Jun 27, 2018

How do you run OS? Explain: Bootstrap and Booting process

Image: How do you run OS? Explain: Bootstrap and Booting process
An OS is described as “the first program,” How does this “first” program get to run?

The OS is loaded through a bootstrapping process, as booting. A bootloader is a program whose task is to load a bigger program, such as the OS. When you turn on a computer, its memory is usually uninitialized. Hence, there is nothing to run. 

Earlier Versions of Bootloader

    Early computers would have hardware that would enable the operator to press a button to load a sequence of bytes from punched cards, punched paper tape, or a tape drive. In some cases, the boot loader software would be hard-wired as non-volatile memory.

Later Versions of Bootloader

    In later systems, read-only memory would contain a small bootloader that would have the basic intelligence to read, say, the first sector (512 bytes) of a disk. Since this initial program had to be as small as possible, it would have minimal capabilities. What often happened is that the bootloader would load another boot loader, called a second stage loader, which was more sophisticated.
    This second stage loader would have error checking, among possibly other features, such as giving the user a choice of OSs to boot, the ability to load diagnostic software, or enabling diagnostic modes in the OS. This multistage boot loader, having a bootloader load a bigger bootloader, is called chain loading.

Main tasks of a bootloader

The bootloader will often perform some core initialization of the system hardware and will then load the OS. Once the OS is loaded, the bootloader transfers control to it and is no longer needed. The OS will initialize itself, configure the system hardware (e.g., set up memory management, set timers, set interrupts), and load device drivers, if needed.


BIOS(Basic Input/Output System, which comprises the bootloader firmware) contains low-level functions for accessing some basic system devices, such as performing disk I/O, reading from the keyboard, and accessing the video display. It also contains code to load a stage 1 boot loader.
This article is simplified version of the Original one

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