A bootable or startable medium (short start medium; boot from English to boot = [to start a computer]) is a storage medium that has both the necessary content and the necessary data structure for one or more computer platforms in order to start the computer using the software located on the medium .
Selection of the start medium
Whether or not a computer can actually start from a certain storage medium depends first and foremost on whether the medium or its reader (e.g. drive ) can also be connected to the computer. Furthermore, the integrated firmware of the computer must support the corresponding reader (drive) and its connection (e.g. via USB ) as a start medium.
In the case of IBM PCs with BIOS as firmware (the most widely used firmware up to approx. 2010, replaced by UEFI ), for example, it is usually possible to configure in the BIOS setup which media are tried in which order, called the boot order. On Apple Macintosh systems, the boot medium to be used can be set via the firmware or selected once by holding the key (the Option key on Apple keyboards corresponds to the Alt key on PC keyboards) after switching on the computer. Other systems often offer similar options or are " locked " by design , e.g. B. mobile operating systems such as Android or iOS on smartphones . ⌥ Alt
Another requirement for a start medium is the correct content: Depending on the computer system, e.g. B. a boot loader is required, which must be available in a certain format and is expected at a predefined location. If the firmware finds such a bootloader , it loads it and then passes control by executing it. The primary task of a boot loader is to start an operating system. Thus, since a software the next software loads and runs, this process is also called chain loading signified the firmware loads the boot loader, which in turn another boot loader loads from a starting medium, in turn, the loader loads an operating system, the other parts ( like the kernel ) of an operating system.
It is also possible to accommodate more than one operating system installed on a single physical storage medium: at this as a multi-booting designated install one of the must chain loading existing Loader a choice offer to the desired by the user after turning on the computer operating system to be able to start. For systems that rely on a boot loader (especially computers with BIOS ), additional configuration or the installation of a boot manager is necessary in order to be able to select the desired operating system for starting. On systems with different firmware , the operating system to be started can usually be selected directly via the firmware ( Open Firmware , UEFI ).
Examples of startup media
In contrast to firmware, which is not exchangeable or which is more or less firmly connected to the computer system, a start medium is usually relatively easy to change or replace. Such a medium can e.g. B. be:
- CD-ROM and other optical storage media - application examples are the installation CD or the live CD
- Floppy disk - on some systems a boot loader is required
- EEPROM or Flash EEPROM - mostly in embedded systems
- Hard disk drive , usually on a partition provided for this purpose - on some systems a boot loader is required for this
- Network card - not a storage medium per se, but a start process can also be initialized via the Bootstrap protocol and / or Preboot Execution Environment (PXE) ("network boot ")
- USB mass storage devices - e.g. B. USB sticks
- Memory card such as B. (Micro) SD card , see for example the single-board computer Raspberry Pi
For repairs, but also for other purposes, media are used that are usually more or less freely available as a memory image (e.g. as an ISO image ) . Also applications, particularly in the backup - and computer security application , offer bootable media, which can create some later ( English boot medium Builder ), but partly with the installation are the same CD.
→ See also: Possible uses of live systems