In computer architecture, 128-bit integers, memory addresses, or other data units are those that are 128 bits (16 octets) wide. Also, 128-bit CPU and ALU architectures are those that are based on registers, address buses, or data buses of that size.
While there are currently no mainstream general-purpose processors built to operate on 128-bit integers or addresses, a number of processors do have specialized ways to operate on 128-bit chunks of data. The IBM System/370 could be considered the first simple 128-bit computer, as it used 128-bit floating-point registers. Most modern CPUs feature single-instruction multiple-data (SIMD) instruction sets (Streaming SIMD Extensions, AltiVec etc.) where 128-bit vector registers are used to store several smaller numbers, such as four 32-bit floating-point numbers. A single instruction can then operate on all these values in parallel. However, these processors do not operate on individual numbers that are 128 binary digits in length; only their registers have the size of 128 bits.
The DEC VAX supported operations on 128-bit integer ('O' or octaword) and 128-bit floating-point ('H-float' or HFLOAT) datatypes. Support for such operations was an upgrade option rather than being a standard feature. Since the VAX's registers were 32 bits wide, a 128-bit operation used four consecutive registers or four longwords in memory.
In the same way that compilers emulate e.g. 64-bit integer arithmetic on architectures with register sizes less than 64 bits, some compilers also support 128-bit integer arithmetic. For example, the GCC C compiler 4.6 and later has a 128-bit integer type
__int128 for some architectures. For the C programming language, this is a compiler-specific extension, as C11 itself does not guarantee support for 128-bit integers.
A 128-bit register can store 2128 (over 3.40 × 1038) different values. The range of integer values that can be stored in 128 bits depends on the integer representation used. With the two most common representations, the range is 0 through 340,282,366,920,938,463,463,374,607,431,768,211,455 (2128 − 1) for representation as an (unsigned) binary number, and −170,141,183,460,469,231,731,687,303,715,884,105,728 (−2127) through 170,141,183,460,469,231,731,687,303,715,884,105,727 (2127 − 1) for representation as two's complement.
A CPU with 128-bit multimedia extensions was designed by researchers in 1999.