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64-bit CPUs have existed in RISC-based servers since the mid-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. Although a CPU may be 64-bit internally, its external data bus or address bus may have a different size, either larger or smaller, 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 (e.g. the original Pentium and later CPUs), and may occasionally be referred to as "64-bit" for this reason. The term may also refer to the size of an instruction in the computer's instruction set or to any other item of data (e.g. 64-bit double-precision floating-point quantities are common). Without further qualification, however, "64-bit" computer architecture generally has integer registers that are 64 bits wide, which allows it to support (both internally and externally) 64-bit "chunks" of integer data. Architectural implicationsRegisters in a processor are generally divided into three groups: integer, floating point, and other. In all common general purpose processors, only the integer registers are capable of storing pointer values (that is, an address of some data in memory). The non-integer registers cannot be used to store pointers for the purpose of reading or writing to memory, and therefore cannot be used to bypass any memory restrictions imposed by the size of the integer registers. Nearly all common general purpose processors (with the notable exception of most ARM and 32-bit MIPS implementations) have integrated floating point hardware, which may or may not use 64 bit registers to hold data for processing. For example, the x86 architecture includes the x87 floating-point instructions which use 8 80-bit registers in a stack configuration; later revisions of x86, and the x86-64 architecture, also include SSE instructions, which use 8 128-bit wide registers (16 registers in x86-64). By contrast, the 64-bit Alpha family of processors defines 32 64-bit wide floating point registers in addition to its 32 64-bit wide integer registers.
Memory limitationsMost CPUs are currently (as of 2005) 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 register meant that 232 addresses, or 4 gigabytes of RAM, could be referenced. At the time these architectures were devised, 4 gigabytes of memory 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. • [List your site here Free!]
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