LINUX:In 1969 AT&T made a decision to withdraw Multics and go with GECOS (General Electric Comprehensive Operating Supervisor / System), with AT & T in Bells Lab when Multics was withdrawn some of the programmers named Ken Thompson and Dennis Ritchie decided to rewrite operating system in order to support low cost computer..To Know More



    MICROSOFT:Microsoft was established to develop and sell BASIC interpreters for the Altair 8800. It rose to dominate the home computer operating system market with MS-DOS in the mid-1980s,followed by the Microsoft Windows line of operating systems. To Know More



    CISCO:During the early 1980s, there was a married couple namely Len and Sandy Bosack who used to work in two different departments of computer located in Stanford University. This couple was facing problem in making their computers communicate with each other To Know More



Booting Process

Booting Process

1) POST (Power On Self Test):- BIOS is group of program and it is written by system programmer to do the POST so it checks the hardware and put the list on memory chip.

2) CMOS (Complimentary Metal Oxide Semiconductor) :- CMOS comes with motherboard and has program in it, we can make changes in. CMOS battery powers the CMOS data which is inside it and to access CMOS data we have to press DEL key or other appropriate key for it.

BIOS reads CMOS and the CMOS in the RAM :-
CMOS:- Why you called me?
BIOS:-To load OS.
CMOS:- Who asked?
CMOS:- Which OS?
BIOS:- Any (then BIOs checks Boot Priority)
CMOS:-  Ok go to the first boot device.
In the first boot device it looks for MBR, it is located at CHS(Cylinder-0, Head-0, Sector-1)

MBR(Master Boot Recorder), MBR is divided into three part:-
1. Boot Partition (446 Bytes)
2. Partition Table (64 Bytes)
3. Magic No. (2 Bytes)

In the system BIOS check the  system  and launches first stage boot loader on the MBR of primary hard disk. The first stage boot loader load itself into memory and launches the second boot stage boot loader from first boot partition.
The first stage boot loader it’s a small machine code on the MBR and its job is to locate the second boot stage loader and load first part into memory once the second boot stage loader is in the memory its resent the user with graphical screen showing different OS and kernel, it has been configured to boot the graphical screen comes because of /etc/grub.conf script which is a link file and actual stored in /boot/grub/grub.conf.

Over here user can use arrow keys to choose which OS or kernel they which to boot and press enter, if  no keys are pressed the boot loader loads the default selected OS after the configuration period has passed.
Now once the second stage boot loader has determine which kernel to boot it locates the corresponding kernel binary ( /boot directory) the kernel binary named used the following format which is located in /boot directory

After selecting OS the kernel is loaded and CPU immediately initialized and configure computer, memory and various hardware attached to the system including all process, input & output system and a storage devices. Then it look for a compressed init.ramfs image in predetermined location in memory. It compresses it directly to /sysroot then it loads all necessary drivers.
Now the init program starts and coordinate the rest of booting process and configure the environment for user. Init is father of all other process and its PID is 1.

When init program start it become parental and grant parent for all other process and all the processes start automatically on that system. Then it first run /etc/rd.d.rc.sysinit script which states the environment path starts the swap memory checks the file system execute all other steps required for system initialization. Then the init command runs /etc/inittab script which describes how the system should be set up in init each runlevel, run levels are more defined by services list.
0 – Halt
1 – single user mode
2 - Multiuser without NFS
3 – Full multiuser
4 - Unused
5 – X11
6 – Rebbot

The init command sets the source function library /etc/init.d/functions script. First system to start, kill and determine the PID of program. The init start all the   processes by looking appropriate rc directory from the runlevels specified as default in file /etc/inittab.
For example when booting to runlevel 5 the init looks in /etc/rc.d/rc5.d script to determine which process to start and stop.
All the files in the /etc/rc.d/rc5.d are symbolic link tp script located in /etc/rc.d/init.d. the name of each symbolic link begain witj k or s where k links are process that kill on runlevel while those beginning with s are started.
After the init command has processed to rc directory for runlevel in /etc/inittab files it directly point to /sbin/mingettys process for each virtual console.
The /sbin/mingettys process opens communication path way to tty devices. Set there mode print a login prompt except the username and password and initialize the login screen.


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