ARM9's and ARM11's exception vectors are hardcoded to point at the CPU's internal memory (0x08000000 region for ARM9, AXIWRAM for ARM11). While the bootrom does set them up to point to an endless loop at some point during boot, it does not do so immediately. As such, a carefully-timed fault injection (via hardware) to trigger an exception (such as an invalid instruction) will cause execution to fall into ARM9 RAM.
Since RAM isn't cleared on boot, one can immediately start execution of their own code here to dump bootrom, OTP, etc. The ARM9 bootrom does the following at reset: reset vector branches to another instruction, then branches to bootrom+0x8000. Hence, there's no way to know for certain when exactly the ARM9 exception-vector data stored in memory gets initialized.
This requires *very* *precise* timing for triggering the hardware fault.
It has been exploited by derrek to dump the ARM9 bootrom as of Summer 2015.
He did not make any Bootrom Public.
hedgeberg and Greg the 2DS are using this method to dump the arm9 Bootrom which is known as boot9
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Sonic Angel Knight:
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K3Nv2:
