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 and arithmetic logic units are those that are based on processor registers, address buses, or data buses of that size. A computer that uses such a processor is a 64-bit computer.
From the software perspective, 64-bit computing means the use of machine code with 64-bit virtual memory addresses. However, not all 64-bit instruction sets support full 64-bit virtual memory addresses; x86-64 and AArch64, for example, support only 48 bits of virtual address, with the remaining 16 bits of the virtual address required to be all zeros or all ones, and several 64-bit instruction sets support fewer than 64 bits of physical memory address.
The term 64-bit also describes a generation of computers in which 64-bit processors are the norm. 64 bits is a word 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 supercomputers since the 1970s and in reduced instruction set computers based workstations and servers since the early 1990s. In 2003, 64-bit CPUs were introduced to the mainstream PC market in the form of x86-64 processors and the PowerPC G5.
A 64-bit register can hold any of 2⁶⁴ different values. The range of integer 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 for representation as an binary number, and −9,223,372,036,854,775,808 through 9,223,372,036,854,775,807 for representation as two's complement. Hence, a processor with 64-bit memory addresses can directly access 2⁶⁴ bytes of byte-addressable memory.
With no further qualification, a 64-bit computer architecture generally has integer and addressing registers 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 buses with different sizes from the registers, even larger.

Architectural implications

Processor registers are typically divided into several groups: integer, floating-point, single instruction, multiple data, 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 and 32-bit MIPS architecture have integrated floating point hardware, which is often, but not always, based on 64-bit units of data. For example, although the x86/x87 architecture has instructions able to load and store 64-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 family uses a 64-bit floating-point data and register format, and 64-bit integer registers.

History

Many computer instruction sets are designed so that a single integer register can store the memory address to any location in the computer's physical or virtual memory. Therefore, the total number of addresses to memory is often determined by the width of these registers. The IBM System/360 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 MiB address space. 32-bit superminicomputers, such as the 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, appeared in the mid-1980s, making 32 bits something of a de facto consensus as a convenient register size.
A 32-bit address register meant that 2³² addresses, or 4 GB of random-access memory, could be referenced. When these architectures were devised, 4 GB of memory was so far beyond the typical amounts 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 databases.
Some supercomputer architectures of the 1970s and 1980s, such as the 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 development began culminating in a 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 and server machines. By the mid-1990s, HAL Computer Systems, Sun Microsystems, IBM, Silicon Graphics, and Hewlett-Packard had developed 64-bit architectures for their workstation and server systems. A notable exception to this trend were mainframes 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 and the 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. 64-bit computing started to trickle down to the personal computer desktop from 2003 onward, when some models in Apple's Macintosh lines switched to PowerPC 970 processors, and Advanced Micro Devices released its first 64-bit x86-64 processor. Physical memory eventually caught up with 32-bit limits. In 2023, laptop computers were commonly equipped with 16GB and servers starting from 64 GB of memory, greatly exceeding the 4 GB address capacity of 32 bits.

64-bit data timeline

; 1961: IBM delivers the IBM 7030 Stretch supercomputer, which uses 64-bit data words and 32- or 64-bit instruction words.
; 1974: Control Data Corporation launches the CDC Star-100 vector supercomputer, which uses a 64-bit word architecture.
; 1976: Cray Research delivers the first Cray-1 supercomputer, which is based on a 64-bit word architecture and will form the basis for later Cray vector supercomputers.
; 1983: Elxsi launches the Elxsi 6400 parallel minisupercomputer. The Elxsi architecture has 64-bit data registers but a 32-bit address space.
; 1989: Intel introduces the Intel i860 reduced instruction set computer 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 introduces the Atari Jaguar video game console, which includes some 64-bit wide data paths in its architecture.

64-bit address timeline

; 1991: MIPS Computer Systems produces the first 64-bit microprocessor, the R4000, which implements the MIPS III architecture, the third revision of its MIPS architecture. The CPU is used in SGI 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 OSF/1.
; 1992: Digital Equipment Corporation introduces the pure 64-bit Alpha architecture which was born from the PRISM project.
; 1994: Intel announces plans for the 64-bit IA-64 architecture as a successor to its 32-bit IA-32 processors. A 1998 to 1999 launch date was targeted.
; 1995: Sun launches a 64-bit SPARC processor, the UltraSPARC. Fujitsu-owned HAL Computer Systems launches workstations based on a 64-bit CPU, HAL's independently designed first-generation 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 video game console, built around a low-cost variant of the MIPS R4000. HP releases the first implementation of its 64-bit PA-RISC 2.0 architecture, the PA-8000.
; 1998: IBM releases the POWER3 line of full-64-bit PowerPC/POWER processors.
; 1999: Intel releases the instruction set for the IA-64 architecture. AMD publicly discloses its set of 64-bit extensions to IA-32, called x86-64.
; 2000: IBM ships its first 64-bit z/Architecture 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 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 also ships the 64-bit "G5" PowerPC 970 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. Intel ships updated versions of its Xeon and Pentium 4 processor families supporting the new 64-bit instruction set.
; 2006: Sony, IBM, and Toshiba begin manufacturing the 64-bit Cell processor for use in the PlayStation 3, servers, workstations, and other appliances. Intel released 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, 64-bit Pentium 4, Pentium D, and Celeron were not into mass production until late 2006 due to poor yield issue 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 family.
; 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, which is the first 64-bit processor in a smartphone, the A7 ARMv8-A-based system-on-a-chip, and the iPad Air and iPad Mini 2, which are the first tablets with 64-bit processors.
; 2014:RISC-V, with both 32-bit and 64-bit support, was published. Google announces the Nexus 9 tablet, the first Android device to run on the 64-bit Tegra K1 chip.
; 2015:Apple announces the iPod Touch, the first iPod Touch to use a 64-bit processor, the A8 ARMv8-A-based system-on-a-chip, alongside the Apple TV, the first Apple TV to use a 64-bit processor.
; 2018:Apple announces the Apple Watch Series 4, the first Apple Watch to use a 64-bit processor, the S4 ARMv8-A-based system-on-a-chip.
; 2020:Synopsis announce the ARCv3 ISA, the first 64-bit version of the ARC ISA. Apple releases the Apple M1, which lacks support for 32-bit applications.
; 2023:Qualcomm releases the Snapdragon 8 Gen 3 and Snapdragon X Elite, which lack support for 32-bit ARM applications.