There are essentially 3 different types of DS/DSi cards:
- Nitro (DS)
- Hybrid (DS & DSi)
- Limited (DSi only)
For Nitro cards, DSi enters backward compatibility mode to ensure an environment similar to an old DS (no cameras, slower clock speed, et cetera). For Hybrid cards, the software can make use of DSi features when running on a DSi, while reverting back to DS only features when running on a DS. For Limited cards, the software will only run on a DSi, and cannot be run on a DS.
The DSi card slot is exactly the same as the old DS card slot (also known as SLOT-1).
However, the DSi has more control over this slot in software than its DS counterpart. More specifically, the following hardware lines can be controlled (indirectly) in software:
- Power supply (VCC) can be enabled or disabled
- Reset (RST) can be cycled
Being able to reset a card using the hardware RST line is essential for the DSi card protocol.
Here's a set of sample card commands that an old DS sends to a DSi enhanced card upon bootup:
|0000||3C02DD38BEC62AC2||ENTER BLOWFISH MODE|
|0910||A75C702DD38EC631||ENTER LFSR MODE|
Note that the blowfish and lfsr commands shown here are already decrypted.
Now here's a set of sample commands that a DSi sends to a DSi enhanced card:
|0000||3CA3BD240F4B7400||ENTER BLOWFISH MODE|
So far, this matches up with a normal DS bootup - with minor differences such as CARDID (90) coming before HEADER (00), the header being queried for 0x1000 bytes, and the size of responses changed for some commands from 0x910 to 0x9F8. After these commands, the card is reset and a new sequence starts. A raw read header command (00) is sent, followed by a raw command that always starts with 0x3D. The following commands are all shown in raw format, as it is impossible to decrypt them yet. However, some obvious similarities exist by simply looking at the response size of the commands.
|0000||3DBA1F0A0E91C100||ENTER BLOWFISH DSi MODE|
|19B8||E1B09DEAABE3D960||SEC DSi 4|
|19B8||082289FB6F52EC75||SEC DSi 5|
|19B8||854F68025AAC4B6D||SEC DSi 6|
|19B8||994FAFFDD8993548||SEC DSi 7|
|09F8||049D1DB7297CCE7F||ENTER LFSR MODE|
The command after 0x3D can not be decrypted using the old DS blowfish keys, it is likely that new blowfish keys are used when entering DSi blowfish mode. From the response lengths and similarity to the normal DS card protocol, a good guesstimate can be made about what each command in the DSi card protocol does.