Core CS

Process Control Block

A Process Control Block (PCB), also known as a Task Control Block, is a fundamental kernel data structure used by an operating system to track and manage the execution state of a specific process. Whenever a process is created, the OS allocates a dedicated PCB that acts as its identification profile, storing all execution metadata—such as the program counter, CPU registers, process state, and memory allocations. When the OS performs a context switch to swap out a running process, it serializes the process's current hardware state into its PCB, allowing the process to resume later exactly where it left off without losing computation progress.

<h2></h2><h2> Definition</h2>In modern multitasking operating systems, a process is dynamic and alternates between active CPU computation, waiting for I/O operations, and sitting in ready queues. The operating system cannot maintain all active processes inside physical CPU registers simultaneously. The PCB serves as an in-memory repository residing in protected kernel space that completely encapsulates a process's hardware and software execution contexts. It is the central mechanism through which schedulers prioritize tasks, memory management units (MMUs) enforce isolation, and resource managers track open handles and I/O devices.<h3> Core Components of a PCB</h3><ul><li>Process Identifier (PID): A unique integer assigned by the OS to identify the process across the system, alongside its Parent Process ID (PPID).</li><li>Process State: The current operational stage of the process, such as <code>New</code>, <code>Ready</code>, <code>Running</code>, <code>Waiting</code>/<code>Blocked</code>, or <code>Terminated</code>.</li><li>Program Counter (PC): The memory address pointing to the next machine instruction the CPU must fetch and execute for this process.</li><li>CPU Registers: A snapshot of processor registers—including accumulators, base/index registers, stack pointers (<code>ESP</code>/<code>RSP</code>), and flags/condition codes—saved during an interrupt or context switch.</li><li>CPU Scheduling Information: Attributes required by the scheduler to allocate CPU time, such as priority levels, scheduling algorithm queue pointers, and quantum/tick counters.</li><li>Memory Management Information: Page tables, segment tables, base and limit registers, and virtual address map pointers defining the process's allocated address space.</li><li>Accounting & Diagnostic Info: Metrics tracked for resource billing and optimization, including total CPU execution time consumed, clock limits, and user credentials.</li><li>I/O Status Information: Pointers and descriptors for allocated devices, pending I/O requests, and the file descriptor table (mapping open files and network sockets).</li></ul><h3>Role in Context Switching</h3> When the CPU switches execution from Process A to Process B:<ol><li>The kernel receives a timer interrupt or system call.</li><li>The current register state, program counter, and stack pointer are copied directly from the CPU hardware into PCB_A.</li><li>Process A's state updates from <code>Running</code> to <code>Ready</code> (or <code>Waiting</code>).</li><li>The scheduler selects Process B and locates PCB_B.</li><li>The CPU registers and program counter are restored from PCB_B into the hardware registers.</li><li>The MMU updates page table pointers to Process B's memory boundaries, and execution resumes.</li></ol>