When I was trying to learn computer architecture the thing that helped me the most was making a toy 4-bit processor (through gate level design of every single thing). Now I want to learn about DSP processors and was wondering if there is a book which takes this approach to teaching DSP architectures.
The features that (used to?) distinguish a DSP from a more general-purpose CPU more-or-less centre around effective execution of a tight loop of MAC (multiply-acumulate) instructions, iterating over two arrays. This implies
In your situation I would dig up a few typical DSP algorithms (convolution, filters, FFT), find the inner loop, dream up the instructions required to do that loop as fast as possible, implement those instructions, and then add other instructions to make it look like a real CPU. For extra bonus points: minimize transistor count in order to minimize battery drain.