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very long instruction word
Very long instruction word (VLIW) refers to processor architectures designed to take advantage of instruction level parallelism (ILP). Whereas conventional processors mostly allow programs only to specify instructions that will be executed in sequence, a VLIW processor allows programs to explicitly specify instructions that will be executed at the same time (that is, in ''parallel''). This type of processor architecture is intended to allow higher performance without the inherent complexity of some other approaches.
== Overview ==
Traditional approaches to improving performance in processor architectures include breaking up instructions into sub-steps so that instructions can be executed partially at the same time (known as ''pipelining''), dispatching individual instructions to be executed completely independently in different parts of the processor (''superscalar architectures''), and even executing instructions in an order different from the program (''out-of-order execution''). These approaches all involve increased hardware complexity (higher cost, larger circuits, higher power consumption) because the processor must intrinsically make all of the decisions internally for these approaches to work. The VLIW approach, by contrast, depends on the programs themselves providing all the decisions regarding which instructions are to be executed simultaneously and how conflicts are to be resolved. As a practical matter this means that the compiler (software used to create the final programs) becomes much more complex, but the hardware is simpler than many other approaches to parallelism.
The acronym VLIW may also refer to ''variable-length instruction word'', a criteria in instruction set design to allow for a more flexible layout of the instruction set and higher code density (depending on the instructions to be used). For example, this approach makes it possible to load an immediate value of the size of a machine word into a processor register, which would not be feasible if each instruction was limited to the size of machine word. The flexibility comes at an additional cost for instruction decoding.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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