As far as we’re concerned, the bootloader is a small program that “wakes up” the STM (STMicroelectronics) chip in the TX when chip detects that the TX is plugged into a USB port with the TX off. After that, OpenTX Companion launches a process called dfu-util that does all the work. The bootloader, written and contributed by Mike Blandford, is what allows us to access the SD card and EEPROM/FIRMWARE in mass storage mode. Without it, we can’t upload the firmware because we can’t see the chip.