What happens when your PC is powered on?


When your PC is powered on a sequence called the bootstrap is run. It occurs in two steps, Power-On Self Test and Operating System Load. 


Power-On Self Test (POST) is a series of tests to make certain that necessary is present and properly operating. 
  1. First the Central Processing Unit (CPU) is reset. 
  2. The CPU checks itself and the POST program stored in the BIOS memory. 
  3. Then the CPU begins using the code located in BIOS to check components in the PC. 
  4. The DRAM is tested by writing and reading back every byte to insure proper operation. 
  5. The Keyboard is checked for attachment and to see if any key has been pressed. 
  6. Pushing certain keys during boot up tells the CPU you want to modify the BIOS code. You would want to do this if a new piece of hardware has been added and you need to set up BIOS to talk to it. 
  7. The Floppy and Hard Drives are checked to make sure they are present. 
  8. The Mouse is also checked to make sure it is attached. 
  9. Finally, the test results are compared to the configuration data in the BIOS. 
  10. Any mismatches causes the POST to fail and an error message appears on the display.

Operating System Load - After the POST is successfully completed the CPU searches Storage devices for special files that indicate the beginning of the OS and then loads it into memory. 
  1. The sequence in which the storage devices are searched is part of the configuration set-up stored in BIOS. 
  2. Once the Operating System (OS) is located, the CPU copies what’s called the 'boot record' from the OS into DRAM. 
  3. Control is then passed to the 'boot record' in DRAM, which then continues loading the rest of the OS. 
  4. This process continues until the OS load is completed. 
  5. When the OS load is completed the Desktop appears in the display and waits for you, the user, to tell it what you want it to do.