The Cisco IOS operational details vary on different internetworking devices, depending on the device’s purpose and feature set. However, Cisco IOS for routers provides the following:

The IOS file itself is several megabytes in size and similar to Cisco IOS switches, is stored in flash memory. Using flash allows the IOS to be upgraded to newer versions or to have new features added. During bootup, the IOS is copied from flash memory into RAM. DRAM is much faster than flash; therefore, copying the IOS into RAM increases the performance of the device.