Operating Systems Overview

Operating System Responsibilities

• The OS manages processes by: creating, deleting, suspending, resuming, and providing mechanisms for synchronizing and communicating between them.
• Allocating and scheduling resources for processes.

Multiprogramming and Multitasking

• Technique where multiple tasks (processes) share common processing resources, like the CPU.
• Aims to maximize CPU utilization by running multiple programs simultaneously.

Process vs. Program

• Program: A set of instructions executed by a computer.
• Process: An instance of a program in execution, active and consuming resources.

Distinction between Program and Process

• Program is static, residing in secondary memory.
• Process is dynamic, existing only during execution in main memory.
• Program is a passive entity, containing instructions.
• Process is an active entity, requiring resources for execution.
• Program has two components: code and data.
• Process has additional information for management and execution.

Process States

• Created (New): Process is first created and awaits admission to the "ready" state.
• Ready: Process is loaded into main memory and awaits execution.
• Running: Process is actively executing on the CPU.
• Waiting: Process is waiting for a resource (e.g., I/O) to be available.
• Terminated: Process has completed execution or been explicitly killed.

Process Control Block (PCB)

• Contains information about a process for management, including state, identification (process ID - PID), parent/child relationships, priority, memory locations, register values, and I/O information.

Location of the PCB

• The PCB is typically placed in a protected area of memory, often at the beginning of the process' kernel stack.

Context Switching

• Reassigning CPU from one task to another.
• Involves saving the context (CPU registers and data) of the current process and loading the context of the new process.
• Triggers when a process exits, exhausts its time slice, requires unavailable resources, or a higher-priority process becomes ready.

Resource Allocation Concepts

• Pre-emptible Resources: Can be taken away and given back later (e.g., CPU).
• Non-pre-emptible Resources: Once assigned, cannot be reused until voluntarily released (e.g., disk blocks).

Scheduling Concepts

• Scheduling: Determining which process in the ready queue will run next.
• Scheduler: Software responsible for assigning processes to CPUs.
• Preemptive Systems: The scheduler can interrupt a process and force a context switch.
• Non-preemptive Systems: The scheduler waits for the running process to voluntarily block or terminate.
• Scheduling Decisions: Occur when a process switches states (ready, running, waiting, terminated).

Scheduling Assumptions

• A pool of ready processes vying for the CPU.
• Processes compete for resources.
• The scheduler aims to allocate the CPU fairly and optimize performance criteria.

Process Behavior and CPU Scheduling

• CPU/IO Burst Cycle: Processes alternate between periods of CPU execution and I/O operations.
• IO-Bound Processes: Characterized by frequent I/O operations and short CPU bursts.
• CPU-Bound Processes: Perform extensive computation and have fewer, longer CPU bursts.

Criteria for CPU Scheduling

• CPU Utilization: Keeping the CPU busy as much as possible.
• Throughput: Number of processes completed per time unit.
• Turnaround Time: Time elapsed from submission to completion of a process.
• Waiting Time: Time spent in the ready queue.
• Response Time: Time from request submission to the first response.
• Fairness: Providing equal CPU time to each process.