Spooling is a technique by which multiple processes can submit their jobs to spool process and can continue other tasks. Printer server is one of the best example to explain better. Multiple computers are connected to same printer and each of them can request to print a document. Spooler as printer receives print request and en-queue all print request and completes one by one. Spool process running at printer can also assign some priority among the en-queued print request. This way serialization goes away from submitter point of view and they can perform other tasks without waiting for the completion of submitted task.

In this section, we will try to trace the evolution of operating system. In particular, we will describe serial processing, batch processing and multiprogramming.