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In computer architecture, 64-bit computing is the use of processors that have datapath widths, integer size, and memory addresses of 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. From the software perspective, 64-bit computing means the use of code with 64-bit virtual memory addresses.

64-bit is a descriptor given to 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 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 (over 18 quintillion) different values. Hence, a processor with 64-bit memory addresses can directly access 264 bytes 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.

64-bit Microsoft operating system timeline

Microsoft releases Windows XP 64-Bit Edition for the Itanium's IA-64 architecture, although it was able to run 32-bit applications through an execution layer.
Microsoft announces plans to create a version of its Windows operating system to support the AMD64 architecture, with backwards compatibility with 32-bit applications.
On April 30, Microsoft releases Windows XP Professional x64 Edition for AMD64 and EM64T processors.
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 plugins.
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. It also releases Windows Server 2008 R2, which is the first 64-bit only operating system released by Microsoft. Apple releases 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.

32-bit vs 64-bit

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. 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.

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 the 64-bit PowerPC instruction set (the IMPI instruction set was quite different from the 32-bit PowerPC instruction set, 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.

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.

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. 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 II onwards allow for 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 4GB of physical memory, even on IA-32 processors. However, drivers and other kernel mode software, particularly older versions, may not be compatible with PAE.
  • Some operating systems reserve portions of 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 very 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—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, 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:
int a, b, c, d, e;
for (a=0; a<100; a++)
  b = a;
  c = b;
  d = c;
  e = d;
If a processor only has the ability to keep 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 as well; this is a process that takes many CPU cycles. A processor that is capable of holding all values and variables in registers can loop through them without needing to move data between registers and memory for each iteration. This behavior can easily be compared with virtual memory, although any effects are contingent upon 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 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, 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 x64 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 these applications do not benefit from these features. The main advantage of 64-bit versions of such applications is the ability to access more registers in the x86-64 architecture.

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. 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 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 a 64-bit version of Windows was considered a challenge. However, the trend has since moved towards 64-bit computing, particularly 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 data models

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. 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.

64-bit data models
Data model short (integer) int long (integer) long long pointers/size_t Sample operating systems
16 32 32 64 64 Microsoft Windows (X64/IA-64)
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.

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