Memory granule terms

Bit

An unambiguous term, short for binary digit, referring to a granule capable of holding two states.

Nibble

A cute term derived from the common homonym of byte--bite. It refers to half of a byte and is exclusively used with 8-bit bytes.

Byte

A granule of size usually corresponding to the size used to store a single character. In some earlier architectures, the size of a byte was only a software convention, sometimes with hardware support for multiple sizes of bytes.

Currently, byte is used synonymously with octet to designate an eight-bit granule.

Octet

Octet refers to a granule of eight bits. The term is often used to avoid the potential ambiguity of byte.

Word

The size of a word usually corresponds to the size of the general purpose registers (GPRs). However, when an architecture extends the size of its general purpose registers, the size of the word is not changed. Likewise, when an architecture is introduced targeting the software base of a previously existing architecture, the word size is often the size of the previous architecture's general purpose registers.

Because the x86 architecture began with the 16-bit 8086, words are 16 bits. MIPS began with 32-bit GPRs and uses 32-bit words. Alpha, despite being introduced with 64-bit GPRs, uses 'word' to refer to a 16-bit granule, perhaps because of continued software legacy from the 16-bit PDP-11. Despite originating with 32-bit GPRs, Renesas SuperH uses word to refer to 16-bit granules, perhaps because of its targeting embedded systems where it might be considered as an upgrade path from 16-bit architectures. The Motorola 68k also uses word to refer to 16-bit granules (again targeting software developers used to 16-bit architectures?).

The term word is also used to refer to an instruction word.

Halfword

Halfword is used by IBM System z mainframe ISA, MIPS, PA-RISC, Power, ARM, and Motorola 88k (half-word) to refer to a granule half the size of a word.

Longword or doubleword

Longword (used by VAX, M68k (long-word), Alpha, SuperH, Renesas RX) and doubleword (used by MIPS, PA-RISC, x86, SPARC, ARM, 88k (double word)) refer to granules the size of two words.

Quadword

Quadword is the generally accepted term for a granule of size equal to four words.

Octaword

Octaword is used by VAX to refer to eight word (16-byte) granules.

Doublet, quadlet, octlet, hexlet (SPARC oddities?)

The terms doublet, octet, and hexlet--referring to two-byte, four-byte, eight-byte, and 16-byte granules, respectively--appears to be unique to SPARC. SPARC uses halfword, word, doubleword, and quadword to refer to naturally aligned doublet, quadlet, octlet, and hexlet granules.

Comments