Largeword

Largeword is a term used in computer science to describe a data unit that is larger than the standard machine word of a given architecture. In practice, a largeword can refer to a single register capable of holding a size greater than the conventional word (for example, 128 bits on a 64-bit machine), or to an abstract data type composed of multiple machine words that together represent a single large value. The term is not a formal standard, but it appears in some technical writings and discussions about extended precision arithmetic, multiword representations, and hardware–software interfaces.

In hardware implementations, largewords may be built from multiple registers with dedicated carry or borrow logic to support arithmetic across limbs. In software, largeword types are typically implemented as structures or classes comprising an array of machine words, providing operations such as addition, subtraction, multiplication, and division with full carry/borrow propagation and normalization rules. Endianness, overflow handling, and memory layout are important design considerations.

Common use cases include cryptography, where large numbers are required for RSA or elliptic-curve operations, and in libraries for arbitrary-precision arithmetic or big-number computation. Largeword concepts also appear in graphics, simulations, and scientific computing where extended precision can be beneficial.

Example: on a 64-bit system, a 128-bit largeword can be represented as two 64-bit limbs; arithmetic is performed limb by limb with cross-limb carry. See also big integer, multi-precision arithmetic, and limb-based representations.