APX Mode (USB Recovery Mode "RCM")
APX Mode (NVIDIA USB Recovery Mode) can be used to run Fusee Gelee or to use Nvidia tools to flash the hardware. https://forum.xda-developers.com/showpost.php?p=77286176&postcount=85

Entering APX Mode

Note - you will need a USB3 port on your development computer.
Power off the SurfaceRT or RT2 (not sleep mode!) When the device is shut off, plug in a USB type A into the Surface, and a USB type A (or type C depending on your development device) into your development computer. Hold VOL-UP; Press PWR and release it. Release VOL-UP if dmesg -Tw shows:
Surface RT
Once connected in APX mode, you can use NVidia tools or a precompiled Fusee Gelee on your device. Nvflash v2.6 - https://docs.toradex.com/103464-nvfl...ry-toolset.zip NvFlash tool for getting uid - https://web.archive.org/web/20160318...ools-win32.zip

Entering APX Mode from anywhere

Connect USB - A-to-A cable Write 0x00000002 to 0x7000E450 <- sets bit in PMC-register to indicate to boot to RCM Write 0x00000010 to 0x7000E400 <- reboot This should force reset in RCM Mode

Enter RCM from UEFI Shell

Boot UEFI Shell as normal. use this startup.nsh
startup.nsh
### Blink screen to indicate the the script has started
mm 0x7000A000 0x8010000E -w 4
stall 1000000
mm 0x7000A000 0x80FF000E -w 4
stall 10000000 # waits 10s; plug in the usb a-a cable in this time
# reboot to RCM is usb a-a cable is connected
mm 0x7000E450 0x02 -w 4 # Write RCM bit
mm 0x7000E400 0x10 -w 4 # reset
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Entering APX Mode
Entering APX Mode from anywhere
Enter RCM from UEFI Shell