ARM9LoaderHax(often shortened to A9LH) is an ARM9 kernel exploit for the Nintendo 3DS. It allows a user to run code milliseconds after pressing the Power button. This exploit is often used to boot CFW at boot time, which in turn patches signature checks and blocks FIRM writes (as to keep the exploit intact even after a system update on SysNAND).
At GS2012 Mirror, multiple builds of A9LH and A9LH variations.
You can check these below.
WARNING: Do not mess with this if you don't know what this is, or you could end up with a brick.
Aurora Wright's ARM9LoaderHax - This is the most used A9LH build.
ShadowNAND - A variation of Aurora Wright's A9LH.
delebile's ARM9LoaderHax - delebile's implementation of A9LH. Aurora Wright's A9LH is built upon this.