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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 were the norm. 64-bit CPUs have existed in supercomputers since the 1960s 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. 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 than 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.
[edit] Architectural implicationsProcessor 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 data and register format is 80 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). [edit] HistoryMost 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, 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 GB addresses were considered an appropriate size to work with for another important reason: 4 billion integers are enough to assign unique references to most physically countable things in applications like databases. Some supercomputer architectures of the 1970s and 80s used registers up to 64 bits wide. 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-gigabyte 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 and Hewlett Packard had developed 64-bit architectures for their workstation and server systems. A notable exclusion to this trend were mainframes from IBM, which remained 32-bit. During the 1990s, several low-cost 64-bit microprocessors were used in consumer electronics and embedded applications. Notably, the Nintendo 64 and PlayStation 2 both had 64-bit microprocessors before its 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, processors based on this architecture becoming common in high-end PCs. The emergence of the 64-bit architecture effectively increases the memory ceiling to 264 addresses, equivalent to approximately 17.2 billion gigabytes, 16.8 million terabytes, or 16 exabytes of RAM. To put this in perspective, in the days when 4 MB of main memory was commonplace, the maximum memory ceiling of 232 addresses was about 1,000 times larger than typical memory configurations. Today, when over 2 GB of main memory is common, the ceiling of 264 addresses is about ten trillion times larger, i.e., ten billion times more headroom than the 232 case. [edit] LimitationsMost 64-bit microprocessors on the market today have an artificial limit on the amount of memory they can address, because physical constraints make it impossible to support the full 16.8 million terabyte capacity. For example, the AMD64 architecture has a 52-bit limit on physical memory and currently only supports a 48-bit virtual address space[1]. Still, that represents 4 petabytes and 256 terabytes respectively, or 2,000,000 and 100,000 times as much as a typical system. [edit] 64-bit processor timeline
[edit] 64-bit software timeline
[edit] 32 vs 64 bitA 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. Other software must also be ported to use the new capabilities; older software is usually 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 x86 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[11]. One significant exception to this is the AS/400, whose software runs on a virtual ISA, called TIMI (Technology Independent Machine Interface) which is translated to native machine code by low-level software before being executed. The low-level 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 (IMPI wasn't anything like 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). While 64-bit architectures indisputably make working with large data sets in applications such as digital video, scientific computing, and large databases easier, there has been considerable debate as to whether they or their 32-bit compatibility modes will be faster than comparably-priced 32-bit systems for other tasks. In x86-64 architecture (AMD64), the majority of the 32-bit operating systems and applications are able to run smoothly on the 64-bit hardware. Sun's 64-bit Java virtual machines are slower to start up than their 32-bit virtual machines because Sun has only implemented the "server" JIT compiler (C2) for 64-bit platforms.[12] The "client" JIT compiler (C1), which produces less efficient code but compiles much faster, is unavailable on 64-bit platforms. 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 HPC (high-performance computing)—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. [edit] Pros and consA common misconception is that 64-bit architectures are no better than 32-bit architectures unless the computer has more than 4 GB of main memory. This is not entirely true:
The main disadvantage of 64-bit architectures is that relative to 32-bit architectures, the same data occupies more space in memory (due to swollen 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. In fact, the highly performance-oriented z/OS operating system takes this approach currently, requiring program code to reside in any number of 32-bit address spaces while data objects can (optionally) reside in 64-bit regions. Currently, most proprietary x86 software is compiled into 32-bit code, not 64-bit code, so it 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 registers in 64-bit mode. 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 nor manipulate 64-bit data items, so they wouldn't 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. [edit] Software availabilityx86-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. Although most software can run in a 32-bit compatibility mode (also known as an emulation mode, e.g. Microsoft WoW64 Technology for IA64) or run in 32-bit mode natively (on AMD64), it is usually impossible to run a driver (or similar software) in that mode since such a program usually runs in between the OS and the hardware, where direct emulation cannot be employed. Because 64-bit drivers for most devices were not available until early 2007, using 64-bit Microsoft Windows operating system was considered impractical. However, the trend is changing towards 64-bit computing as most manufacturers provide both 32-bit and 64-bit drivers nowadays, so this issue is most likely to occur when attempting to use older peripherals. It should be noted that Linux/Unix operating systems do not have such problems with open source drivers that are already available for a 32-bit OS, as 64 bit builds can be made from them. Because device drivers in operating systems with monolithic kernels, and in many operating systems with hybrid kernels, execute within the operating system kernel, it is possible to run the kernel as a 32-bit process while still supporting 64-bit user processes. This provides the memory and performance benefits of 64-bit for users without breaking binary compatibility with existing 32-bit device drivers, at the cost of some additional overhead within the kernel. This is the mechanism by which older versions of Mac OS X enables 64-bit processes while still supporting 32-bit device drivers. See also: List of native x86-64 Windows software [edit] 64-bit data modelsConverting application software written in a high-level language from a 32-bit architecture to a 64-bit architecture varies in difficulty. One common recurring problem is that some programmers assume that pointers have the same length as some other data type. These programmers assume they can transfer quantities between these data types without losing information. Those assumptions happen to be true on some 32-bit machines (and even some 16-bit machines), but they are no longer true on 64-bit machines. The C programming language and its descendant C++ make it particularly easy to make this sort of mistake. Differences between the C89 and C99 language standards also exacerbate the problem[13]. To avoid this mistake in C and C++, the One needs to be careful to use the Neither C nor C++ define the length of a pointer, int, or long to be a specific number of bits. C99, however, stdint.h provides names for integer types with certain numbers of bits where those types are available. [edit] Specific data modelsIn most programming environments on 32-bit machines, pointers, "int" types, and "long" types are all 32 bits wide. However, in many programming environments on 64-bit machines, "int" variables are still 32 bits wide, but "long"s 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" variables are also 64 bits wide[citation needed]. 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" type, which is at least 64 bits on all platforms, including 32-bit environments.
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 VC++ 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. [edit] Current 64-bit microprocessor architectures
64-bit microprocessor architectures (as of 2008[update]) include:
Most 64-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. [edit] ImagesIn digital imaging, 64-bit refers to 48-bit images with a 16-bit alpha channel. [edit] See also[edit] References
[edit] External linksThis article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL.
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