It handles ROM bankswitching as well as sound generation. All sound registers are accessed through only two physical registers. This is the index register. You write the desired register number here. This is the data register.
Data written here is stored in the register pointed to by the above index register.
There are 6 channels, each containing three registers, and 8 custom instrument control registers. See below for info. To program the custom instrument, you load registers 00h-07h with the desired parameters for it. All channels set to instrument 00h will then use this instrument. Note that you can only program one custom instrument at a time.