BIOS 为启动机器提供了基本的功能，并允许您的操作系统可以访问硬件。系统提供了 BIOS 设置菜单，通过它可以配置 BIOS。要进入 BIOS 设置菜单，您需要在打开计算机的时候按下某组合键。通常这会是 Delete 或 F2 键，但也有厂商使用其他的键。一般在开机的时候会有信息显示使用哪些键可以进入设置界面。

在 BIOS 设置菜单上，您可以选择用什么次序来检测设备上的可引导操作系统。供选择的通常有内置硬盘、CD/DVD-ROM 驱动器和 USB 存储设备，如 U 盘和外置硬盘。在新的系统上还可能会有使用 PXE 打开网络引导。

根据不同的安装介质(CD/DVD ROM、U 盘、网络引导)，您可以选择打开哪些没有打开的引导设备。

多数 BIOS 版本允许在启动时调出引导菜单选择本次会话中所使用的引导设备。如果有这项功能，BIOS 通常启动时会显示一条类似 “press F12 for boot menu” 的信息。具体的按键会因系统的不同而不同；常用的键有 F12、F11 和 F8。从这个菜单选择的设备不会影响 BIOS 中默认的引导次序，就是说，可以从 U 盘启动一次，而将内置硬盘作为主引导设备。

如果您的 BIOS 不提供引导菜单选择当前的启动设备，您只好从 BIOS 设置中将 debian-installer 设备设为主引导设备。

很不幸，有些计算机的 BIOS 版本有 bug。即使将 BIOS 的主设备改成从 U 盘引导，也无法另 debian-installer 成功启动。有些系统完全不能从 U 盘介质进行引导；其他的引导需要将 U 盘引导默认从 “USB harddisk” 或 “USB stick” 改成 “USB ZIP” 或 “USB CDROM” 才行。 特别是在您在 U 盘上使用 isohybrid CD/DVD 映像(参阅 第 4.3.1 节 “使用混合 CD 或 DVD 映像制作 U 盘”)，将设备类型改成 “USB CDROM” 有助于解决不能从 U 盘硬盘模式下的引导问题。

If you cannot manipulate the BIOS to boot directly from a USB stick you still have the option of using an ISO copied to the stick. Boot debian-installer using 第 4.4 节 “为从硬盘引导准备文件” and, after scanning the hard drives for an installer ISO image, select the USB device and choose an installation image.

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 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 支持与键盘. 假如您没有 PS/2 规格的键盘，而只有 USB 键盘，在一些很老的 PC 上您需要在 BIOS 设置里面打开 legacy keyboard emulation 设置，使得在引导菜单中可以使用键盘。当然新系统中不会遇到这种问题。如果您的键盘无法在引导菜单下工作，请参考主板手册，并在 BIOS 中查看 “Legacy keyboard emulation” 或 “USB keyboard support” 选项。