dell table only can boot from UEFI driver and the bios setting has no legacy boot option, i do this :in bios ->Boot Sequence->add boot option->Add a file name by clicking on the browse button, navigate and select EFI\Microsoft\Boot\BOOTMGFW.EFI and click OK.

it can not go into the LEGACY boot USB drive which installed PE system after i do this. i have try update a BIOS.exe, but it doesn't work,too. this DELL table has a win8.1 system.

@Fernando
Thank you for your reply. i have a USB dirver which installed Hirens.BootCD.15.2 .it is usefull for me but the Hirens.BootCD.15.2 only works in LEGACY mode (I can't be absolutely certain of it，butI did a lot of research on the Internet to show that it couldn't).in the other hand,i need to flash a full SPI.bin to the COMS chip in windows system because it is hard to dismantle the CMOS chip from the motherboard.I know one way to unlock the Access permissions:Format a USB stick to FAT32 and add the bootx64.efi file into EFI/BOOT. Boot from the USB driver.then is show command prompt 'grub> prompt' setup_var 0x75 0x00.I think this can only work in LEGACY mode after I tried a few failures but It worked on other computers which has LEGACY MODE.Although I have solved this problem by dismantling the chip again, I still want to know what I should do so that I can do this next time.

thank you .I will try later.