At last month’s Embedded Processor Forum, ARC described extensions to its ARCtangent customizable processor core that target VoIP applications. The extensions include enhanced saturation and rounding support for existing ALU, shifter, and multiplier operations, and new operations like absolute value and negate. These instructions are intended to improve performance on applications that conform to the bit-exact ITU and ETSI specifications for voice compression algorithms such as G.729.
Although the new instructions are useful mainly for compliance with bit-exact standards, other added features will be useful for a broad range of signal-processing applications. Most notably, the extensions greatly improve the processor’s address-generation capabilities. The existing core, the ARCtangent-A4, contains four address registers; each of these registers has a corresponding modifier register. The extensions expand the address registers to a total of eight and expand the modifier registers to two per address register.
While expanding the address registers may seem like a trivial improvement, BDTI’s analysis shows otherwise. BDTI analyzed the existing ARCtangent-A4 core (which lacks the extensions described at the Forum) and found that restrictions on the address registers were a significant limitation when optimizing signal-processing algorithms. For example, four address pointers are barely sufficient for common signal-processing algorithms like FIR filters. Even fairly simple FIR filters may require four separate address pointers, one each for the input buffer, output buffer, delay line, and coefficients.
By recognizing that signal-processing performance depends not only on fast multiplier hardware but also on features like specialized arithmetic operations and flexible addressing, ARC may gain an advantage over its competitors. For example, BDTI’s analysis of ARC and ARM processors with similar multiplier hardware shows that a 170 MHz ARCtangent-A4 is about 25% faster than a 200 MHz ARM9E on typical DSP tasks. (This analysis applies only to the older version of the ARCtangent-A4 without the extensions described at the Forum.) It will be interesting to see if ARC can widen this performance gap through the addition of DSP features like those described at the Forum.
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