System-Level Design:Word Length Determination

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Word Length Determination

Functional specifications for system tasks are frequently detailed enough to contain the algorithm to be implemented. To determine the implementation costs of each system task, knowledge of the word widths to be used is important as system cost varies almost quadratically with word width.

Tools to automatically select task word width are currently experimental, but the potential for future commercial tools exists.

In typical hardware implementations of an arithmetic-intensive algorithm, designers must determine the word lengths of resources such as adders, multipliers, and registers. Wadekar and Parker [6] in a recent publication, present algorithm-level optimization techniques to select distinct word lengths for each computation. These techniques meet the desired accuracy and minimize the design cost for the given performance constraints. The cost reduction is possible by avoiding unnecessary bit-level computations that do not contribute significantly to the accuracy of the final results. At the algorithm level, determining the necessary and sufficient precision of an individual computation is a difficult task since the precision of various predecessor/successor operations can be traded off to achieve the same desired precision in the final result. This is achieved using a mathematical model [7] and a genetic selection mechanism [6]. There is a distinct advantage to word-length optimization at the algorithmic level. The optimized operation word lengths can be used to guide high-level synthesis or designers to achieve an efficient utilization of resources of distinct word lengths and costs. Specifically, only a few resources of larger word lengths and high cost may be needed for operations requiring high precision to meet the final accuracy requirement. Other relatively low-precision operations may be executed by resources of smaller word lengths. If there is no timing conflict, a large word length resource can also execute a small word length operation, thus improving the overall resource utilization further. These high-level design decisions cannot be made without the knowledge of word lengths prior to synthesis.

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