Router Memory

RAM: The RAM stores the OS, running config file, IP routing table, Arp cache and packet buffer

ROM: Permanent storage – Scaled down version of IOS, Basic software

Flash Memory: Permanent storage of for OS, Cisco IOS

NVRAM: Used as permanent storage for startup config files.

3 Configuration Registry

0 * 2142 For password recovery.
0 * 2102 The default. Router looks for config file in NVRAM and for a valid IOS image.
0 * 2100 Router boots into ROM monitor mode.

If start up config is not found in the RAM memory what does it do ?

It searches the NVRAM where all the it is permanently stored!