Except with that new 64 bit only thing Intel is trying to do, all x86_64 processors have 16-bit and 32-bit mode for backwards compatibility reasons, the processors actually start at 16-bit mode and are raised by the UEFI (or previously, the OS/bootloader) to 64-bit mode. In fact, if your UEFI supports CSM, you can flash MS-DOS 6.22 onto a USB stick and boot from it, and it'll treat it as a floppy drive (many BIOS implementations use floppy disks to emulate usb mass storage devices).