Operating Systems Architecture

Operating Systems Architecture

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Learning objective: Describe the events of the booting sequence

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Every time you turn on your computer, its as if its the first time your computer has ever been turned on since there are no instructions or data in memory. This allows for multiple boots to allow different operating systems to be loaded and used on the same computer. For example, Macs allow users to run OS X, Windows, and Linux using the Boot Camp boot manager that comes as a OS X utility.

The booting process of DOS begins with the examination of the first sector of a diskette or hard disk is loaded into memory. From the time we switch on the computer to the booting process, a series of the following events occur. [UNESCO]

In Windows, when the pointer goes from the hour glass to the normal pointer, the operating system has been loaded and all the subsystems are synchronized and ready for user requests to process data.


The BIOS of a PC software is built into the PC, and is the first code run by a PC when powered on ('boot firmware'). The primary function of the BIOS is to load and start an operating system. When the PC starts up, the first job for the BIOS is to initialize and identify system devices such as the video display card, keyboard and mouse, hard disk, CD/DVD drive and other hardware. The BIOS then locates software held on a peripheral device (designated as a 'boot device'), such as a hard disk or a CD, and loads and executes that software, giving it control of the PC. This process is known as booting, or booting up, which is short for bootstrapping. [Wikipedia]


Program execution starts after the computer is turned on at memory location F000:FFF0. This is part of the ROM-BIOS and contains a jump command to a BIOS routine which takes over system initialization. The location of this routine may differ from one computer to another, however the task this routine performs remains identical for nearly all PCs. [UNESCO]

System Check

First the BIOS tests individual functions of the processor, its register and some instructions. If an error occurs during this test, the system stops without displaying an error message. If the CPU passes the test, BIOS tests the ROMs by computing checksum. Each chip on the main board goes through tests and initialization. [UNESCO]

Peripheral testing

After determining the function of the main board, the computer tests the peripherals (keyboard, disk drive, etc.). Then the computer initializes the BIOS variables and the interrupt vector table. [UNESCO]

The bootstrap loader

A computer's central processor can only execute program code found in Read-Only Memory (ROM), Random Access Memory (RAM) or an operator's console. Modern operating systems and application program code and data are stored on nonvolatile data storage devices, such as hard disk drives, CD, DVD, flash memory cards (like an SD card), USB flash drive, and floppy disk. When a computer is first powered on, it does not have an operating system in ROM or RAM. The computer must initially execute a 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. [Wikipedia]

✏ Self Quiz!

Select the best response(s) for each question/statement. Use this opportunity to test *your* knowledge and not just move on...

1) Booting...
Tests the computer's processor
Tests the computer's Internet connection
Is controlled by the BIOS

2) The booting process usually involves...
Peripheral testing
Application discovery


Thinking: Why not put the entire OS on a ROM chip?

Key terms: BIOS, booting, bootstrap loader, initialization

To maximize your learning, please visit these Web sites and review their content to help reinforce the concepts presented in this section.

Quick links:
BIOS @ Wikipedia
Booting @ Wikipedia
Comparison of boot loaders @ Wikipedia
Chapter 2 Operating Systems on PC @ UNESCO (www.netnam.vn/unescocourse/os/21.htm)

Embedded Resources

Notes on navigation: Click inside the frame to navigate the embedded Web page. - Click outside the frame to navigate this page to scroll up/down between the embedded Web pages. - Click on the frame title to open that page in a new tab in most browsers. - Click on the the "Reload page" link to reload the original page for that frame.

BIOS @ Wikipedia | Reload page | If frame is empty, click on the link to view the page in a new tab or window

Booting @ Wikipedia | Reload page | If frame is empty, click on the link to view the page in a new tab or window

Comparison of boot loaders @ Wikipedia | Reload page | If frame is empty, click on the link to view the page in a new tab or window

Chapter 2 Operating Systems on PC @ UNESCO (www.netnam.vn/unescocourse/os/21.htm) | Reload page | If frame is empty, click on the link to view the page in a new tab or window

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