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Flow Diagram

PCB (Process Control Block)

  • Process ID
  • Program Counter
  • Process Priority
  • PCB Pointer
  • List of Open Files
  • CPU Register
  • Memory Management Information
  • I/O Status Information
  • Scheduling Information

2. Program Counter

The program counter stores information about which next instructions are going to be executed by CPU.

Priority Example in Arithmetic Operations:

"Priority Example"

3. Process Priority

The priority is a numerical value which is assigned while a process is created.

Note: The lesser number, the higher its value / priority.

  • Priority makes CPU/attention to process execution.

Why does the execution prioritize processes?

For two reasons:

  1. Starvation: The situation or condition where a process never gets CPU time.

  2. Consumption Resources

Age/Aging

  • It is a technique or method of giving priority to a process until the process gets CPU Time.

Note: Age increases the priority.

4. PCB Pointer

  • The PCB pointer stores information about the next PCB to be executed.

5. List of Open Files

  • The information of open files is stored into PCB. As once the process terminates, the open files need to be closed by OS.

6. CPU Register

  • Register is a memory.
  • Register is a temporary memory.
  • Register is the fastest memory among all types of memories.
  • Register memory is close (near) to CPU.
  • CPU time unit is millisecond.
  • Register time unit is nanosecond.

[!NOTE] Time Units

  • 1 second = 1000 milliseconds = 1,000,000,000 nanoseconds
  • 1 millisecond = 1,000,000 nanoseconds
  • 1 second = 1000 milliseconds
  • 1 millisecond = 1000 microseconds
  • 1 microsecond = 1000 nanoseconds

Register Types

  • AX = Accumulator Register: Arithmetic operations or mathematical operations are stored.
  • BX = Base Register: Logical operations are stored in here.

Register Attributes

  1. Stack Pointer:

    • It shows the temporary free space to CPU so that CPU can save the PCB in there.

  2. Index Register:

    • The movement through the list.

  3. Status Flag:

    • Flag the issues.

In very simple words we can say that registers are used to store the information of different tasks which are not currently processed like if we have a task that is currently running and due to need for input output devices it is sent to register it is basically like a checkpoint so whenever the process is loaded again to the CPU it starts from where it left off instead of restarting the whole task.

7. Memory Management Information

  • The memory management information store information about process allocated how much memory or RAM.

E.g:

  • P1 = 200MB
  • P2 = 10MB
  • P3 = 1.4GB
  • P4 = 5MB

8. I/O Status Information

  • It stores which I/O devices are allocated to a process.

E.g:

  • P1 = Printer

  • Requests of I/O by the processes.

9. Scheduling Information

  • Priority
  • Scheduling Queue Pointer
  • Parameters (FCFS, SJF, RR)

Queue = Lines

Types

1. Job Queue

  • All processes waits in line for getting RAM. "Job Queue"

2. Ready Queue

  • All processes are waiting to get CPU time. "Ready Queue"

3. Device I/O Queue

  • All processes are waiting to get requested I/O devices. "Device I/O Queue"

4. Blocked/Waiting Queue

  • When a program or a process is not ready to be scheduled to run.

E.g:

  • A process is waiting for some data to be received.