nomount mode
only parameter file is accessible.
this mode is only use to modify parameter file content/paramter.
mount mode
read control file but not validate its file content.
certain maintenance operation requires mount mode. ( RMAN backup restore recovery)
Note:- ******* its mandatory for "recover database" that db should be in mount mode*******
parameter file, processes gets started and memory allocated for the instance.
open mode
start to read data files and redo logs
