I assume they all use a similar method... this section of code is from the reinx bootloader.c source file
Code:
void bootloader() {
mbist_workaround();
clock_enable_se();
// This makes fuse registers visible
clock_enable_fuse(0x01);
check_sku();
// Check configuration fuses
check_config_fuses();
// Disables fuse programming until next reboot
FUSE(FUSE_PRIVATEKEYDISABLE) = 0x10;
// Setup memory controllers
mc_enable();
// Pre-Firmware setup
setup();
}
Beyond that I have no clue XD