SPARC64 XIfx, SPARC64 XIfx processors *IBM's z/Architecture, a 64-bit version of the ESA/390 architecture, used in IBM's IBM System z, eServer zSeries and System z IBM mainframes, mainframes: **IBM IBM z13 (microprocessor), z13 and IBM z14 (microprocessor), z14 **Hitachi AP8000E *HP-Intel's IA-64 architecture: **Intel's Itanium processors *MIPS Technologies' MIPS architecture, MIPS64 architecture *ARM Holdings' AArch64 architecture *Elbrus (computer), Elbrus architecture: **Elbrus-8S *NEC SX architecture **SX-Aurora TSUBASA *RISC-V Most architectures of 64 bits that are derived from the same architecture of 32 bits can execute code written for the 32-bit versions natively, with no performance penalty. This kind of support is commonly called ''bi-arch support'' or more generally ''multi-arch support''.


See also

*Computer memory


Notes

*


References


External links


64-bit Transition Guide, Mac Developer Library
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Lessons on development of 64-bit C/C++ applicationsAMD64 (EM64T) architecture
{{Processor technologies 64-bit computers Data unit