One of the standard ways of viewing the operating system is as a service provider. This is in contrast to viewing it as a governor or controller, although it must be this, too. Thinking of the operating system as a bundle of services and subroutines that all user programs can, and in fact must, use makes it easier to visualize what a trap does and why it is needed. The services provided by an operating system are many and varied. Here is a small list:

• stop the program when it is done and clean up after it
• initiate and monitor input and output
• change program characteristics like its memory size or its priority
• send and received messages to other user programs
• create children programs