This page documents the format of NCSD.
There are two known specialisations of the NCSD container format. The CTR Cart Image (CCI) format and the 3DS' raw NAND format. CCI is the format of game ROM images.
CTR System Update (CSU) is a variant of CCI, where the only difference is in the file extension. This is used with developer System Updates and associated Tools.
NCSD images start with a NCSD header, followed by up to a maximum of 8 NCCH partitions.
For CCI images, the partitions are reserved as follows:
|NCCH Index||Reserved Use|
|0||Executable Content (CXI)|
|2||Download Play Child container (CFA)|
|6||New3DS Update Data (CFA)|
|7||Update Data (CFA)|
The format of partitions can be determined from the partition FS flags (normally these are zero for CCI/CSU NCSD Images).
 NCSD header
|0x000||0x100||RSA-2048 SHA-256 signature of the NCSD header|
|0x100||4||Magic Number 'NCSD'|
|0x104||4||Size of the NCSD image, in media units (1 media unit = 0x200 bytes)|
|0x110||8||Partitions FS type (0=None, 1=Normal, 3=FIRM, 4=AGB_FIRM save)|
|0x118||8||Partitions crypt type|
|0x120||0x40=(4+4)*8||Offset & Length partition table, in media units|
|0x160||0x20||Exheader SHA-256 hash|
|0x180||0x4||Additional header size|
|0x184||0x4||Sector zero offset|
|0x188||8||Partition Flags (See Below)|
|0x190||0x40=8*8||Partition ID table|
|0x1FE||0x1||Support for this was implemented with 9.6.0-X FIRM. Bit0=1 enables using bits 1-2, it's unknown what these two bits are actually used for(the value of these two bits get compared with some other value during NCSD verification/loading).|
|0x1FF||0x1||Support for this was implemented with 9.6.0-X FIRM, see below regarding save crypto.|
|0x1BE||0x42||Encrypted MBR partition-table, for the TWL partitions(key-data used for this keyslot is console-unique).|
 Partition Flags
|0||Backup Write Wait Time (The time to wait to write save to backup after the card is recognized (0-255 seconds)).NATIVE_FIRM loads this flag from the gamecard NCSD header starting with 6.0.0-11.|
|3||Media Card Device (1 = NOR Flash, 2 = None, 3 = BT) (SDK 3.X+)|
|4||Media Platform Index (1 = CTR)|
|5||Media Type Index (0 = Inner Device, 1 = Card1, 2 = Card2, 3 = Extended Device)|
|6||Media Unit Size i.e. u32 MediaUnitSize = 0x200*2^flags;|
|7||Media Card Device (1 = NOR Flash, 2 = None, 3 = BT) (Only SDK 2.X)|
 Partition Flags (In Terms of Save Crypto Determination)
|1|| Starting with 6.0.0-11 NATIVE_FIRM will use this flag to determine the gamecard savegame keyY method, when flag is set. 0 = 2.0.0-2 hashed keyY, 1 = new keyY method implemented with 6.0.0-11. 0x0A = implemented with 9.3.0-X. On Old3DS this is identical to the 2.2.0-4 crypto. On New3DS this is identical to the 2.2.0-4 crypto, except with New3DS-only gamecard savedata keyslots.
Starting with 9.6.0-X FIRM, Process9 now sets <savecrypto_stateval> to partitionflag + <the u8 value from NCSD+0x1FF>, instead of just setting it to partitionflag.
|3||Support for this flag was implemented in NATIVE_FIRM with 2.0.0-2. When this flag is set the hashed gamecard savegame keyY method is used, this likely still uses the repeating-CTR however. With 6.0.0-11 the system will determine the gamecard savegame keyY method via flag, instead of just using the hashed keyY via this flag.|
|7||This flag enables using the hashed gamecard savegame keyY method, support for this flag was implemented in NATIVE_FIRM with 2.2.0-4. All games with the NCSD image finalized since 2.2.0-4(and contains 2.2.0-4+ in the system update partition) have this flag set, this flag also enables using new CTR method as well.|
Starting with 9.6.0-X FIRM, Process9 will just write val0 to a state field then return 0, instead of returning an error when the save crypto type isn't recognized. This was the *only* actual functionality change in the Old3DS Process9 function for gamecard savedata crypto init.
 Card Info Header
|0x200||4||CARD2: Writable Address In Media Units (For 'On-Chip' Savedata). CARD1: Always 0xFFFFFFFF.|
|0x204||4||Card Info Bitmask|
|0x1000||8||Media ID (same as first NCCH partitionId)|
|0x1100||0x100||Copy of first NCCH header (excluding RSA signature)|
 Development Card Info Header Extension
ctrtool - (CMD)(Windows/Linux) Parsing NCSD files
3DSExplorer - (GUI)(Windows Only) Parsing NCSD files