64-bit computing

In computing, a 64-bit component is one in which data are stored in chunks (computer words) of sixty-four bits (binary digits) each. The term often describes a computer's CPU: e.g. "The Sun UltraSPARC is a 64-bit processor". Its external data bus or address bus may be narrower (i.e. pass data in chunks of fewer bits), and the term is often used to describe the size of these buses as well. For instance, many current machines with 32-bit processors use 64-bit buses. The term may also refer to the size of an instruction in the computer's instruction set or to any other item of data. Without further qualification, however, a computer architecture described as "64-bit" generally has registers that are 64 bits wide and thus directly supports dealing both internally and externally with 64-bit "chunks" of data.

Most CPUs are designed so that the contents of a single 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 register meant that 2³² addresses, or 4 gigabytes of memory, could be referenced. At the time all of these architectures were settled on, 4 gigabytes of RAM was so far beyond the typical quantities available in installations that this was considered to be enough "headroom" for addressing. 4-gigabyte 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.

However, with the march of time and the continual reductions in the cost of memory (see Moore's Law), by the early 1990s installations with quantities of RAM approaching 4 gigabytes began to appear, and the use of virtual memory spaces greater than the four gigabyte limit became desirable for handling certain types of problems. In response, a number of companies began releasing new families of chips with 64-bit architectures, initially for supercomputers and high-end server machines. 64-bit computing has gradually drifted down to the personal computer desktop, with Apple's desktop line using a 64-bit processor as of 2004, and AMD's x86-64 architecture becoming common in high-end Windows PCs and Intel adopting the same architecture for their future desktop CPUs.

A change to a 64-bit architecture is a fundamental alteration, as operating systems must be 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 an older 32-bit instruction set as well as the new modes), or through software emulation.

While 64-bit architectures indisputably make working with huge 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.

Theoretically, some programs could well be faster in 32-bit mode. Instructions for 64-bit computing take up more storage space than the earlier 32-bit ones, so it is possible that some 32-bit programs will fit into the CPU's high-speed cache while equivalent 64-bit programs will not. However, in applications like scientific computing, the data being processed often fits naturally in 64-bit chunks, and will be faster on a 64-bit architecture because the CPU will be designed to process such information directly rather than requiring the program to perform multiple steps. Such assessments are complicated by the fact that in the process of designing the new 64-bit architectures, the instruction set designers have taken the opportunity to make other changes that address some of the deficiencies in older instruction sets by adding new performance-enhancing facilities (such as the extra register in the x86-64 design).

pro/cons

An often made mistake is that 64-bit architectures are considered useless till one adds more than 4 GB of memory to a computer. This is not entirely true:

• Some OSes map in parts of the kernel and/or userland OS parts into userland space, effectively reducing the addressspace available for mapping memory for the userland program. (e.g. Windows DLLs and userland OS components. Free usable memory under XP: 3 GB, 2000 (non server): 2GB)
• A lot of C programs use signed values for buffersizes, which wrap around if they get above 2GB. (However it is doubtful that such badly written programs will run on 64-bits systems unmodified)
• Memorymapping of files is getting dangerous with 32-bit architectures, specially with DVD becoming cheaper quickly, and videoediting becoming available to the masses. A 2 or 4 GB file is not an exception anymore, and such large files can't be memory mapped on 32-bits architectures. Memory mapping is still one of the most efficient disk-to-memory methods, when properly implemented by the OS.

The big disadvantage of 64-bit architectures is that the same data (due to swollen pointers and possibly other types) is slightly larger. This increases both memory requirements (typically 10-50% on my Alpha) and processor cache utilisation. (cache is also a very expensive resource nowadays)

Keeping a partial 32-bit datamodel (see next section) can be a way to remedy this reasonably effective.

64-bit data models

Converting application software written in a high-level language from a 32-bit architecture to a 64-bit architecture varies in difficulty. One common problem that occurs is that in the C programming language and its descendant C++ on 32-bit machines, pointers (variables that store memory addresses), and two built-in "data types" for storing numbers - "int" and "long" - all refer to a 32-bit wide data chunk. Some (usually poorly-written) programs transfer quantities between these data types with the assumption that no information will be lost. In many programming environments on 64-bit machines, however, "int" variables are still 32 bits wide, but "long"s and pointers are 64-bits wide. These are described as having a LP64 data model. Another alternative is the ILP64 data model in which all three data types are 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 any changes at all. Besides this there is also a LLP64 model that allows for maximum compability, since both int and long stay 32-bit, and a new 64-bit long long type is defined that is compatible with pointer.

Note that a programming model is a choice made on a per compiler basis, and several can coexist on the same OS. However typically the programming model chosen by the OS API as primary model dominates.

References

64-bit processor architectures include:

• The DEC Alpha architecture
• Intel's IA-64 architecture (used in Intels Itanium CPUs)
• AMD's AMD64 architecture (used in AMD's Opteron and Athlon 64 CPUs).
  • Intel now markets the same architecture for its own processors as EM64T.
• Sun's UltraSPARC architecture
• IBM's POWER architecture
• MIPS Technologies' MIPS IV, MIPS V, and MIPS64 architectures
• IBM/Motorola's PowerPC architecture (originally the PowerPC 620, more recently the PowerPC 970 µP)
• HP's PA-RISC family

See also:

• computer memory
• Win64

External links

• Data Size Neutrality and 64-bit Support

This article is based on material taken from the Free On-line Dictionary of Computing prior to 1 November 2008 and incorporated under the "relicensing" terms of the GFDL, version 1.3 or later.