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Archive for the ‘general’ Category

How Your Computer Boot?

Posted by aboelnour on July 30, 2009


Peace be upon you:

How are you? in this article we will talk about the BOOT operation on your computer so read and enjoy 🙂


History :

* The Computer word boot is short for “bootstrap” (short for ‘bootstrap load”)

* The term bootstrap began as a metaphor derived from pull straps sewn onto the backs of leather boots with which a person could
pull on their boots without outside help

* In 1950’s when you press a bootstrap button caused a hardwired program to read a bootstrap program from apunched card
and then execute the loaded boot program which load ed a larger system of programs from a punched cards into memory ,
without further help from the human operator
so this word has been used at least 1958

Introduction :

* A computer’s central processor can only execute program code found in Read-Only Memory (ROM) and Random Access Memory (RAM).
Modern operating systems and application program code and data are stored on nonvolatile data storage devices, such as hard disk drives,
CD, DVD, 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
evices 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.

Second-stage boot loader :

* The small program which loaded is most often not itself an operating system, but only a second-stage boot loader, such as GRUB, BOOTMGR, LILO or NTLDR.
It will then be able to load the operating system properly, 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.

* Many bootloaders (like GRUB, BOOTMGR, LILO, and NTLDR) can be configured to give the user multiple booting choices. These choices can include
different operating systems (for dual or multi-booting from different partitions or drives), different kernel versions of the same operating system
(in case a new version has unexpected problems), different kernel options (e.g., booting into a rescue or safe mode) or some standalone program that can
function without an operating system, such as memory testers (e.g., memtest86+) or even games.[9] Usually a default choice is preselected with a time delay
during which you can press a key to change the choice, after which the default choice is automatically run, so normal booting can occur without interaction.

* 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 one minute (of which about 15 seconds are taken by a power-on self test (POST) and a preliminary boot loader
, and the rest by loading the operating system, pre-OS time can be considerably shortened by bringing the system with all cores at once, as with coreboot[10]
in as little as 3 seconds[11]; whereas, large servers may take several minutes to boot and start all their services.

Boot device :

* The boot device is the device from which the operating system is loaded. A modern PC BIOS supports booting from various devices,
typically a local hard disk drive (or one of several partitions on such a disk), an optical disc drive, a USB device
(flash drive, hard disk drive, optical disc drive, etc.), or a network interface card (using PXE). Older, less common bootable devices include
floppy disk drives, SCSI devices, Zip drives, and LS-120 drives.

* Typically, the BIOS will allow the user to configure a boot order. If the boot order is set to “firstly, the DVD drive; secondly, the hard disk drive”,
then the BIOS will try to boot from the DVD drive, and if this fails (e.g. because there is no DVD in the drive), it will try to boot from the local hard drive.

Booting Sequence  :

* Upon starting CPU runs the instruction located at the memory location  of the BIOS It typically contains a jump instruction that transfers
execution to the location of the BIOS start-up program. This program runs a power-on self test (POST) to check and initialize required devices.
The BIOS goes through a pre-configured list of non-volatile storage devices (“boot device sequence”) until it finds one that is bootable.
A bootable device is defined as one that can be read from, and the last two bytes of the first sector contain the word 0xAA55 (also known as the boot signature).

* Once the BIOS has found a bootable device it loads the boot sector and transfers execution to the boot code.
In the case of a hard disk, this is referred to as the master boot record (MBR) and is often not operating system specific The conventional MBR code
checks the MBR’s partition table for a partition set as bootable (the one with active flag set)[12]. If an active partition is found, the MBR code loads
the boot sector code from that partition and executes it. The boot sector is often operating system specific, however in most operating systems its main
function is to load and execute the operating system 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 which will select a partition (often via user input) and load its boot sector, which usually loads the corresponding operating system kernel.

Rebooting  :
Hard reboot :

A hard reboot is when power to a computer is cycled (turned off and then on) or a special reset signal to the processor is triggered. This restarts the
computer without first performing any shut-down procedure. (With many operating systems, especially those using disk caches, after a hard reboot the
filesystem may be in an “unclean” state, and an automatic scan of on-disk filesystem structures will be done before normal operation can begin.)

Soft reboot :

A soft reboot is restarting a computer under software control, without removing power or (directly) triggering a reset line. It usually,
though not always, refers to an orderly shutdown and restarting of the machine.
This kind of reboot will not usually reset the hard disks, so that they have time to update their write cache to permanent storage.
Hard disks will also keep their configuration (like C/H/S adjustments, HPA, DCO, internal passwords…) over these reboots.


For the old days take this one 😀


so that’s the end for now any feed back are welcomed

best wishes,



Posted in general | Leave a Comment »

Hello world!

Posted by aboelnour on June 12, 2009

Peace be upon you
this my new blog  it will be for computer Science which most of them will be written by me isa so i hope you enjoy this blog and any comment or feedback please make me know about it

Posted in general | 1 Comment »

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