Figure 1 shows the boot up process.  First the router performs a POST and loads the bootstrap.  Both are found in ROM.  Secondly the router loads the IOS found in flash or by TFTP. Finally the router loads the startup configuration, found in NVRAM or by TFTP or console. Figure 2 highlights the POST section. Figure 3 highlights the loading of the IOS section. Figure 4 highlights the loading of the startup configuration section.