In this article at Linux Journal, Julian Rose explains dsp_K, an "open-source, real-time kernel and library for digital signal processors (DSPs)". It covers both technical and historical ground, paying particular attention to the board support package (BSP) layer that is at the kernel's heart; and explains how dsp_K handles funcionality that is essential in any kernel, like context switching, and interrupt handling.

The BSP comprises the minimal services required to provide a functional software platform for an application to run, along with a minimal kernel personality, interrupts, context switching, scheduler sanity checker and task functions

From the article:

The purpose of the dsp_K project is to provide developers with a small real-time kernel suitable for DSPs, serve as an illustration of embedded open-source software, propose and implement an EL/IX profile for DSPs and serve as an exercise in porting pieces of Linux code to DSPs.

dsp_K Project Home Page
dsp_K Working Mirror
EL/IX Homepage