Encyclopedia
- This article is about booting, in the sense of starting a computer. For other meanings, see Booting .
In computing,
booting is a bootstrapping process that starts
operating systems when the user turns on a computer system. A
boot sequence is the set of operations the computer performs when it is switched on that load an operating system.
Boot loader
Most typwriter systems can only execute code found in the memory . Modern operating systems are stored on
hard disks, or occasionally on
LiveCDs,
USB flash drives, or other non-volatile storage devices. When a computer is first powered on, it doesn't have an operating system in memory. The computer's
hardware alone cannot perform complex actions such as loading a program from disk, so a
paradox exists: to load the operating system into memory, one appears to need to have an operating system already loaded.
The solution to the paradox is to use a special small program, called a
bootstrap loader or
boot loader. This program's only job is to load other software for the operating system to start. Often, multiple-stage boot loaders are used, in which several small programs summon each other, until the last of them loads the operating system. The name
bootstrap loader comes from the image of one pulling oneself up by one's bootstraps .
Early programmable computers had a row of
toggle switches on the front panel to allow the operator to manually enter the
binary boot instructions into memory before transferring control to the
CPU. The boot loader would then read the operating system in from an outside storage medium such as
paper tape,
punched card, or an old fixed head disk drive.
Pseudo-assembly code for the bootloader might be as simple as the following eight instructions:
0: set the P register to 8
1: check paper tape reader ready
2: if not ready, jump to 1
3: read a byte from paper tape reader to accumulator
4: if end of tape, jump to 8
5: store accumulator to address in P register
6: increment the P register
7: jump to 1
A related example is based on a loader for a 1970's Nicolet Instrument Corporation minicomputer. Note that the bytes of the second-stage loader are read from paper tape in reverse order.
0: set the P register to 106
1: check paper tape reader ready
2: if not ready, jump to 1
3: read a byte from paper tape reader to accumulator
4: store accumulator to address in P register
5: decrement the P register
6: jump to 1
The length of the second stage loader is such that the final byte overwrites location 6. After the instruction in location 5 executes, location 6 starts the second stage loader executing. The second stage loader then waits for the much longer tape containing the operating system to be placed in the tape reader. The difference between the boot loader and second stage loader is the addition of checking code to trap paper tape read errors, a frequent occurrence with the hardware of the time, which in this case was an
ASR-33 teletype.
In modern computers the bootstrapping process begins with the
CPU executing software contained in ROM at a predefined address . This software contains rudimentary functionality to search for devices eligible to participate in booting, and load a small program from a special section of the most promising device.
Boot loaders may face peculiar constraints, especially in size; for instance, on the IBM PC and compatibles, the first stage of boot loaders must fit into the first 446 bytes of the Master Boot Record, in order to leave room for the 64-byte partition table and the 2-byte AA55h 'signature', which the BIOS requires for a proper boot loader.
Some operating systems, most notably pre-1995
Macintosh systems from
Apple Computer, are so closely interwoven with their hardware that it is impossible to natively boot an operating system other than the standard one. A common solution in such situations is to design a bootloader that works as a program belonging to the standard OS that hijacks the system and loads the alternative OS. This technique was used by Apple for its
A/UX Unix implementation and copied by various freeware operating systems and
BeOS Personal Edition 5.
Second-stage boot loader
The small program is most often not itself an operating system, but only a second-stage boot loader, such as
NTLDR,
LILO or
GRUB. It will then be able to load the operating system proper, and finally transfer execution to it. The system will initialize itself, and may load
device drivers and other programs that are needed for the normal operation of the OS.
The boot process is considered complete when the computer is ready to interact with the user or the
operating system is capable of running ordinary applications. Typical modern
PCs boot in about a minute , while large servers may take several minutes to boot and to start all services. To ensure high availability, they bring up some services before others.
Most
embedded systems must boot immediately. For example, waiting a minute for a digital television to come up is not acceptable. Therefore they have their complete operating system in ROM or
flash memory, so it can be executed directly.
BIOS boot devices
A boot device is any device that must be initialized prior to loading the operating system. This includes the primary input device , the primary output device , and the initial program load device .
In a modern
BIOS, the user can select one of several interfaces from which to boot. These include:
hard disk, floppy,
SCSI,
CDROM,
Zip, LS-120, a network interface card using PXE, or
USB .
For example, one can install
Microsoft Windows on the first hard disk and
Linux on the second. By changing the BIOS boot device, the user can select the
operating system to load.
Boot sequence on standard PC
Upon starting, a
personal computer's x86
CPU runs the instruction located at the memory location F000:FF00 of the
BIOS. This memory location is close to the end of system memory. It contains a jump instruction that transfers execution to the location of the BIOS start-up program. This program runs a Power-on self test to check that devices the computer will rely on are functioning; it also initializes these devices. Then, the BIOS goes through a preconfigured list of devices until it finds one that is bootable. On the original IBM PC, if it finds no such device, control is transferred to IBM Cassette BASIC. On more modern machines with no built-in operating system, an error message is generated and the boot process stops. If the BIOS finds a bootable device, it loads and executes its boot sector. In the case of a hard drive, this is referred to as the master boot record and is often not
operating system specific. Usually, the MBR code checks the partition table for an active partition. If one is found, the MBR code loads that partition's boot sector and executes it. The boot sector is often
operating system specific, however in most operating systems its main function is to load and execute a kernel, which continues startup. If there is no active partition or the active partition's boot sector is invalid, the MBR may load a secondary boot loader and pass control to it and this secondary boot loader will select a partition and load its boot sector, which usually loads the corresponding
operating system Kernel.
Other kinds of boot sequence
Some other processors have other kinds of boot modes; most
digital signal processors have the following boot modes:
- Serial mode boot
- Parallel mode boot
- HPI boot
- Warm boot or soft reboot refers to an abridged start up which does not require that power be removed and reapplied.
Random reboot
Random reboot is a non-technical term referring to an unintended reboot for which the cause is not immediately evident to the user. Such reboots may occur due to a multitude of software and / or hardware problems.
See also
Further reading
-
- at OSDEV Community
-
- FreeBSD
- Linux
- Mac OS X
- Windows
