Advanced Linux Sound Architecture

The lowest layer is probably the open source sound system, which has slightly formed by the wayside by now.

But the most prominent one is probably known as ELSA, as in the Advanced Linux Sound Architecture.

So ELSA is the idea of, and OSS was similar in that regard, the idea was to abstract away from device registers, interrupts, and all the rest of it, basically to provide a unified view of audio hardware to higher layer abstractions.

So the idea is basically that you have devices in ALSA which have certain features and characteristics, and ALSA abstracts away from the individual hardware in terms of the hardware registers, the interrupts, and all the rest of it.