Boot menu

from Wikipedia, the free encyclopedia

A boot menu is the starting ( English boot ) of a computer system shown menu in which a user between multiple boot options to select and possibly other functions can. Displaying this menu is often either optional, as it only appears if the user presses or holds down a certain key after switching on or restarting the computer, or one of the options is automatically selected after an adjustable time-out if the User not responding.

species

A boot menu can be implemented on different software levels. It is possible u. a. a selection menu at firmware level, as an independent boot manager or as part of an operating system . In addition to the firmware, a boot menu is implemented as a boot loader .

Firmware

Selection menu according to the BIOS Boot Specification (BBS).
Selection menu of rEFIt on a MacBook with EFI.

The very first software a computer runs after it is turned on is firmware . In modern PCs, this has mostly been a UEFI since 2010 . Depending on the manufacturer, a boot menu can be displayed by pressing one of the function keys . Since UEFI has replaced the BIOS on IBM PC-compatible computers , it can also start there in a BIOS-compatible mode. In this case, loads the UEFI a BIOS emulator, the CSM ( English Compatibility Support Module ) is called. Both a BIOS and a UEFI-CSM search for bootable media on found data storage media and then display them as a selection in the boot menu. Although the BIOS basically only has to have a boot sector with the correct signature, more modern specifications require a more detailed check of the medium. With the El Torito standard, for example, optical data media such as B. CD-ROMs can be started either from a hard disk or from a floppy disk image. In order to display the boot menu correctly, the BIOS must first check the data carrier for the method used. Depending on the media available, there may be different delays before the boot menu is displayed. Special recovery partition ( English recovery partition ) to be examined by the BIOS of individual producers and z Press. B. started directly with a given function key. Some BIOS implementations also check the MBR for correctness, such as the presence of an active partition . In the BIOS, the boot menu is also called BBS, from the English BIOS Boot Specification . The BBS menu is not displayed automatically, but after pressing a specific key during BIOS POST . Because there is no standard, which key to press varies depending on the BIOS, but usually one of the following keys: , , , , , or , on tablets (without hardware keyboard) usually one of the volume buttons or . EscEntfF1F2F10F11F12Volume upVolume down

With UEFI, if a CSM is available and activated, a BIOS-compatible check of possible boot media is also available. EFI can also start from an MBR-partitioned data carrier, but in principle uses the GUID partition table as the designated MBR successor. The firmware then searches for UEFI-compatible bootloaders that must be on an EFI system partition (ESP for short) formatted with the FAT file system. The processor architecture must match that of the EFI, which is why older 32-bit (U) EFI implementations cannot start 64-bit UEFI bootloaders and vice versa. On the other hand, on permanently installed data storage devices such as hard disks and SSDs, a boot option entered in NVRAM is the norm and provided by UEFI specification. Operating systems are registered during installation, which enables a firmware-controlled multi-boot system. By Hybrid MBR both a BIOS-based and UEFI-based operating system on the same disk can, with limitations, to be started; since the CSM was only intended as a transitional module, it will no longer be present in modern UEFI implementations.

The Open Firmware used by Apple until 2006 on PowerPC-based Macintosh computers also has a boot menu that appears when you hold down Wahltaste ⌥or Alton the PC keyboards. To do this, the Open Firmware searches all supported media for a boot loader, which on Apple computers must be on an HFS file system partitioned with the partition table type APM and must be marked as the start program. Only one launcher per partition is supported. Under Mac OS X , this is BootX: . However, the program code can also be available in the platform-independent Forth . /System/Library/CoreServices/BootX

On Intel-Macs, which inherited the Power-Macs from 2006 onwards, you still have Wahltaste ⌥to keep the button pressed to display the boot menu. Apple uses an adapted version of EFI which, in addition to the UEFI standard, can also start directly from Apple's own partitions. The boot menu therefore also shows all Apple EFI boot loaders ( /System/Library/CoreServices/boot.efifrom macOS ) found on supported file systems (originally HFS + ) without a boot loader installed on the EFI system partition and without any entry in the NVRAM.

What all firmware variants have in common is that they usually also recognize network boot options. The network search required for this usually creates a delay of a few seconds before the boot menu can be displayed or used. With BIOS and UEFI, the network boot option usually has to be activated in the BIOS setup.

support

Since firmware usually implements limited support for external, exchangeable storage media, it is often not possible to start from all possible connected devices. Although a bootable medium is available, it does not appear in the boot menu. Some examples are:

  • On Power Macs, media connected via USB do not appear in the boot menu as the Open Firmware does not search for bootloaders on USB data storage media. However, it is possible, via a detour to the Open Firmware Console, to start directly from a USB storage medium such as a CD-ROM in a USB CD drive or from a hard drive connected via USB by entering the access path.
  • Some (mostly older) BIOS implementations cannot start from USB sticks. You can recognize the data storage device by the USB connection, but you cannot start from it.
  • With some older UEFI implementations, it was not possible to start NVMe SSDs because the firmware did not support the standard and could therefore not access NVMe data storage.

Bootloader

If the firmware transfers to a recognized start program, this in turn can provide a boot menu. This is usually the case with modern operating systems, which implement extended functions and different start modes. There are also numerous boot managers that, on the one hand, allow many different operating systems to be started regardless of the operating system, and on the other hand offer additional functions. Most boot managers focus on multi-boot systems , but some can also change or expand the functionality of the firmware before starting the operating system.

Operating system's own boot manager

Windows Boot Manager selection menu ( Bootmgr ).
A GRUB2 boot menu.
Windows advanced startup options.

In order for an operating system to start at all, it must have a suitable boot loader, which is therefore installed on the target medium during the operating system installation and configured appropriately. In addition, the installation media often offer a repair program that can correct a faulty boot configuration or reinstall a new boot loader including the configuration for a recognized operating system on a storage medium specified by the user. The simplest form of a boot loader is a start program without a selection menu and without the option of selecting options for the operating system to be started. Most boot sectors are designed in such a way that they start the operating system directly without a selection menu. Boot managers, on the other hand, usually offer the option of influencing the start of the operating system. With GRUB, for example, the kernel command line can be edited before starting Linux and other Unix-like operating systems, and with Windows (from Windows 9x to Windows 7 ), a selection menu with extended start options can be displayed by pressing the function F8 key when starting . a. allows you to boot into Safe Mode .

Examples of simple boot sectors without a boot menu:

Examples of boot managers of operating systems with (optional) boot menu:

A submarine selection menu.

Boot sectors

Some utility programs offer either boot sectors or small start programs that also offer a rudimentary selection of the medium to be started. For example, there are formatting programs such. B. FDFORMAT or VGA-COPY / 386, which write a boot sector with boot menu on floppy disks when formatting. The selection is limited to a few standards that can be addressed via BIOS routines. A corresponding boot sector can start from the first hard disk, for example, if there are no system files on the floppy disk.

Third party boot managers

Especially under DOS and Windows there were boot managers from various manufacturers. These are installed in the MBR on IBM PC-compatible computers with BIOS and support a large number of operating systems for multi-boot systems . Some also offer additional functions, such as modifying partition data (such as hiding or marking as active), or even upgrading the LBA-48 access mode if the BIOS does not support this.

An incomplete list of boot managers for IBM PC-compatible computers:

  • BootIt, proprietary ; also available in UEFI version
  • Bootstar, proprietary
  • Boot-US, proprietary
  • Data Becker BootManager, proprietary
  • Ontrack Disk Manager, proprietary ( abandonware )
  • OSL2000 Boot Manager, proprietary
  • Paragon BootManager, proprietary
  • Plop boot manager, freeware
  • XFDisk, the boot manager contained in eXtended FDisk , is free
  • XOSL (for English Extended Operating System Loader )
  • egoot manager, shareware

An incomplete list of boot managers for (U) EFI systems:

  • Clover EFI bootloader, free; can also provide a UEFI on PCs with BIOS and, conversely, start operating systems for BIOS-based PCs on UEFI systems
  • rEFIT, free; Boot menu for Intel Macs with Apple EFI; Development stopped
  • rEFInd, free; Successor to rEFIT, can also be used on PCs with UEFI

Localization

The most widely used language for firmware is English and, as the storage space is limited, there is usually no localization in other languages. However, some more modern firmware implementations, such as UEFI, offer a few additional languages ​​to choose from, which can be set in the firmware setup (also known as “BIOS setup”).

The boot loader and boot manager are partly in the language of the installed operating system - but they are often also in English, although the operating system is e.g. B. was installed in German.

See also

Individual evidence

  1. ↑ Boot to UEFI mode or legacy BIOS mode. In: Hardware Dev Center. Microsoft, May 14, 2018, accessed on February 1, 2019 (English, section "To boot to UEFI or BIOS: 1. Open the firmware menus."): "Boot the PC, and press the manufacturer's key to open the menus." Common keys used: Esc, Delete, F1, F2, F10, F11, or F12. On tablets, common buttons are Volume up or Volume down ... "