The easiest place to start in writing your Audio HW DLL is the analog mixer, because it's the simplest part of controlling a card; it doesn't involve any realtime constraints and small mistakes generally don't crash the machine. When you're using DMA, it's possible to overwrite the kernel, so we'll save PCM programming for later (see the Handling PCM Audio Data chapter).
In addition, if your card has an analog input (e.g. CD or Line In), it's very easy to test the analog mixer in isolation from the rest of the sound card. In contrast, without volume controls to adjust, it's very difficult to verify that your PCM playback (and capture) code is functioning correctly.
If your card uses one of the standard codecs (listed in the Supported Codecs appendix), see "Using a standard mixer DLL," later in this chapter.