Very Long Instruction Word - Motivation

Motivation

A processor that executes every instruction one after the other (i.e. a non-pipelined scalar architecture) may use processor resources inefficiently, potentially leading to poor performance. The performance can be improved by executing different sub-steps of sequential instructions simultaneously (this is pipelining), or even executing multiple instructions entirely simultaneously as in superscalar architectures. Further improvement can be achieved by executing instructions in an order different from the order they appear in the program; this is called out-of-order execution.

As often implemented, these three techniques all come at a cost: increased hardware complexity. Before executing any operations in parallel, the processor must verify that the instructions do not have interdependencies, for example, a first instruction's result is used as a second instruction's input. Clearly, they cannot execute at the same time, and the second instruction can't be executed before the first. Modern out-of-order processors have increased the hardware resources which do the scheduling of instructions and determining of interdependencies.

The VLIW approach, on the other hand, executes operations in parallel based on a fixed schedule determined when programs are compiled. Since determining the order of execution of operations (including which operations can execute simultaneously) is handled by the compiler, the processor does not need the scheduling hardware that the three techniques described above require. As a result, VLIW CPUs offer significant computational power with less hardware complexity (but greater compiler complexity) than is associated with most superscalar CPUs.