As described earlier, your Audio HW DLL must provide an entry point called ctrl_init(). The Organization of a Driver chapter describes the initialization that this function must do no matter what features your DLL supports.
In much the same way that we created a mixer device in the previous chapter, we now create a PCM device using the ado_pcm_create() function. This informs the upper levels of software that this card now supports a PCM device. If you call this function again, it creates additional devices.