Lessons · IT support · POST and the boot order
POST and the boot order: what a machine does before the OS exists
When power arrives, the firmware runs a power-on self-test (POST) of the memory, the processor and the basic hardware, then reads the boot order and starts the first device on the list that has something to start. The operating system is not involved in any of that.
Hone is a place to practise a career, one idea a day. This is one of its lessons, written out in full and free to read without an account.
What it is for
A desktop shows a black screen with a blinking cursor and the user says 'Windows is broken'. Windows has not started yet. A USB stick left in the front port is first in the boot order and has nothing on it, so the firmware stopped there. Pulling the stick fixes it. Knowing where the OS begins is knowing where to stop blaming it.
How to think about it
Read the screen in the order it happens. No lights, no fans: power. Fans, no picture: POST, and listen for beeps or watch for a code. A firmware logo then a message about no boot device: the boot order, or the disk. A logo that spins and stops: now, and only now, the OS. Each stage rules out everything before it.
Worked example
Power light on, fans spin, no beepsPower is fine and the self-test passed. The fault is later than POST.
Screen: 'No bootable device found'The firmware looked down the boot order and found nothing it could start.
Boot order: 1. USB, 2. internal disk. A USB stick is in the front portFirst in the list, and empty. The firmware stopped at it and never reached the disk.
Stick removed, restart: the OS loadsOne change, verified. The note says 'USB stick in boot order; removed', and next time it is a ten-second ticket.
Your turn
The fans spin and the firmware logo shows, then 'no bootable device'. Write which stage to check next.
Power OK → POST OK → check the
Solve one, graded on the server
The trap
Reinstalling the operating system for a fault that happens before the operating system starts. If there is no logo and no spinner, the OS has not been reached, and no reinstall can fix a stage it never runs in.