When your PC is powered on asequence called the bootstrap is run. It occurs in two steps, Power-On SelfTest and Operating System Load.
Power-On Self Test (POST) is a series of tests to make certain that necessary is present andproperly operating.
- First the Central Processing Unit (CPU) is reset.
- The CPU checks itself and the POST program stored in the BIOS memory.
- Then the CPU begins using the code located in BIOS to check components in the PC.
- The DRAM is tested by writing and reading back every byte to insure proper operation.
- The Keyboard is checked for attachment and to see if any key has been pressed.
- 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.
- The Floppy and Hard Drives are checked to make sure they are present.
- The Mouse is also checked to make sure it is attached.
- Finally, the test results are compared to the configuration data in the BIOS.
- Any mismatches causes the POST to fail and an error message appears on the display.
Operating System Load - After the POST is successfully completedthe CPU searches Storage devices for special files that indicate the beginning of the OS and thenloads it into memory.
- The sequence in which the storage devices are searched is part of the configuration set-up stored in BIOS.
- Once the Operating System (OS) is located, the CPU copies what’s called the 'boot record' from the OS into DRAM.
- Control is then passed to the 'boot record' in DRAM, which then continues loading the rest of the OS.
- This process continues until the OS load is completed.
- 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.