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.
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 in 2012 even 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).
A 64-bit register can store 2 (over 18 quintillion or 1.8×10) different values. Hence, a processor with 64-bit memory addresses can directly access 2 bytes (=16 exbibytes) of byte-addressable memory.
With no further qualification, a 64-bit computer architecture generally has integer and addressing processor 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 from 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.