boot process has five distinct phases

1-The boot PROM phase 
2-The boot programs phase 
3-The kernel initialization phase 
4-The init phase 
5-The svc.startd phase

it is main point of Router 
(1) ROM is Chip 
where two types POST and Bootstrap
when router goes up the first of all Rom works Power on 
self test and recoll the IOS via Flash memory and Flash 
Memory is where IOS are installed  
(2)LOad the configuration files form NVRAM