The Power-On Self Test (POST) is a process which occurs on almost every computer during bootup. This process is used to test the router hardware. When the router is powered on, software on the ROM chip performs tests on several hardware components, such as CPU, RAM and NVRAM. After the POST is completed, the router executes the bootstrap program.
After the POST has passed all the tests, the router copies the bootstrap program from ROM into RAM. After the program got into RAM, the CPU executes the instructions in the bootstrap. The main task of the bootstrap is to locate the Cisco IOS and load it into RAM. If you are connected with a console cable, at this point you will see text like the following on the console, depending on your platform:
System Bootstrap, Version 12.3(8r)T8, RELEASE SOFTWARE (fcl)
Cisco 1841 (revision 5.0) with 114688K/16384K bytes of memory.
Generally, the IOS is stored in the flash memory, but it can be also stored in other places, such as a TFTP server. At the third step, the router loads the IOS image into RAM for execution by the CPU. If the IOS image can’t be located, a minimal version of the IOS is loaded from ROM into RAM. This version can be used to diagnose problems and load a complete version of IOS. When the IOS begins to load, you may see a string of pound signs (#), while the image decompresses.