Process Control Block (PCB); Process States; CPU Scheduling

Process Control Block (PCB)

What is a PCB?

The PCB (Process Control Block) is the data structure used by the OS to store all information about a process.

Also called: Task Control Block (TCB)

PCB Contents

Field	Description
Process ID (PID)	Unique identifier for the process
Process State	New, Ready, Running, Waiting, Terminated
Program Counter (PC)	Address of next instruction to execute
CPU Registers	Contents of all CPU registers (saved on context switch)
CPU Scheduling Info	Priority, scheduling queue pointers
Memory Management Info	Base register, limit register, page/segment tables
I/O Status Info	List of open files, I/O devices allocated
Accounting Info	CPU time used, elapsed time, job limits
Parent PID (PPID)	PID of parent process
User ID (UID)	Owner of the process

Process States

Five-State Process Model

                    +--------+
                    |  NEW   |  ← Process created
                    +--------+
                         | admitted
                         ↓
              +------READY------+
              |    ←scheduler   |
              |   dispatch      |
              ↓                 |  interrupt / time slice
         +--------+             |
         |RUNNING |→→→→→→→→→→→→+
         +--------+
              |
              | Wait for I/O or event
              ↓
         +--------+
         |WAITING |  ← process blocked for I/O
         | (BLOCKED)|
         +--------+
              |
              | I/O or event complete
              ↓
         READY (returns to Ready queue)

         RUNNING → TERMINATED (exit)

Process States Explained

State	Description	Transitions
New	Process is being created	→ Ready (after creation)
Ready	In memory, waiting for CPU	→ Running (scheduler dispatches)
Running	Actually being executed on CPU	→ Ready (time slice expires) / Waiting (I/O) / Terminated (exit)
Waiting (Blocked)	Waiting for I/O completion or event	→ Ready (I/O/event done)
Terminated	Process has finished execution	End state

Context Switch

When CPU switches from one process to another:

Process A running on CPU
    |
    | Interrupt / time expired
    ↓
Save CPU state of A → PCB_A
Load CPU state of B ← PCB_B
    |
    | Process B now running on CPU

Context switch overhead: Saving and restoring registers, cache flush — expensive!

CPU Scheduling

What is CPU Scheduling?

CPU Scheduling selects which process in the Ready queue gets the CPU next.

Why scheduling?

CPU can only run one process at a time
Multiple processes compete for CPU
OS must decide who runs next

Scheduling Queues

New Process → Job Queue
                 ↓
         Ready Queue (in RAM)
                 ↓
         CPU Scheduler picks one
                 ↓
         CPU executes process
                 ↓
     I/O request? → I/O Wait Queue → I/O done → Ready Queue
     Time slice?  → Preempted → Ready Queue
     Exit?        → Terminated

Scheduling Criteria (Performance Metrics)

Metric	Description	Optimize
CPU Utilization	Percentage of time CPU is busy	Maximize
Throughput	Number of processes completed per time unit	Maximize
Turnaround Time	Total time from submission to completion	Minimize
Waiting Time	Total time process spends in ready queue	Minimize
Response Time	Time from submission to first response	Minimize

Formulas:

Turnaround Time = Completion Time - Arrival Time
Waiting Time    = Turnaround Time - Burst Time
Response Time   = First CPU given - Arrival Time

Preemptive vs Non-Preemptive Scheduling

Type	Description
Non-Preemptive	Once CPU is given to a process, it keeps it until done or waiting for I/O
Preemptive	OS can forcibly take CPU from running process (time slice, priority)