Stream Registers

$2188 : Stream 1 - C1L2 - Low 5-bit Logical Channel + Data Group ID (R/W)
$2189 : Stream 1 - C1L1 - High 6-bit Logical Channel (R/W)
$218A : Stream 1 - PFCNT1 - Packet Prefix Counter (R)
$218B : Stream 1 - C1POD - Packet Prefix Data (R/W)
$218C : Stream 1 - C1DOD - Packet Data (R/W)
$218D : Stream 1 - Packet Prefix OR Gate (R)

$218E : Stream 2 - C2L2 - Low 5-bit Logical Channel + Data Group ID (R/W)
$218F : Stream 2 - C2L1 - High 6-bit Logical Channel (R/W)
$2190 : Stream 2 - PFCNT2 - Packet Prefix Counter (R)
$2191 : Stream 2 - C2POD - Packet Prefix Data (R/W)
$2192 : Stream 2 - C2DOD - Packet Data (R/W)
$2193 : Stream 2 - Packet Prefix OR Gate (R)

Other Registers

$2194 : 4-bit Satellaview Settings (R/W)
        bit0: Enables Streams and $2199
        bit1: Unknown
        bit2: ACCESS LED
        bit3: Unknown/Unused
$2195 : Unknown (?)
        Always sends back 0x00.
$2196 : Satellaview Status (R)
        bit0: Toggles on/off when $2194.0 or 1 is set.
        bit1: Tested for bitstream cable (1 = unplugged)
        bit2: Always 0
        bit3: Toggles on/off when $2194.0 or 1 is set.
        bit4: Always 1
        bit5: Always 0
        bit6: Always 0
        bit7: Always 0
$2197 : Satellaview Control (R/W)
        This register sends back what was written on it like RAM.
        On reset, it's initialized as 0x80.
        bit7: If set, mute Soundlink Radio.
        bit1: Satellaview EXT Pin 33
        bit0: Satellaview EXT Pin 34
$2198 : Serial I/O - Serial Number SPR-BSA EEPROM (R/W)
        Uses Serial Communication. Chip is S-2913CR, a 16-bit Serial EEPROM.
        Input : Command that could be found in datasheet below.
        Data in EEPROM: "BSA00???????" 0xA4 at sent offset (ASCII).
                ??????? being the Serial Number under the Satellaview.
                It sends the same byte two times before going to the next one.
                After 0xA4, and any invalid input sent, it outputs 0xFF.
$2199 : Serial I/O - Unknown (R/W)
        Uses Serial Communication.
        Enabled when $2194.0 is on.
        Input : Send 16-bit size to be sent.
                Then send 8-bit Increment (from 0x00) and 8-bit Data.
        Output: Outputs 0xE00000 if enabled.
                If not, it sends back 0xFFFFFF.

        BS-X inits $2199 at boot by sending these:
        0008
        8800
        8001
        0402
        0003
        0804
        0005
        7006
        
        BS-X can also send these depending on the Town Data:
        8800
        0000
$219A : Always 0x10
$219B : Always 0x00
$219C : Always 0x00
$219D : Always 0x00
$219E :  Always 0x00
$219F : Always 0x00
$21A0-21AF : EXT Port Access

S-2913CR Datasheet or S-2913CR Datasheet from datasheetarchive.com