Skip to main content

Operating Systems | Concept of Process

Hard disk drive of the system in called primary memory and secondary memory is Random Access Memory(RAM). Either a program written in High Level Language(HLL) or a executable code generated by the sequence of works done by pre-processor, compiler and assembler, resides in secondary memory of the system.

To start an execution, operating system allocates some space in the main memory, for the program to be executed and loads the program in the secondary memory to the allocated space. The piece of work which is loaded by the operating system to the main memory in order to execute program is called process

Every program loaded by operating system will create a focus boundary(or process body), a partitioned memory area where all memory requirements for the execution of program is satisfied.

  • Variable which will not change its value through out life time of process is called static variable and variables which are globally accessible in a process known as global variable.
  • Heap area is reserved for dynamic allocation of memories
  • Stack area for the stacking of recursive function calls.
  • Since heap and stack have a dynamic behavior they are defined to grow in opposite direction as shown

After creating the focus boundary CPU will be executing the line available in the executable code and depending on the nature of execution line(dynamic allocation request refers to heap, recursive calls refers to stack, request to access static or global variable refers to respective areas) it may have to refer to the remaining areas of the process boundary. Referring to a memory location beyond the scope of process boundary will lead to segmentation fault.

Operating system maintains a linked list of information regrading all the processes. A node of this linked list includes data necessary to control and execute the process, such a node is called Process Control Block(PCB or context)

A Process Control Block have the following attributes:

Process IDHolds unique identification number given by OS for each process
Program CounterHolds information about the address of next instruction, that has to be executed
Process StateHolds the present state of the process(example: Ready, Runn, Block...)
PriorityHolds a number which represents the importance of the process, high priority means probability to get processed is high
General Purpose RegisterIt hold values of general purpose registers, so that change of register values will while switching between processes, can be avoided by reassigning values stored in this attribute.
List of FilesHold list of all files that are opened by the process in any mode(Reading mode, Writing mode, Read/Write mode)
List of Open DevicesHolds data about the open devices, which are in use at the that instant of process
ProtectionContains security information to avoid illegal data access


Popular posts from this blog

Operating Systems | Scheduling Algorithms : Round Robin

| FCFS | SJF | SRTS | Round Robin | LJF | Priority Scheduling | HRRN | Round RobinFeatures : Most popular algorithm of allPractically implementableImplementable with basic data structures like queueExtremely lesser starvationOptimum efficiency can be set by controlling time quantumThe round robin algorithm will continuously switch between processes if a process in CPU (under execution) exceeds a time limit set by OS called time quantum.Flow Chart :Scheduler allocated process to for execution.CPU starts monitoring the execution time right after from allocation.If the process completes its execution before exceeding time quantum, OS forwards the process to termination state.else, the processes gets preempted once the time quantum limit exceeded and if the process finished at this moment, OS moves the process to termination state, else it moves to ready queue and iterates over the whole process listed above.Example : Consider the following table of processes and calculate complet…

Operating Systems | Scheduling Algorithms : SJF

| FCFS | SJF | SRTS | Round Robin | LJF | Priority Scheduling | HRRN | Shortest Job First(SJF)SJF algorithm schedules the shortest job(low burst time) available on main memory at the time of context switching in ready state . It can be either preemptive or non-preemptive depending on the nature of scheduler.Since the SJF select the job which is shorted among available jobs on main memory, Non-preemptive SJF algorithm will have larger waiting time if a job with longer burst time get scheduled first. In short SJF is also vulnerable to Convoy EffectTo find shortest process among min-heap would be the best data structure to be usedExample : Consider the following table of processes and calculate completion time, turn around time and waiting time using SJF algorithm with the following assumptions. No preemption.No I/O request will be there from any process.Arrival time is relative to CPU on time.Process NumberATBT103222324442View Answer

The execution of processes can be visualized …