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64-bit computing
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{{Short description|Computer architecture bit width}} {{Redirect|64-bit|64-bit images in computer graphics|Deep color}} {{More citations needed|date=April 2023}} {{Computer architecture bit widths}} [[File:Hex dump of the Section Table in a 64 bit PE File.jpg|thumb|[[Hex dump]] of the section table in a 64-bit [[Portable Executable]] File. A 64-bit [[Word (computer architecture)|word]] can be expressed as a sequence of 16 [[hexadecimal]] digits.]] In [[computer architecture]], '''64-bit''' [[Integer (computer science)|integers]], [[memory address]]es, or other [[Data (computing)|data]] units{{efn|such as [[floating-point arithmetic|floating-point]] numbers.}} are those that are 64 [[bit]]s wide. Also, 64-bit [[central processing unit]]s (CPU) and [[arithmetic logic unit]]s (ALU) are those that are based on [[processor register]]s, [[address bus]]es, or [[Bus (computing)|data buses]] of that size. A [[computer]] that uses such a processor is a 64-bit computer. From the software perspective, 64-bit computing means the use of [[machine code]] with 64-bit [[virtual memory]] addresses. However, not all 64-bit instruction sets support full 64-bit virtual memory addresses; [[x86-64]] and [[AArch64]], for example, support only 48 bits of virtual address, with the remaining 16 bits of the virtual address required to be all zeros (000...) or all ones (111...), and several 64-bit instruction sets support fewer than 64 bits of physical memory address. The term ''64-bit'' also describes a generation of computers in which 64-bit processors are the norm. 64 bits is a [[Word (computer architecture)|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 been used in [[supercomputer]]s since the 1970s ([[Cray-1]], 1975) and in [[reduced instruction set computer]]s (RISC) based [[workstation]]s and [[server (computing)|server]]s since the early 1990s. In 2003, 64-bit CPUs were introduced to the mainstream [[personal computer|PC]] market in the form of x86-64 processors and the [[PowerPC 970|PowerPC G5]]. A 64-bit register can hold any of 2<sup>64</sup> (over 18 [[quintillion]] or 1.8Γ10<sup>19</sup>) different values. The range of [[integer]] values that can be stored in 64 bits depends on the [[Integer (computer science)#Value and representation|integer representation]] used. With the two most common representations, the range is 0 through 18,446,744,073,709,551,615 (equal to 2<sup>64</sup> β 1) for representation as an ([[signedness|unsigned]]) [[binary number]], and β9,223,372,036,854,775,808 (β2<sup>63</sup>) through 9,223,372,036,854,775,807 (2<sup>63</sup> β 1) for representation as [[two's complement]]. Hence, a processor with 64-bit memory addresses can directly access 2<sup>64</sup> bytes (16 [[exabytes]] or EB) of [[byte addressing|byte-addressable]] memory. With no further qualification, a ''64-bit computer architecture'' generally has integer and addressing [[processor register|registers]] that are 64 bits wide, allowing direct support for 64-bit data types and addresses. However, a CPU might have external [[Bus (computing)|data buses]] or [[address bus]]es with different sizes from the registers, even larger (the 32-bit [[Pentium]] had a 64-bit data bus, for instance).<ref>{{cite book |url=https://bitsavers.org/components/intel/pentium/1993_Intel_Pentium_Processor_Users_Manual_Volume_1.pdf |title=Pentium Processor User's Manual Volume 1: Pentium Processor Data Book |publisher=[[Intel]] |year=1993}}</ref>
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