What is need to boot a computer?

Answer 1

Applying power to a computer's circuitry, normally by depressing the "On" switch, gives the microprocessor the voltage it, and its associated clock circuits, need to function. After any internal self-initialization the processor might do, it loads a fixed address onto its address bus, determined by the manufacturer, and starts reading instructions from memory at that address, incrementing its instruction counter as it goes. In small microcontroller circuits, that may be the end of the story; it just keeps reading and executing instructions from ROM (Read-Only Memory) until the program ends (if ever), or some unexpected interrupt or exception occurs which halts it.

In the case of modern desktop computers, however, typically the BIOS (Basic Input/Output System) is the ROM which is addressed at initialization, and this can consist of many megabytes of initialization and test code until some means of long-term storage (disk, USB flash device, CD- or DVD-ROM) is loaded. This will typically have several stages of loaders, where the first 512-byte sector has code which knows where to fetch a secondary loader; eventually a loader is capable of reading the filesystem of the disk well enough to locate the operating system itself.

The operating system then typically has its own test and initialization routines to go through, and while some OSes such as Windows may by default hide all this behind a "splash screen", others like some Linux distributions will display screen after screen of quickly-scrolling information letting you know exactly what's going on. Once it has mapped all the computer's circuitry: memory (RAM), disks, sound cards, and other hardware to its internal representations of these devices, it typically displays a login prompt. The computer is now said to be "booted": lifted up by its own bootstraps.

In the case of a "turnkey" or "kiosk" system, the login may be bypassed, and the system might not be considered "booted" until it starts executing the process for which it was designed. This would be the case at info-kiosks at airports, or point-of-sale (POS) machines (modern cash registers) in stores.

Answer 2

In computing, booting (also known as "booting up") is a bootstrapping process that starts operating systems when the user turns on a computer system. A boot sequence is the initial set of operations that the computer performs when power is switched on. The boot loader typically loads the main operating system for the computer.

Boot loader

A computer's central processor can only execute program code found in read-only memory (ROM), random access memory (RAM) or in the 1950s from an operator's console. Modern operating systems and application program code and data are stored on nonvolatile data storage devices, such as hard drives, CD, DVD, flash memory cards (like an SD card), USB flash drive, and floppy disk. When a computer is first powered on, it usually does not have an operating system in ROM or RAM. The computer must execute a relatively small program stored in ROM along with the bare minimum of data needed to access the nonvolatile devices from which the operating system programs and data are loaded into RAM.

The small program that starts this sequence of loading into RAM is known as a bootstrap loader, bootstrap or boot loader. This small boot loader program's only job is to load other data and programs which are then executed from RAM. Often, multiple-stage boot loaders are used, during which several programs of increasing complexity sequentially load one after the other in a process of chain loading.

Early computers (such as the IBM 650, DEC PDP-5 through PDP-8 and early models of the PDP-11) had a row of dials or toggle switches on the front panel to allow the operator to manually enter the boot instructions into memory before transferring control to the CPU, or to directly pass an instruction to the CPU. (Interestingly, the PDP-1 had a hardware built-in loader, such that an operator only needed to push the "load" switch and the paper tape reader would read a program directly into core memory.) The boot loader would then read in either the second-stage boot loader (called Binary Loader of paper tape with checksum), or the operating system from an outside storage medium such as paper tape, punched card, or a disk drive.

Pseudo-assembly code for the bootloader might be as simple as the following eight instructions:

Set the P register to 9

Check paper tape reader ready

If not ready, jump to 2

Read a byte from paper tape reader to accumulator

If end of tape, jump to 9

Store accumulator to address in P register

Increment the P register

Jump to 2

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.

Set the P register to 106

Check paper tape reader ready

If not ready, jump to 2

Read a byte from paper tape reader to accumulator

Store accumulator to address in P register

Decrement the P register

Jump to 2

The length of the second stage loader is such that the final byte overwrites location 7. After the instruction in location 6 executes, location 7 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.

Some computer systems, upon receiving a boot signal from a human operator or a peripheral device, may load a very small number of fixed instructions into memory at a specific location, initialize at least one CPU, and then point the CPU to the instructions and start their execution. These instructions typically start an input operation from some peripheral device (which may be switch-selectable by the operator). Other systems may send hardware commands directly to peripheral devices or I/O controllers that cause an extremely simple input operation (such as "read sector zero of the system device into memory starting at location 1000") to be carried out, effectively loading a small number of bootload instructions into memory; a completion signal from the I/O device may then be used to start execution of the instructions by the CPU.

Smaller computers often use less flexible but more automatic bootload mechanisms to ensure that the computer starts quickly and with a predetermined software configuration. In many desktop computers, for example, the bootstrapping process begins with the CPU executing software contained in ROM (for example, the BIOS of an IBM PC) at a predefined address (some CPUs, including the Intel x86 series are designed to execute this software after reset without outside help). This software contains rudimentary functionality to search for devices eligible to participate in booting, and load a small program from a special section (most commonly the boot sector) 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 located on hard drives must fit into the first 446 bytes (or 440 bytes if Windows NT or above has to be supported because NT put 6 byte disk-signature starting from offset 440) of the Master Boot Record, in order to leave room for the 64-byte partition table and the 2-byte 0xAA55 'signature', which the BIOS requires for a proper boot loader.

Some operating systems, most notably pre-1995 Macintosh systems from Apple, are so closely interwoven with their hardware that it is impossible to natively boot an operating system other than the standard one. This is the opposite extreme of the bootload using switches mentioned above; it is highly inflexible but relatively error-proof and foolproof as long as all hardware is working normally. 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.

The Atari ST also had its operating system in ROM, but by inserting the Spectre GCR cartridge with the Macintosh system ROM in the game slot and turning the Atari on, it could "natively boot" the Macintosh operating system rather than Atari's own TOS system. The Atari ST hardware was designed so the cartridge slot could provide native program execution for gaming purposes as a holdover from Atari's legacy making electronic games.