The CSND service talks to the DSP using the DSP FIFO to negotiate what CSND channels are allocated to the DSP, and this seems to be channels 0-7 (usually). The rest are dedicated to CSND services.

CSND service "csnd:SND"

Command Header Description
0x00010140 Initialize
0x00020000 Shutdown
0x00030040 ExecuteType0Commands
0x00040080 ExecuteType1Commands
0x00050000 AcquireSoundChannels. This function tries to acquire sound channels. Response cmdbuf[2] holds a bitmask to which channels were allocated to this process.
0x00060000 ReleaseSoundChannels.
0x00070000 AcquireUnknown. This function tries to acquire unknown channels (16).
0x00080040 ReleaseUnknown. This function tries to release the channels corresponding to the given bitmask in cmdbuf[1].
0x00090082 FlushDCache
0x000A0082 StoreDCache
0x000B0082 InvalidateDCache
0x000C0000

A maximum of 4 service sessions can be opened at the same time for this service.

For cache operations addresses must be in 0x14000000-0x1C000000 range. Starting with 8.0.0-18 addresses in the range 0x30000000-0x40000000 are also allowed.

This service is normally used for playing CWAVs. The audio data must be located in the 0x14000000 virtual memory region, since CSND requires the physical address for playing the audio.