Multitasking Basics

Multitasking is essentially the ability to do many things concurrently. For example, you may be working on a project, eating lunch, and talking to a colleague at the same time. Not everything may be happening simultaneously, but you are jumping back and forth, devoting your attention to each task as necessary.

In programming, a task is simply an independent execution path. On a computer, the system software can handle multiple tasks, which may be applications or even smaller units of execution. For example, the system may execute multiple applications, and each application may have independently executing tasks within it. Each such task has its own stack and register set.

Multitasking may be either cooperative or preemptive. Cooperative multitasking requires that each task voluntarily give up control so that other tasks can execute. An example of cooperative multitasking is an unsupervised group of children wanting to look at a book. Each child can theoretically get a chance to look at the book. However, if a child is greedy, he or she may spend an inordinate amount of time looking at the book or refuse to give it up altogether. In such cases, the other children are deprived.

Preemptive multitasking allows an external authority to delegate execution time to the available tasks. Preemptive multitasking would be the case where a teacher (or other supervisor) was in charge of letting the children look at the book. He or she would assign the book to each child in turn, making sure that each one got a chance to look at it. The teacher could vary the amount of time each child got to look at the book depending on various circumstances (for example, some children may read more slowly and therefore need more time).

The Mac OS 8 operating system implements cooperative multitasking between applications. The Process Manager can keep track of the actions of several applications. However, each application must voluntarily yield its processor time in order for another application to gain it. An application does so by calling WaitNextEvent , which cedes control of the processor until an event occurs that requires the application's attention.

Multiprocessing Services allows you to create preemptive tasks within an application (or process). For example, you can create tasks within an application to process information in the background (such as manipulating an image or retrieving data over the network) while still allowing the user to interact with the application using the user interface.

Note

The definition of task in this document is analogous to the use of the term thread in some other operating systems such as UNIX^®. In older documentation, Apple has sometimes referred to separate units of execution as threads . For example, the Thread Manager allows you to create cooperatively scheduled threads within a task. You should not confuse these threads with the preemptively scheduled tasks created by Multiprocessing Services.