Very long instruction word

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Very long instruction word (VLIW) refers to instruction set architectures that are designed to exploit instruction-level parallelism (ILP). A VLIW processor allows programs to explicitly specify instructions to execute in parallel, whereas conventional central processing units (CPUs) mostly allow programs to specify instructions to execute in sequence only. VLIW is intended to allow higher performance without the complexity inherent in some other designs.

The traditional means to improve performance in processors include dividing instructions into substeps so the instructions can be executed partly at the same time (termed pipelining), dispatching individual instructions to be executed independently, in different parts of the processor (superscalar architectures), and even executing instructions in an order different from the program (out-of-order execution).^[1] These methods all complicate hardware (larger circuits, higher cost and energy use) because the processor must make all of the decisions internally for these methods to work.

In contrast, the VLIW method depends on the programs providing all the decisions regarding which instructions to execute simultaneously and how to resolve conflicts. As a practical matter, this means that the compiler (software used to create the final programs) becomes more complex, but the hardware is simpler than in many other means of parallelism.