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| 1-bit | 4-bit | 8-bit | 12-bit | 16-bit | 18-bit | 24-bit | 31-bit | 32-bit | 36-bit | 48-bit | 60-bit | 64-bit | 128-bit |
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| Single precision floating-point format (32-bit) Double precision floating-point format (64-bit) Quadruple precision floating-point format (128-bit) | |||||||||||||
In computer architecture, 64-bit integers, memory addresses, or other data units are those that are at most 64 bits (8 octets) wide. Also, 64-bit CPU and ALU architectures are those that are based on registers, address buses, or data buses of that size. 64-bit is also a term given to a generation of computers in which 64-bit processors are the norm.
64-bit 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 existed in supercomputers since the 1970s (Cray-1, 1975) and in RISC-based workstations and servers since the early 1990s. In 2003 they were introduced to the (previously 32-bit) mainstream personal computer arena in the form of the x86-64 and 64-bit PowerPC processor architectures.
A 64-bit register can store 264 = 18446744073709551616 different values, a number in excess of 18 quintillion. Hence, a processor with 64-bit memory addresses can directly access 16 EiB (exbibytes) of byte-addressable memory.
Without 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 (the 32-bit Pentium 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 numbers.
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Processor registers are typically divided into several groups: integer, floating-point, 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 and/or address-registers can be used to address data in memory, the other types 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 most ARM and 32-bit 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/x87 architecture has instructions capable of loading and storing 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 family uses a 64-bit floating-point data and register format (as well as 64-bit integer registers).
Most CPUs are designed so that the contents of a single integer register can store the address (location) of any datum in the computer's virtual memory. Therefore, the total number of addresses in the virtual memory — the total amount of data the computer can keep in its working area — is determined by the width of these registers. Beginning in the 1960s with the IBM System/360 (which was an exception, in that it used the low order 24 bits of a word for addresses, resulting in a 16 MB [16 × 10242 bytes] address space size), then (amongst many others) the DEC VAX minicomputer in the 1970s, and then with the Intel 80386 in the mid-1980s, a de facto consensus developed that 32 bits was a convenient register size. A 32-bit address register meant that 232 addresses, or 4 GB of RAM, could be referenced. At the time these architectures were devised, 4 GB of memory was so far beyond the typical quantities (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 databases.
Some supercomputer architectures of the 1970s and 1980s used registers up to 64 bits wide. In the mid-1980s, Intel i860[1] development began culminating in a (too late[2] for Windows NT) 1989 release. However, 32 bits remained the norm until the early 1990s, when the continual reductions in the cost of memory led to installations with quantities 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[3] and the PlayStation 2 had 64-bit microprocessors before their introduction in personal computers. High-end printers and network equipment, as well as industrial computers also used 64-bit microprocessors such as the Quantum Effect Devices R5000. 64-bit computing started to drift down to the personal computer desktop from 2003 onwards, when some models in Apple's Macintosh lines switched to PowerPC 970 processors (termed "G5" by Apple) and the launch of AMD's 64-bit x86-64 extension to the x86 architecture, itself a response to Intel's Itanium gaining early operating systems support.
In principle a 64-bit microprocessor can address 16 exabytes of memory. Practical 64-bit processors have lower physical limits.
For example, the AMD64 architecture as of 2011[update] had a 52-bit limit on physical memory and supported a 48-bit virtual address space.[4] This is 4 PB (4 × 10245 bytes) and 256 TB (256 × 10244 bytes), respectively. A PC cannot contain 4 petabytes of memory (due to the physical size of the memory chips if nothing else) but AMD envisioned large servers, shared memory clusters, and other uses of physical address space that might approach this in the foreseeable future, and the 52-bit physical address provides ample room for expansion while not incurring the cost of implementing 64-bit physical addresses. Similarly, the 48-bit virtual address space was designed to provide more than 65,000 times the 32-bit limit of 4 GB (4 × 10243 bytes), allowing room for later expansion without incurring the overhead of translating full 64-bit addresses.
A change from a 32-bit 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.[17] Other software must also be ported to use the new capabilities; older 32-bit software may be supported through either a hardware compatibility mode in which the new processors support the older 32-bit version of the instruction set as well as the 64-bit version, through software emulation, or by the actual implementation of a 32-bit processor core within the 64-bit processor, as with the Itanium processors from Intel, which include an IA-32 processor core to run 32-bit x86 applications. The operating systems for those 64-bit architectures generally support both 32-bit and 64-bit applications.[18]
One significant exception to this is the AS/400, whose software runs on a virtual Instruction Set Architecture (ISA) called TIMI (Technology Independent Machine Interface), which is translated to native machine code by low-level software before being executed. The translation software is all that has to be rewritten to move the entire OS and all software to a new platform, such as when IBM transitioned their line from the older 32/48-bit "IMPI" instruction set to 64-bit PowerPC (the IMPI set was quite different from 32-bit PowerPC, so this was an even bigger transition than from a 32-bit version of an instruction set to a 64-bit version of the same instruction set).
On 64-bit hardware with x86-64 architecture (AMD64), most 32-bit operating systems and applications can run without 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 databases 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 without modification. The lengths and precision of all the built in types 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.[19]
Speed is not the only factor to consider in a comparison of 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. 64-bit clusters have been widely deployed in large organizations, such as IBM, HP, and Microsoft, for this reason.
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.[20] This is not entirely true:
int a, b, c, d, e; for (a=0; a<100; a++) { b = a; c = b; d = c; e = d; }
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 utilization. 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.
As of June 2011[update], most proprietary x86 software is compiled into 32-bit code, with less being also compiled into 64-bit code (although the trend is rapidly equalizing ), so most of that software does not take advantage of the larger 64-bit address space or wider 64-bit registers and data paths on x86 processors, or the additional general-purpose registers. However, users of most RISC platforms, and users of free or open source operating systems (where the source code is available for recompiling with a 64-bit compiler) have been able to use exclusive 64-bit computing environments for years. Not all such applications require a large address space or manipulate 64-bit data items, so do not benefit from the larger address space or wider registers and data paths. The main advantage to 64-bit versions of such applications is the ability to access more registers in the x86-64 architecture.
x86-based 64-bit systems sometimes lack equivalents to software that is written for 32-bit architectures. The most severe problem in Microsoft Windows is incompatible device drivers. Most 32-bit application software can run on a 64-bit operating system in a compatibility mode, also known as an emulation mode, e.g. Microsoft WoW64 Technology for IA-64 and AMD64. The 64-bit Windows Native Mode[22] 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 not available until early 2007 (Vista x64), using 64-bit Microsoft Windows operating system was considered a challenge. However, the trend since then moved towards 64-bit computing, particular as memory prices dropped and the use of more than 4 GB of RAM, not usable by 32-bit systems, 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 problematical for open source platforms due to the relatively small number of users.
On most Macs, Mac OS X runs with a 32-bit kernel even on 64-bit-capable processors, but the 32-bit kernel can run 64-bit user-mode code; this allows those Macs to support 64-bit processes while still supporting 32-bit device drivers - although not 64-bit drivers and performance advantages that would come with them. On systems with 64-bit processors, both the 32- and 64-bit Mac OS X kernels can run 32-bit user-mode code, and all versions of Mac OS X include 32-bit versions of libraries that 32-bit applications would use, so 32-bit user-mode software for Mac OS X will run on those systems.
Linux and most other Unix-like operating systems, and the C and C++ toolchains for them, have supported 64-bit processors for many years, releasing 64-bit versions of their operating system before official Microsoft releases. Many applications and libraries for those platforms are open source, written in C and C++, so that, if it is 64-bit-safe, they can be compiled into 64-bit versions. This source-based distribution model with an emphasis on frequent releases and cutting-edge code makes availability of application software for those operating systems less of an issue.
In 32-bit programs, pointers and data types such as integers generally have the same length; this is not necessarily true on 64-bit machines.[23][24][25] Mixing data types in programming languages such as C and its descendants such as C++ and Objective-C may thus function 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. Another alternative is the ILP64 data model in which all three data types are 64 bits wide, and even SILP64 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 without 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.
| Data model | short (integer) | int | long (integer) | long long | pointers/size_t | Sample operating systems |
|---|---|---|---|---|---|---|
| LLP64/ IL32P64 |
16 | 32 | 32 | 64 | 64 | Microsoft Windows (X64/IA-64) |
| LP64/ I32LP64 |
16 | 32 | 64 | 64 | 64 | Most Unix and Unix-like systems, e.g. Solaris, Linux, and Mac OS X; z/OS |
| ILP64 | 16 | 64 | 64 | 64 | 64 | HAL Computer Systems port of Solaris to SPARC64 |
| SILP64 | 64 | 64 | 64 | 64 | 64 | Unicos |
Many 64-bit compilers today use the LP64 model (including Solaris, AIX, HP-UX, Linux, Mac OS X, FreeBSD, and IBM z/OS native compilers). Microsoft's Visual C++ compiler uses the LLP64 model. The disadvantage of the LP64 model is that storing a long into an int may overflow. On the other hand, casting a pointer to a long will work. In the LLP 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 integer types.
Note that a programming model is a choice made on a per-compiler basis, and several can coexist on the same OS. However, the programming model chosen as the primary model for the OS API typically dominates.
Another consideration is the data model used for 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 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 gigabyte 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 IOMMU.
64-bit microprocessor architectures for which processors are currently being manufactured (as of January 2011[update]) include:
Most 64-bit processor architectures that are derived from 32-bit processor architectures can execute code for the 32-bit version of the architecture natively without any performance penalty. This kind of support is commonly called bi-arch support or more generally multi-arch support.
In digital imaging, 64-bit refers to 48-bit images with a 16-bit alpha channel.
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