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UEFI (“Unified Extensible Firmware Interface”) is a new kind of system firmware that is used on many modern systems and is - among other uses - intended to replace the classic PC BIOS.
Currently most PC systems that use UEFI also have a so-called “Compatibility Support Module” (CSM) in the firmware, which provides excatly the same interfaces to an operating system as a classic PC BIOS, so that software written for the classic PC BIOS can be used unchanged. Nonetheless UEFI is intended to one day completely replace the old PC BIOS without being fully backwards-compatible and there are already a lot of systems with UEFI but without CSM.
On systems with UEFI there are a few things to take into consideration when installing an operating system. The way the firmware loads an operating system is fundamentally different between the clas- sic BIOS (or UEFI in CSM mode) and native UEFI. One major difference is the way the harddisk partitions are recorded on the harddisk. While the classic BIOS and UEFI in CSM mode use a DOS partition table, native UEFI uses a different partitioning scheme called “GUID Partition Table” (GPT). On a single disk, for all practical purposes only one of the two can be used and in case of a multi-boot setup with different operating systems on one disk, all of them must therefore use the same type of partition table. Booting from a disk with GPT is only possible in native UEFI mode, but using GPT becomes more and more common as hard disk sizes grow, because the classic DOS partition table cannot address disks larger than about 2 Terabytes while GPT allows for far larger disks. The other major difference between BIOS (or UEFI in CSM mode) and native UEFI is the location where boot code is stored and in which format it has to be. This means that different bootloaders are needed for each system.
The latter becomes important when booting debian-installer on a UEFI system with CSM be- cause debian-installer checks whether it was started on a BIOS- or on a native UEFI system and installs the corresponding bootloader. Normally this simply works but there can be a problem in multi- boot environments. On some UEFI systems with CSM the default boot mode for removable devices can be different from what is actually used when booting from hard disk, so when booting the in- staller from a USB stick in a different mode from what is used when booting another already installed operating system from the hard disk, the wrong bootloader might be installed and the system might be unbootable after finishing the installation. When choosing the boot device from a firmware boot menu, some systems offer two seperate choices for each device, so that the user can select whether booting shall happen in CSM or in native UEFI mode.