Queue (printing)

In computing, spooling is a specialized form of multi-programming for the purpose of copying data between different devices. In contemporary systems it is usually used for mediating between a computer application and a slow peripheral, such as a printer. Spooling allows programs to "hand off" work to be done by the peripheral and then proceed to other tasks, or do not begin until input has been transcribed. A dedicated program, the spooler, maintains an orderly sequence of jobs for the peripheral and feeds it data at its own rate. Conversely, for slow input peripherals, such as a card reader, a spooler can maintain a sequence of computational jobs waiting for data, starting each job when all of the relevant input is available; see batch processing. The spool itself refers to the sequence of jobs, or the storage area where they are held. In many cases the spooler is able to drive devices at their full rated speed with minimal impact on other processing.

Spooling is a combination of buffering and queueing.

Nowadays, the most common use of spooling is printing: documents formatted for printing are stored in a queue at the speed of the computer, then retrieved and printed at the speed of the printer. Multiple processes can write documents to the spool without waiting, and can then perform other tasks, while the "spooler" process operates the printer.

For example, when a large organization prepares payroll checks, the computation takes only a few minutes or even seconds, but the printing process might take hours. If the payroll program printed checks directly, it would be unable to proceed to other computations until all the checks were printed. Similarly, before spooling was added to PC operating systems, word processors were unable to do anything else, including interact with the user, while printing.

Spooler or print management software often includes a variety of related features, such as allowing priorities to be assigned to print jobs, notifying users when their documents have been printed, distributing print jobs among several printers, selecting appropriate paper for each document, etc.

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