Startup Oracle Data base

Startup involve three steps:
Instance Started:

Read the parameter file.
Memory structure allocated based on parameter file
Background process will be started

Example:
Total system Global area 503316480 bytes
Fixed Size 1259712 bytes
Variable Size 184551232 bytes
Database Buffers 314572800 bytes
Redo Buffers 2932736 bytes

Mount the Data Base:

Read the control file and Locate datafiles and redo logfiles

Open the Data Base:

Open the datafiles and redo log files

Why we are using Mount database command? When DB is opened with Nomount?

SQL> Alter database mount;

Rename the data files
Enable / Disable Archive redo log files
Full Database recovery