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 computers (RISC) based workstations and servers since the early 1990s, notably the MIPS R4000, R8000, and R10000, the Digital Equipment Corporation (DEC) Alpha, the Sun Microsystems 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).