64-bit

In computer architecture, 64-bit computing is the use of processors that have datapath widths, integer size, and memory addresses widths of 64 bits (eight octets). Also, 64-bit CPU and ALU architectures are those that are based on 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 is a descriptor given to 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 RISC-based workstations and servers since the early 1990s, notably the DEC Alpha, Sun UltraSPARC, Fujitsu SPARC64, and IBM RS64 and POWER3 and later POWER microprocessors. In 2001 NEC released a 64 bit RISC CPU for PDA’s, notably the low cost Casio BE-300. In 2003 64-bit CPUs were introduced to the (previously 32-bit) mainstream personal computer arena in the form of the x86-64 and 64-bit PowerPC processor architectures 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 SoC. A 64-bit register can store 264 (over 18 quintillion or 1.8 1019) different values. Hence, a processor with 64-bit memory addresses can directly access 264 bytes (=16 exbibytes) of byte-addressable memory. Without further qualification, a 64-bit computer architecture generally has integer and addressing 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.