lecture 6

Chapter 3: Process Management
3,1 Process Concept
A question that arises in discussing operating systems involves what to call all the CPU activities, A batch system executes jobs, whereas time-shared system has user programs or tasks, Even on a single-user system such as Microsoft Windows, a user may be able to run several programs at one time: a word processor, a Web browser and an e-mail package. And even if the user can execute only one program at a time, the operating system may need to support its own internal programmed activities, such as memory management. In many respects, all these activities are similar, so we call all of them processes. The terms Job and process are used almost interchangeably in this text. Although prefer the term process,


3.1.1 The Process
A process is a program in execution. A process is more than the program code. It also includes the current activity, as represented by the value of the program counter and the contents of the processor s registers. A process generally also includes the process stack, which contains temporary data (such as function parameters, return addresses, and local variables), and a data section, which contains global variables. A process may also include a heap, which is memory That is dynamically allocated during process run time. The structure of a process in memory is shown in Figure 3.1.
We emphasize that a program by itself is not a process; a program is a passive entity, such as a file containing a list of instructions stored on disk (often called an executable file), whereas a process is an active entity, with a program counter specifying the next instruction to execute and a set of associated resources. A program becomes a process when an executable file is loaded into memory. Two common techniques for loading executable files are double-clicking an icon representing the executable file and entering the name of the executable file on the command line (as in prog.exe or a.out.)


Although two processes may be associated with the same program, they are nevertheless considered two separate execution sequences. For instance, several users may be running different copies of the mail program, or the same user may invoke many copies of the Web browser program. Each of these is a separate process; and although the text sections are equivalent, the data, heap, and stack sections vary. It is also common to have a process that spawns many processes as it runs. .