Power On Self Test (POST)

The Power On Self Test or POST is the first piece of software to run after power on. It both tests and initializes various hardware components. The specific order varies depending on the creator of the BIOS but generally follows this sequence:

  1. The CPU
  2. The Video Screen. This is the first time you'll see any evidence of the system starting.
  3. Memory. This step is time-consuming so it is often optional.
  4. Floppy disks.
  5. Peripheral Components. Each controller card plugged into the system bus (usually the PCI bus) can have its own piece of BIOS code. The system BIOS looks for a special signature in memory for these add-ons and if found calls them.
The system cannot communicate on the video screen until after step 2 above is finished. Therefore if there are problems found before that, the POST beeps the system loudspeaker and halts. Of course, if the CPU isn't functioning properly, it's unlikely the POST will get far enough to beep or do anything else useful.

When the POST finishes, the BIOS looks to see if a special key was pressed and if so the Setup routine is called.

Otherwise, control is transfered to the boot routine.

Next: boot routine

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