Full custom. That is, not only is it our own DSP architechture and own instruction set, but the firmware running on it.

Normally we'd have the firmware loaded from elsewhere on system boot at the cost of a few hundred miliseconds, but for the current project we need a much larger firmware and the bootup time is prohibitive. So we've ended up with a three-way partition of functionality. A "standard library" of mature functions in ROM, an application-specific set of medium maturity functions in OTP (one time programmable) which can be changed without needing an expensive metal layer respin, and finally the actual application in RAM.