This document is about the format of CTR cart images (CCI), which is also the format of 3DS ROM dumps.
CCI files start with a NCSD header which is 4096 bytes long, followed by 4 CXI block. While it is unknown what each CXI block is for, the second one is most likely the main program code since it is much bigger than the others.
The first CXI block always begins at 0x1000, and appears to lack a RSA-2048 signature. The offsets to the other blocks are found inside the NCSD header.
|0x000||0x100||RSA-2048 signature of the NCSD header, using SHA-256 (needs to be confirmed)|
|0x100||4||Magic ID, always 'NCSD'|
|0x104||4||Size of the CCI image, in media units (1 media unit = 0x200 bytes)|
|0x120||4||Offset to the second CXI in media units|
|0x124||4||Size of the second CXI|
|0x130||4||Offset to the third CXI|
|0x134||4||Size of the third CXI|
|0x158||4||Offset to the fourth CXI|
|0x15C||4||Size of the fourth CXI|
|0x18C||4||Unknown (always 0x00000101)|
|0x190||8||Partition ID of the second CXI|
|0x1A0||8||Partition ID of the third CXI|
|0x1C8||8||Partition ID of the fourth CXI|
|0x300||4||Used ROM size in bytes|
CCI images may not include all four CXI blocks. For example, Ghost Recon: Shadow Wars lacks the third CXI. Entries at 0x130, 0x134 and 0x1A0 in its NCSD header are zero.