Hit performance goals with configurable processors

By Steven Leibson, Tensilica  
October 17, 2006 -- dspdesignline.com

With mainstream DSPs, code must be hand-tuned using assembly code in order to meet performance goals. A more productive approach is to tailor the processor to the algorithm. This article explains why, using FFT, Viterbi, and MPEG4 examples.
  
For more than 30 years, the fixed-ISA (instruction set architecture) microprocessor has largely defined electronic system design. Similarly, single-chip DSPs have dominated DSP system design since they were introduced 20 years ago. Fixed-ISA processors have addressed application performance problems in two ways. The first approach has been to increase clock rate, mirroring the same trend in PC processors. If power dissipation is not important, rapidly escalating clock rates can go a long way towards force-fitting a particular processor to an application. The alternative approach for fixed-ISA processors is to provide more computational resources that operate in parallel so that the processor can perform more work per clock.

Designers of fixed-ISA microprocessors and DSPs attempt to develop architectures that are good at executing a wide range of algorithms, but not tailored to any specific application. This design approach can reduce clock-rate requirements, but it doesn't match the clock-rate or gate-usage efficiencies of a tailored solution. The design of complex portable and battery-powered processor-based systems increasingly calls for taking a different approach, one that more closely matches the processing resources to the application tasks so that very high clock rates (and correspondingly high power and heat dissipations) are not required. FPGA, ASIC and SOC technologies provide an ideal medium for tailoring processors to specific applications, using newly available configurable and extensible processor technology.

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