3.3.1.5 Spooling Describe spooling, explaining why it is used.

Spooling Spooling is the process of sending output (intended for a printer or other peripheral) to a disk file, temporarily, so that it can be forwarded to the printer (or other peripheral) when it free.

Spooling Note that a spool file is effectively a buffer between the fast processor and the much slower printer. Spooling: avoids delays – printers are relatively slow and so spooling frees up the processor quickly and allows it to get on with other processing jobs; allows more than one print job to be submitted at a time – each job will be held in a queue and printed one at a time – the use of a queue also allows priorities to be set. in a multi-user system, provides a method of keeping print-jobs separate – it means that printouts will not be muddled up. It is important to realise that the output itself is not placed in the queue. The queue simply contains the details of the files that need to be printed. Place input and output on a fast access device, such as disk, so that slow peripheral devices do not hold up the processor. In a multi-programming, multi-access or network system, several jobs may wish to use the peripheral devices at the same time. It is essential that the input and output for different jobs do not become mixed up. This can be achieved by using Simultaneous Peripheral Operations On-Line (spooling). Suppose two jobs, in a network system, are producing output that is to go to a single printer. The output is being produced in sections and must be kept separate for each job. Opening two files on a disk, one for each job, can do this. Suppose we call these files File1 and File2. As the files are on disk, job 1 can write to File1 whenever it wishes and job 2 can write to File2. When the output from a job is finished, the name (and other details) of the file can be placed in a queue. This means that the OS now can send the output to the printer in the order in which the file details enter the queue. As the name of a file does not enter the queue until all output from the job to the corresponding file is complete, the output from different jobs is kept separate. Spooling can be used for any number of jobs. It is important to realise that the output itself is not placed in the queue. The queue simply contains the details of the files that need to be printed so that the OS sends the contents of the files to the printer only when the file is complete. The part of the OS that handles this task is called the spooler or print spooler. It should be noted that spooling not only keeps output from different jobs separate It also saves the user having to wait for the processor until the output is actually printed by a printer (a relatively slow device). Spooling is used on personal computers as well as large computer systems capable of multi-programming.