The amount of time in a given period that a computing system can be used by its normal users. During available time the system must be functioning correctly, have power supplied to it, and not be undergoing repair or maintenance. Available time is comprised of productive time and idle time. Productive time is the amount of time in a given period that a system is performing useful work for the users. Idle time is the amount of time in a given period that a system is performing no useful function. It usually occurs when waiting for completion of some I/ O function or backing-store transfer.