Process in Operating System:- A process is a program in execution. A process is more than the program code, which is sometimes known as the text section. It also includes the current activity, as represented by the value of the program counter and the contents of the processor’s registers.
Process in Operating System
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 a memory that is dynamically allocated during process run time.
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, 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.
The structure of a process in memory.
Although two processes may be associated with the same program, they have 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.
Process States in Operating System
As a process executes, it changes state. The state of a process is defined in part by the current activity of that process. Each process may be in one of the following states:
- New. The process is being created.
- Running. Instructions are being executed.
- Waiting. The process is waiting for some event to occur.
- Ready. The process is waiting to be assigned to a processor.
- Terminated. The process has finished execution.
These names are arbitrary, and they vary across operating systems. The states that they represent are found on all systems, however. Certain operating systems also more finely delineate process states. It is important to realize that only one process can be running on any processor at any instant.
Process Control Block in Operating System
- Process state. The state may be new, ready, running, waiting, halted, and so on.
- Program counter. The counter indicates the address of the next instruction to be executed for this process.
- CPU registers. The registers vary in number and type, depending on the computer architecture. They include accumulators, index registers, stack pointers, and general-purpose registers, plus any condition-code information. Along with the program counter, this state information must be saved when an interrupt occurs, to allow the process to be continued correctly afterwards.
- CPU-scheduling information. This information includes a process priority pointer to scheduling queues and any other scheduling parameters.
- Memory-management information. This information may include such information as the value of the base and limit registers, the page tables or the segment tables, depending on the memory system used by the operating system.
- Accounting information. This information includes the amount of CPU and real-time used, time limits, account numbers, job or process numbers, and so on.
- I/O status information. This information includes the list of 1/0 devices allocated to the process, a list of open files, and so on.
In brief, the PCB simply serves as the repository for any information that may vary from process to process.