The BIOS provides the basic functions needed to boot your machine and to allow your operating system to access your hardware. Your system provides a BIOS setup menu, which is used to configure the BIOS. To enter the BIOS setup menu you have to press a key or key combination after turning on the computer. Often it is the Delete or the F2 key, but some manufacturers use other keys. Usually upon starting the computer there will be a message stating which key to press to enter the setup screen.

I BIOS-opsætningsmenuen kan du vælge hvilke enheder, der skal kontrolleres i hvilken sekvens af det startende operativsystem. Mulige valg inkluderer normalt interne harddiske, cd/dvd-rom-drev og USB-enheder enten i form af små drev eller eksterne USB-harddiske. På moderne systemer er det ofte også muligt at aktivere netværksopstart via PXE.

Afhængig af dit valgte installationsmedie (cd/dvd rom, usb-drev, netværksopstart) skal du aktivere de passende opstartsenheder, hvis de ikke allerede er det.

Most BIOS versions allow you to call up a boot menu on system startup in which you select from which device the computer should start for the current session. If this option is available, the BIOS usually displays a short message like “press F12 for boot menu” on system startup. The actual key used to select this menu varies from system to system; commonly used keys are F12, F11 and F8. Choosing a device from this menu does not change the default boot order of the BIOS, i.e. you can start once from a USB stick while having configured the internal harddisk as the normal primary boot device.

Hvis din BIOS ikke giver dig en opstartsmenu til ad hoc-valg for din nuværende opstartsenhed, så skal du ændre din BIOS-opsætning, så enheden hvorfra debian-installer startes op er den primære opstartsenhed.

Unfortunately some computers contain buggy BIOS versions. Booting debian-installer from a USB stick might not work even if there is an appropriate option in the BIOS setup menu and the stick is selected as the primary boot device. On some of these systems using a USB stick as boot medium is impossible; others can be tricked into booting from the stick by changing the device type in the BIOS setup from the default “USB harddisk” or “USB stick” to “USB ZIP” or “USB CDROM”. In particular if you use an isohybrid CD/DVD image on a USB stick (see Afsnit 4.3.1, “Forberedning af et USB-drev via en hybrid cd eller et dvd-aftryk”), changing the device type to “USB CDROM” helps on some BIOSes which will not boot from a USB stick in USB harddisk mode. You may need to configure your BIOS to enable “USB legacy support”.

Hvis du ikke kan manipulere BIOS'en til at starte op direkte fra et USB-drev, så har du stadig mulighed for at at bruge en ISO kopieret til drevet. Start d-i; op via Afsnit 4.4, “Forberedelse af filer for harddiskopstart” og efter skanning af harddisken efter et ISO-aftryk med et installationsprogram, vælg USB-enheden og vælg et installationsaftryk.

UEFI (“Unified Extensible Firmware Interface”) er en ny slags systemfirmware, som bruges på mange moderne systemer og er - blandt andre ting - lavet til at erstatte den klassiske PC BIOS.

Currently most PC systems that use UEFI also have a so-called “Compatibility Support Module” (CSM) in the firmware, which provides exactly 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 classic 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 because 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 installer 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.

Another UEFI-related topic is the so-called “secure boot” mechanism. Secure boot means a function of UEFI implementations that allows the firmware to only load and execute code that is cryptographically signed with certain keys and thereby blocking any (potentially malicious) boot code that is unsigned or signed with unknown keys. In practice the only key accepted by default on most UEFI systems with secure boot is a key from Microsoft used for signing the Windows bootloader. As the boot code used by debian-installer is not signed by Microsoft, booting the installer requires prior deactivation of secure boot in case it is enabled. Secure boot is often enabled by default on systems that come preinstalled with a 64-bit version of Windows 8 and there is unfortunately no standard way to disable it in the UEFI setup. On some systems, the option to disable secure boot is only made visible when a BIOS password has been set by the user, so if you have a system with secure boot enabled, but cannot find an option to disable it, try setting a BIOS password, powercycle the machine and look again for an appropriate option.

Windows 8 offers a feature called “fast boot” to cut down system startup time. Technically, when this feature is enabled, Windows 8 does not do a real shutdown and a real cold boot afterwards when ordered to shut down, but instead does something resembling a partial suspend to disk to reduce the “boot” time. As long as Windows 8 is the only operating system on the machine, this is unproblematic, but it can result in problems and data loss when you have a dual boot setup in which another operating system accesses the same filesystems as Windows 8 does. In that case the real state of the filesystem can be different from what Windows 8 believes it to be after the “boot” and this could cause filesystem corruption upon further write accesses to the filesystem. Therefore in a dual boot setup, to avoid filesystem corruption the “fast boot” feature has to be disabled within Windows.

It may also be necessary to disable “fast boot” to even allow access to UEFI setup to choose to boot another operating system or debian-installer. On some UEFI systems, the firmware will reduce “boot” time by not initialising the keyboard controller or USB hardware; in these cases, it is necessary to boot into Windows and disable this feature to allow for a change of boot order.

USB BIOS-understøttelse og tastaturer. Hvis du ikke har et tastatur i PS/2-stil, men kun en USB-model, så skal du måske aktivere forældet tastaturemulering i din BIOS-opsætning for at kunne bruge dit tastatur i opstartsindlæsernes menu, men dette er ikke en problemstilling for moderne systemer. Hvis dit tastatur ikke fungerer i opstartsindlæserens menu, så kig i dit bundkorts manual og find i BIOS'en indstillingerne for “Legacy keyboard emulation” eller “USB keyboard support”.