What's the deal with 17- and 40-bit math in TI DSPs?

Question

The TMS320C55x has a 17-bit MAC unit and a 40-bit accumulator. Why the non-power-of-2-width units?

Answer 1

I may be talking through my hat here, but I'd expect to see the 17-bit stuff used to avoid the need for a separate carry bit when adding/subtracting 16-bit samples.

Answer 2

The 40-bit accumulator is common in a few TI DSPs. The idea is basically that you can accumulate up to 256 arbitrary 32-bit products without overflow. (vs. in C where if you take a 32-bit product, you can overflow fairly quickly unless you resort to using 64-bit integers.)

The only way you access these features is by assembly code or special compiler intrinsics. If you use regular C/C++ code, the accumulator is invisible. You can't get a pointer to it.

So there's not any real need to adhere to a power-of-2 scheme. DSP cores have been fairly optimized for power/performance tradeoffs.