Start up

Start up

• Before the UNIX operating system can perform correctly, there are a number of steps that must be followed
• The failure of any one of these can mean that the system will not start. 
• It is important for the Systems Administrator to be aware of what happens during system startup so that any problems that occur can be remedied.

Booting - consists of the following steps

• ROM Phase
• Boot Program Phase
• Kernel initialization Phase
• Init Phase

Boot ROM Phase

• After turn on power to the system, the ROM will display system identification and run self-test diagnostics to verify system hardware

• Then the ROM will loads primary boot program called bootstrap program that store in bootblock of a boot device

Boot Program Phase

• The boot strap program find and execute the secondary boot program from the file system and load it into memory

• After the secondary boot program is loaded, it will load the kernel

Kernel initialization Phase

• The kernel initializes itself (Create process control and other data structures)
• Load device driver modules
• Mounts the necessary file system
• Create the process 0 (swapper)
• Then, bring the system to “initdefault” state specified in /etc/inittab

Swapper

• The first task for the kernel is to start the swapper process.
• The swapper process is the part of the kernel that schedules all other process.
• The swapper has process ID of 0
• It’s first job is to start up the init process

Init Phase

• The kernel start /sbin/init process, which starts other process by reading the /etc/inittab file
• The /sbin/init process start the run control scripts

Run Control Script

• Each init state has a corresponding series of run control scripts 
• The init process executes the /sbin/rc<n> script which execute a series of other scripts located in /etc/rc<n>.d directory
• All run control scripts are also located in the /etc/init.d directory. These file are linked to run control scripts in /etc/rc<n>.d directory