Research and Development
PCs have two boot modes - BIOS (Basic Input Output System) and UEFI (Unified Extensible Firmware Interface). BIOS was installed in IBM PCs and compatibles from the 1980s. UEFI mode has become prevalent since Microsoft introduced it in Windows 7 and later began requiring it on new PCs to meet the Windows Logo License Agreement requirements. Most PCs since 2010 have UEFI.
Apple Macintosh/iMac devices have their own EFI (Extensible Firmware Interface) which is almost, but not quite, the same as UEFI but do not have a BIOS equivalent. This guide doesn't (currently) address installation on Apple devices.
BIOS is also known as Legacy or CSM (Compatibility Support Module) when part of UEFI.
If the target system is BIOS-only you can disregard the rest of this section.
In order to support UEFI Secure Boot, or to install alongside another operating system that uses UEFI boot mode (e.g. Windows 10), the system motherboard's firmware boot-manager has to be told to start the Ubuntu installer in UEFI mode.
Unfortunately, there is no consistency between different PC manufacturers on how motherboard firmware boot-managers should indicate boot-mode so we, as users, have to figure it out from what clues we can see when the PC's boot menu is displayed and lists boot devices.
Let's assume we're using a USB Flash device. The boot menu may list that device twice (once for UEFI mode, and again for BIOS/CSM/Legacy mode). It may make it explicit that one is "UEFI" and the other not, or it may use some hard-to-spot code such as a single letter abbreviation (e.g. "U" vs "B").
If we want to guarantee UEFI mode and avoid BIOS/CSM/Legacy mode then by entering firmware Setup at power-on we should be able to find an option to disable CSM/Legacy mode.
After doing that we can be sure the installer will boot in UEFI mode.
There is a quick way to confirm the installer has started in UEFI mode - it will be using GRUB, so see the following section First Boot Screen > GRUB (UEFI mode) for what it will look like.
Once Linux has started it is possible to check. The presence of the efivarfs filesystem means the system booted in UEFI mode:
$ mount | grep efivars
efivarfs on /sys/firmware/efi/efivars type efivarfs (rw,nosuid,nodev,noexec,relatime)