Operating System : Process Concept

Process Concept

Process: A process is a program in execution. It is more than the program code, which is sometimes known as Text Section. It also includes the current activity, as represented by the value of the program counter & the contents of the processor’s register. It includes process stack, which contains Temporary data (such as function parameters, return address & local variables) & data section which contains global variable. It also includes heap, which is memory that is dynamically allocated during process runtime. A program is a passive entity, such as a file containing a list of instructions stored on a disk. A process is an active entity, with a program counter specifying the next instruction to execute & a set of associated resources. A program becomes a process when an executable file is loaded into memory.

Two common techniques:

For loading executable file

·        Double-clicking an icon representing the executable file.

·        Entering the name of the executable files 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 sequenced. The text sections are equivalent, the data, heap & stack sections vary.

Process State:

As a process executes, it changes state. The state of a person is defined in part by the current activity of that process.

New- The process is being created.

Running- Instructions are being executed.

Waiting- Process is waiting for some event to occur.

Ready- Process is waiting to be assigned to a processor.

Terminated- Process has finished execution.

Process Control Block:

Each process is represented in the OS by a PCB, also called Task Control Block.

Process State: State may be new, ready, running, waiting, halted & so on.

Program Counter: It indicates the address of next instruction to be executed.

CPU Registers: Registers vary in number & type. They include accumulators, index registers, stack pointers & general purpose registers plus any condition code.

CPU Scheduling Information: It includes a process priority, pointers to scheduling queues & other scheduling parameters.

Memory Management Information: It includes such items as the value of the base and limit registers & the page table, or the segment tables, depending on the memory system used by OS.

Accounting Information: It includes the amount of CPU & real time used, time limits, account number, job or process numbers.

I/O Status Information: It includes the list of I/O devices allocated to the process, a list of open files.

THREADS:

A process is a program that performs a single thread of execution. For example, when a process running a word-processor program, a single thread of instructions is being executed. The user cannot simultaneously type in character & run spell checker within the same process. Process with multiple threads, perform than one task at a time.

Diagram showing CPU switch from process to process