Process Information

Process Image

• Refers to the snapshot of a process’ state at a specific point in time
• Contains all necessary information for the process to execute:
  • User memory: stack, heap, code segment, .bss
  • Process Control Block (PCB): data needed to control the process
  • Assuming VM provides separation and swapping
• Must be (at least partially) loaded in main memory to execute
• i.e., Process image is essential for OS to control and manage process effectively

Process Control Block (PCB)

• Contains essential information about a process, including:
  • Registers (program counter, status word, etc.)
  • FPU state
  • Descriptor tables (e.g., IDT — Interrupt Descriptor Table)
  • Memory pointers to code and data, shared memory blocks
  • Fault handler
  • Flags
  • Process Identification: PID, parent, descendants, user
  • Processor state: PCB: registers, status word
  • Process information:
    • Process information (state, priority, event)
    • Signals and Timers
    • Credentials
    • File descriptors
    • System call vector
• Serves as a data structure that holds all necessary details for the OS to manage and control the process effectively

Process Creation

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1. Building the PCB to store process information
2. Allocate memory for the process to execute
3. Create an execution context (thread) for the process
4. Run the process

Process Switch

• Saves context of currently running process into PCB
• Updates process with new state and accounting information
• Moves process to appropriate queue (ready, blocked, etc.)
• Selects another process to continue its execution
• Updates MMU (Memory Management Unit)
• Restores PCB of selected process to continue execution
• i.e., allows OS to efficiently manage multiple processes by switching between them to utilise system resources effectively

Process Termination

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Occurs when a process exits due to reasons such as:

• Calling exit(2) function
• Receiving a SIGKILL signal
• Encountering an unhandled signal
• Exceeding resource limits

Queueing Model

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Analysis and optimisation of processes waiting to be executed

Process models

• Progresses from simple to realistic process execution models.

Two-state model:

• Simplest process model; divides process execution into two states:
  • Running
  • Exit
• In this model, a process is either actively running, or has been terminated.
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Five-state model:

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• New: Created but not ready to execute
• Ready: Can execute when scheduled by the OS
• Running Actively executing
• Blocked: Waiting for an event (e.g., I/O completion)
• Exit: Terminated but metadata remains in memory

Process State Transitions

• Defines the movement of a process from one state to another:
  • Null $\to$ New: Process creation
  • New $\to$ Ready: OS prepared to make process runnable
  • Ready $\to$ Running: Dispatch ready process to execute
  • Ready $\to$ Exit: Terminated by other process (e.g., parent)
  • Running $\to$ Exit: Termination
  • Running $\to$ Ready:
    • Process has had its time continuously running
    • Higher priority process has become ready — preemption
    • Voluntary yield (e.g., sleep(3), nanosleep(2))
  • Running $\to$ Blocked: Process waits for syscall, I/O, etc.
  • Blocked $\to$ Ready: Event that process is waiting for occurs
  • Blocked $\to$ Exit: As above

Seven-state model:

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• A more detailed representation of process behaviour, including specific conditions and events for transitions between states
• Introduces additional process state transitions:
  • Ready $\to$ Suspend: Manual suspension (SIGSTOP or SIGTSTP)
  • Suspend $\to$ Ready: No ready process or suspended process has highest priority
  • Blocked $\to$ Blocked/Suspend: Manual suspension (SIGSTOP or SIGTSTP)
  • Blocked/Suspend $\to$ Suspend: Event being waited for occurs
  • New $\to$ Ready/Suspend: Depends on available resources
  • Blocked/Suspend $\to$ Blocked: Higher priority than any in Ready queue; resources available
  • Running $\to$ Ready/Suspend: Pre-empted by Suspended process and memory ended
  • Any $\to$ Exit: Termination

Reasons for suspension:

Lists various reasons for process suspension.

Reason	Comment
Swapping	OS needs to release resources for another process
User Request	e.g., debugging or manual suspension (Ctrl+Z)
Timing	Suspension of periodic process while awaiting next run
Parent Request	Debugging, examination, coordination of descendants