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= Memory Map = From a software standpoint, extensions are accessed through register addresses 0xA40000 - 0xA400FF. These registers are mirrored every 0x100 bytes up to 0xA5FFFF. For official extensions, the layout of this address range is: {{MemMapStart|Register}} {{MemMapItem|0xA40000|0xA40007|Unknown/Not Used}} {{MemMapItem|0xA40008|0xA4000D|Input Data}} {{MemMapItem|0xA4000E|0xA4001F|Unknown}} {{MemMapItem|0xA40020|0xA4002F|Calibration}} {{MemMapItem|0xA40030|0xA4003F|Calibration<br>(Backup)}} {{MemMapByte|0xA40040|Encryption Key}} {{MemMapSkip}} {{MemMapItem|0xA400F0|0xA400FD|Unknown}} {{MemMapItem|0xA400FE|0xA400FF|Identifier}} {{MemMapEnd}} = Known Identifiers = {| |00 00 |[[Nunchuck]] |- |01 01 |[[Classic Controller]] |- |01 03 |[[Guitar]] |} = Encryption = Official extensions encrypt their output, using a single-byte key written to 0xA40040. The general algorithm used for this encryption is unknown. However, when the key is 0x00, the output bytes can be decrypted using the following formula: P = ((C ⨁ 0x17) + 0x17) mod 0x100 In C-like languages, this can be implemented as: out = ((in ^ 0x17) + 0x17) & 0xFF Further research into this encryption algorithm, while welcome, is not encouraged. There are other unknowns of much greater significance that could be worked on instead. = Communication Protocol = {{Unknown}} The extension is connected to the Wii Remote via an I²C bus with a nominal bit rate of 400kbit/sec. Commands are sent to device ID 0x52. When handling a write to the extension address range ??? When handling a read from the extension address range ??? When filling in a User Input report, the Wii Remote writes a single byte, 0x00, and then issues 1 - 3 reads to fill the report field with all but the last read being 8 bytes.

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