Do they repeatedly check for the condition and execute if the condition is met. Ex, how the OS knows exactly when a USB device is plugged in, or how MSN knows exactly when you get an email. How does this work?
Thanks
Share
Sign Up to our social questions and Answers Engine to ask questions, answer people’s questions, and connect with other people.
Login to our social questions & Answers Engine to ask questions answer people’s questions & connect with other people.
Lost your password? Please enter your email address. You will receive a link and will create a new password via email.
Please briefly explain why you feel this question should be reported.
Please briefly explain why you feel this answer should be reported.
Please briefly explain why you feel this user should be reported.
Do they repeatedly check for the condition and execute if the condition is met. Ex, how the OS knows exactly when a USB device is plugged in, or how MSN knows exactly when you get an email. How does this work?
Thanks
At the low level, the OS kernel “knows” when something happens, because the device in question sends the CPU a hardware interrupt.
So when, say a network packet arrives, the network controller sends an interrupt, and the OS kernel responds as appropriate.
At the program level, it works quite differently – most application programs run an “event loop”, where they fetch a message (say, a message from the OS saying that “the mouse was clicked on this point in your application”), perform the appropriate actions in response to that, and then, listen for more messages. If there is no message, the OS sleeps the thread until it has a message to deliver.