What happens when your PC
is powered on?
by John Anthony
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
- 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 Hard Drive(s) 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 completed the CPU searches Storage devices for
special files that indicate the beginning of the OS and
then loads 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 whatís called the 'boot record' from the OS into
- 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.
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