Translations:Syslinux/85/en
From Gentoo Wiki
If the system has not been booted in EFI mode, then the Syslinux files need to be copied to the /boot/efi/EFI/Boot directory instead and syslinux.efi needs to be renamed to bootx64.efi. If this is the case then skip the next section concerning the efibootmgr utility.