boot9.bin is a dumped firmware file containing the BootROM 9 (also known as B9) of the Nintendo 3DS family of systems (including 3DS, 3DS XL, 2DS, New 3DS, New 3DS XL, New 2DS XL). It is the first code executed by the ARM9 processor after the system powers on.
Unlocking Your 3DS: Why the File is Your Golden Ticket If you’ve spent any time in the 3DS homebrew scene, you’ve likely seen the name Boot9.bin 3ds
Emulators require boot9.bin to decrypt games. Technical Report: boot9
In the context of the Nintendo 3DS, is a backup dump of the system’s "boot9" firmware. This firmware is used by the ARM9 security processor during early system initialization and cryptographic functions. Purpose and Importance Decryption & Installation : It is a critical file for PC-based tools like Custom-Install Permanent CFW: It allows for "boot-time" exploits