Inside Macintosh: Processes

Chapter 1 - Introduction to Processes and Tasks

About Processes

The Process Manager manages the scheduling of processes. A process is an open application or, in some cases, an open desk accessory. (Desk accessories that are opened in the context of an application are not considered processes.) The number of processes is limited only by available memory.

The Process Manager maintains information about each process--for example, the current state of the process, the address and size of its partition, its type, its creator, a copy of all process-specific system global variables, information about its 'SIZE' resource, and a process serial number. This process information is referred to as the context of a process. The Process Manager assigns a process serial number to identify each process. A process serial number identifies a particular instance of an application; this number is unique during a single boot of the local machine.

The foreground process is the one currently interacting with the user; it appears to the user as the active application. The foreground process displays its menu bar, and its windows are in front of the windows of all other applications.

A background process is a process that isn't currently interacting with the user. At any given time a process is either in the foreground or the background; a process can switch between the two states at well-defined times.

The foreground process has first priority for accessing the CPU. Other processes can access the CPU only when the foreground process yields time to them. There is only one foreground process at any one time. However, multiple processes can exist in the background.

An application that is in the background can get CPU time but can't interact with the user while it is in the background. (However, the user can bring the application to the foreground--for example, by clicking in one of the application's windows.) Any application that has the canBackground flag set in its 'SIZE' resource is eligible to obtain access to the CPU when it is in the background.

Applications can be designed without a user interface; these are called background-only applications. A background-only application does not call the Window Manager InitWindows routine and is identified by having the onlyBackground flag set in its 'SIZE' resource. Background-only applications do not display windows or a menu bar and are not listed in the Application menu.

Background-only applications and applications that can run in the background should be designed to relinquish the CPU often enough so that the foreground process can perform its work and respond to the user.

Once an application is running, in either the foreground or the background, the CPU is available only to that application. That application can be interrupted only by hardware interrupts, which are transparent to the scheduling of the application. However, the application that is running must periodically relinquish control of the CPU. This yielding of the CPU allows background applications access to processing time and lets users interact with the foreground application or switch to another application.

Your application can relinquish control of the CPU each time you call the Event Manager functions WaitNextEvent or EventAvail. If, at that time, there are no events pending for your application, the Process Manager may schedule other processes for execution. (You can also call the GetNextEvent function; however, you should use WaitNextEvent to provide greater support for cooperative multitasking.)

---

Subtopics

Process Creation

Process Scheduling