<Ul> <Li> </Li> <Li> </Li> <Li> </Li> </Ul> <P> In computer architecture, 64 - bit computing is the use of processors that have datapath widths, integer size, and memory address widths of 64 bits (eight octets). Also, 64 - bit computer architectures for central processing units (CPUs) and arithmetic logic units (ALUs) are those that are based on processor 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 . However, not all 64 - bit instruction sets support full 64 - bit virtual memory addresses; x86 - 64 and ARMv8, for example, support only 48 bits of virtual address, with the remaining 16 bits of the virtual address required to be all 0's or all 1's, and several 64 - bit instruction sets support fewer than 64 bits of physical memory address . </P> <P> The term 64 - bit describes 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 been used in supercomputers since the 1970s (Cray - 1, 1975) and in reduced instruction set computing (RISC) based workstations and servers since the early 1990s, notably the MIPS R4000, R8000, and R10000, the DEC Alpha, the Sun UltraSPARC, and the IBM RS64 and POWER3 and later POWER microprocessors . In 2003, 64 - bit CPUs were introduced to the (formerly 32 - bit) mainstream personal computer market in the form of x86 - 64 processors and the PowerPC G5, and were introduced in 2012 into the ARM architecture targeting smartphones and tablet computers, first sold on September 20, 2013, in the iPhone 5S powered by the ARMv8 - A Apple A7 system on a chip (SoC). </P> <P> A 64 - bit register can store 2 (over 18 quintillion or 1.8 × 10) different values . The range of integer values that can be stored in 64 bits depends on the integer representation used . With the two most common representations, the range is 0 through 18,446,744,073,709,551,615 (2 − 1) for representation as an (unsigned) binary number, and − 9,223,372,036,854,775,808 (− 2) through 9,223,372,036,854,775,807 (2 − 1) for representation as two's complement . Hence, a processor with 64 - bit memory addresses can directly access 2 bytes (= 16 exabytes) of byte - addressable memory . </P>

When did 64 bit operating systems come out
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