Suppose you sit down at your computer to check your email. One of the messages includes an attached document, which you are to edit. You click the attachment, and it opens up in another window. After you start editing the document, you realize you need to leave for a trip. You save the document in its partially edited state and shut down the computer to save energy while you are gone. Upon returning, you boot the computer back up, open the document, and continue editing.
This scenario illustrates that computations interact. In fact, it demonstrates at least three kinds of interactions between computations. In each case, one computation provides data to another. First, your email program retrieves new mail from the server, using the Internet to bridge space. Second, your email program provides the attachment to the word processor, using the operating system’s services to couple the two application pro grams. Third, the invocation of the word processor that is running befo …
