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    Processes, Threads, System Calls and Context Switching Short Notes for GATE CS

    Processes, Threads, System Calls and Context Switching short notes for GATE CS: 5 study cards covering concepts, formulas, shortcuts and exam traps, plus solv

    processes threads system calls and context switching short notes

    State Transition Cheat Sheet: Quick Reference

    Process States and Context Switching: Quick Reference

    Five States

    1. New: Being created
    2. Ready: Waiting for CPU
    3. Running: Executing instructions
    4. Blocked: Waiting for event (I/O, signal, resource)
    5. Terminated: Finished execution

    Allowed Transitions (6 total)

            New -> Ready (admit)
        Ready -> Running (dispatch)
        Running -> Ready (preempt)
        Running -> Blocked (wait)
        Running -> Terminated (exit)
        Blocked -> Ready (wake up)

    Impossible Transitions (The Big 3)

    • Blocked -> Running: must go through Ready
    • Blocked -> Terminated: must wake up first
    • Ready -> Blocked: must be Running to block

    Context Switch Triggers (Always)

    • Timer interrupt (preemption)
    • I/O request (blocking)
    • Wait for event (blocking)
    • Higher priority process ready (preemption)
    • Process termination (exit)

    Ready Queue Entry Counting

    Formula: Ready entries = Number of I/O operations (+ 1 if initial entry counted)

    I/O operations include: scanf, printf, fread, fwrite, fopen, close, etc.

    Key Distinctions

    • Ready versus Blocked: Ready = waiting for CPU (eligible to run). Blocked = waiting for event (NOT eligible to run).
    • Mode Switch versus Context Switch: Mode switch = User <-> Kernel (same process). Context switch = Process A <-> Process B (different processes).
    Memory Hooks:
    Five states: "Never Rush Really Big Tasks" (New, Ready, Running, Blocked, Terminated)
    Impossible transitions: "Blocked cannot Run, Blocked cannot Terminate, Ready cannot Block"
    Context switch: "Save state, load state, pure waste"

    Final Exam Checklist: Process States and Context Switching

    Final Exam Checklist

    Can You...?

    • Draw the Five-State Model: New, Ready, Running, Blocked, Terminated. All six allowed transitions. Three impossible transitions marked.

    Threads: Final Checklist

    Final Checklist

    Core Facts

    • Shared: Code, Data, Heap, Files
    • Private: PC, Registers, Stack, Thread ID

    Context Switch

    Save/Restore: PC, Registers, Stack. Do NOT touch: Page Table.

    Models

    Many-to-One (fast, fragile), One-to-One (parallel, heavy), Many-to-Many (balanced).

    One-line exam method: For any thread option, ask: per-thread or per-process, and does this cross into the kernel?

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    Processes, Threads, System Calls and Context Switching Short Notes for GATE CS

    Processes, Threads, System Calls and Context Switching short notes for GATE CS: 5 study cards covering concepts, formulas, shortcuts and exam traps, plus solved practice questions.

    State Transition Cheat Sheet: Quick Reference

    Process States and Context Switching: Quick Reference

    Five States

    1. New: Being created
    2. Ready: Waiting for CPU
    3. Running: Executing instructions
    4. Blocked: Waiting for event (I/O, signal, resource)
    5. Terminated: Finished execution

    Allowed Transitions (6 total)

            New -> Ready (admit)
        Ready -> Running (dispatch)
        Running -> Ready (preempt)
        Running -> Blocked (wait)
        Running -> Terminated (exit)
        Blocked -> Ready (wake up)

    Impossible Transitions (The Big 3)

    • Blocked -> Running: must go through Ready
    • Blocked -> Terminated: must wake up first
    • Ready -> Blocked: must be Running to block

    Context Switch Triggers (Always)

    • Timer interrupt (preemption)
    • I/O request (blocking)
    • Wait for event (blocking)
    • Higher priority process ready (preemption)
    • Process termination (exit)

    Ready Queue Entry Counting

    Formula: Ready entries = Number of I/O operations (+ 1 if initial entry counted)

    I/O operations include: scanf, printf, fread, fwrite, fopen, close, etc.

    Key Distinctions

    • Ready versus Blocked: Ready = waiting for CPU (eligible to run). Blocked = waiting for event (NOT eligible to run).
    • Mode Switch versus Context Switch: Mode switch = User <-> Kernel (same process). Context switch = Process A <-> Process B (different processes).
    Memory Hooks:
    Five states: "Never Rush Really Big Tasks" (New, Ready, Running, Blocked, Terminated)
    Impossible transitions: "Blocked cannot Run, Blocked cannot Terminate, Ready cannot Block"
    Context switch: "Save state, load state, pure waste"

    Final Exam Checklist: Process States and Context Switching

    Final Exam Checklist

    Can You...?

    • Draw the Five-State Model: New, Ready, Running, Blocked, Terminated. All six allowed transitions. Three impossible transitions marked.

    Threads: Final Checklist

    Final Checklist

    Core Facts

    • Shared: Code, Data, Heap, Files
    • Private: PC, Registers, Stack, Thread ID

    Context Switch

    Save/Restore: PC, Registers, Stack. Do NOT touch: Page Table.

    Models

    Many-to-One (fast, fragile), One-to-One (parallel, heavy), Many-to-Many (balanced).

    One-line exam method: For any thread option, ask: per-thread or per-process, and does this cross into the kernel?

    System Calls: Final Checklist

    System Calls: Final Checklist

    Core Facts

    • System Call: Synchronous, software-initiated request.
    • Mode Bit: = Kernel, = User.
    • Transition: Trap instruction mode bit flips jump to vector.

    Critical Distinctions

    • Procedure Call: User User (No mode change).
    • System Call: User Kernel (Synchronous).
    • Interrupt: Hardware Kernel (Asynchronous).

    C Library Rules

    Never: strlen, abs.
    Conditional: printf, malloc.
    Always: open, read, fork, exit.

    One-Line Exam Method: To determine if an event guarantees a transition, ask: Does this absolutely require privileged hardware access or OS intervention?

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