Definition

VLIW (very long instruction word)

Part of the Microprocessors glossary:

Very long instruction word (VLIW) describes a computer processing architecture in which a language compiler or pre-processor breaks program instruction down into basic operations that can be performed by the processor in parallel (that is, at the same time). These operations are put into a very long instruction word which the processor can then take apart without further analysis, handing each operation to an appropriate functional unit.

VLIW is sometimes viewed as the next step beyond the reduced instruction set computing ( RISC ) architecture, which also works with a limited set of relatively basic instructions and can usually execute more than one instruction at a time (a characteristic referred to as superscalar ). The main advantage of VLIW processors is that complexity is moved from the hardware to the software, which means that the hardware can be smaller, cheaper, and require less power to operate. The challenge is to design a compiler or pre-processor that is intelligent enough to decide how to build the very long instruction words. If dynamic pre-processing is done as the program is run, performance may be a concern.

The Crusoe family of processors from Transmeta uses very long instruction words that are assembled by a pre-processor that is located in a flash memory chip. Because the processor does not need to have the ability to discover and schedule parallel operations, the processor contains only about a fourth of the transistor s of a regular processor. The lower power requirement enables computers based on Crusoe technology to be operated by battery almost all day without a recharge. The Crusoe processors emulate Intel's x86 processor instruction set. Theoretically, pre-processors could be designed to emulate other processor architectures.

This was last updated in March 2011
Posted by: Margaret Rouse

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