Computer systems are designed to shutdown when they are no longer needed. However, in some cases, the computer may not shut down properly. This can lead to data loss or even a system crash. To prevent these problems, computer systems often rely on a shutdown process known as “power off.” When the computer shuts down, it first disconnects all of its cords and shuts down all of its components. This process can take some time, so it’s important to keep your computer running while it’s doing this. After the shutdown is complete, the computer will restart with new software and settings. If you notice that your computer is not shutting down properly after the power off process is complete, you can try turning it off and on again. If that doesn’t work, you may need to call your computer manufacturer or service provider to troubleshoot the issue.
Today’s Question & Answer session comes to us courtesy of SuperUser—a subdivision of Stack Exchange, a community-drive grouping of Q&A web sites.
The Question
SuperUser reader FrozenKing wants to know how his computer knows when it has crashed or otherwise had an improper shutdown. He writes:
It’s an interesting question; clearly modern operating systems are well aware that something has gone wrong, but by what mechanism are they made aware?
How does the computer know that the shutdown was not properly done?
The Answers
SuperUser contributor David Schwartz offers a general answer and insight into different operating systems:
Another contributor, ChrisF answers more specifically for the Windows operating system:
Have something to add to the explanation? Sound off in the the comments. Want to read more answers from other tech-savvy Stack Exchange users? Check out the full discussion thread here.
Windows also uses the Dirty Bit method to detect whether the PC was shut down properly:
When powered off as normal, the bit is switched off. However, in case of power outage or improper (forced) shutdown, the bit will still be on the next time the PC is started.
This means that Windows can suggest remedial action – such as booting into Safe Mode.
In addition a Windows based PC will write an entry into event log detailing when and why (if known) it was shut down. It also writes an event when the PC starts up.
