Process Management

A user typically has many applications installed on his or her computer. These are purely passive entities. The programs on disk contain data that is needed only when the program is run, consisting of the executable code and application data. When the user launches an application, the operating system loads the program's code and data into memory from disk and begins executing its code. A program being executed is known as a “process.” Unlike a program, a process is an active entity, and consists of a snapshot of the state of the program at a single instance during execution. This includes the program's code, the memory that the program has allocated, and the current state of its execution, such as the CPU instruction of the function that the program is currently executing, and the contents of its variables and memory allocations.

There are typically many processes running on a system at once. These include applications that the user has launched (such as iTunes or Safari), as well as processes that are started automatically by the operating system and that run with no indication to the user. For example, the Time Machine backup service will automatically run a background process every hour to perform a backup of your data. There may even be multiple instances of the same program being executed at any one time, each of which is considered a distinct process by the operating system. Figure 1-3 shows the Activity Monitor utility that is included with Mac OS X, which allows all of the processes running on the system to be examined.

Figure 1-3. Activity Monitor on Mac OS X showing all processes running on the system. Compare this to the Dock, which shows the visible user applications.