Q1Operating System

The Process Control Block (PCB), also called the Task Control Block, is the data structure used by the OS kernel to store all information about a process. Each process has exactly one PCB. The OS uses PCBs to manage, schedule, and control processes. When a context switch occurs, the current process's state is saved in its PCB and the next process's state is restored from its PCB.

• Process State: Current state of the process (new, ready, running, waiting, terminated).
• Process ID (PID): Unique identifier for each process.
• Program Counter (PC): Address of the next instruction to be executed.
• CPU Registers: Contents of all CPU registers (accumulators, index registers, stack pointer, general-purpose registers) that must be saved and restored on context switch.
• CPU Scheduling Information: Process priority, scheduling queue pointers, and other scheduling parameters.
• Memory Management Information: Base and limit registers, page tables, or segment tables depending on the memory system used.
• Accounting Information: CPU time used, elapsed real time, time limits, account numbers, job or process numbers.
• I/O Status Information: List of I/O devices allocated to the process, list of open files, pending I/O requests.

A process passes through a series of distinct states during its lifetime. The process state is stored in the PCB and is updated by the OS.

• New: The process is being created. The OS is setting up the PCB and allocating resources. Transition: New → Ready (after admission by the OS long-term scheduler).
• Ready: The process is waiting to be assigned to the CPU. It has all resources except the CPU. Multiple processes can be in the ready state simultaneously (ready queue). Transition: Ready → Running (short-term scheduler dispatches it).
• Running: The process is currently executing on the CPU. Only one process per CPU core can be in running state. Transitions: Running → Waiting (process requests I/O or event), Running → Ready (preempted by scheduler or time quantum expires), Running → Terminated (process finishes).
• Waiting (Blocked): The process is waiting for an event (I/O completion, signal, resource). Cannot proceed until the event occurs. Transition: Waiting → Ready (I/O completes or event occurs).
• Terminated: The process has finished execution. The OS deallocates its resources and removes its PCB. Also called the exit state.

In some models, two additional states are added: Suspended Ready and Suspended Waiting, to handle swapping of processes to disk when memory is insufficient. The medium-term scheduler handles these transitions (swap in/swap out).