Operating Systems Windows LinuxOperating Systems Windows Linux

An operating system acts as an intermediary between the user and the computer hardware, managing system resources efficiently.

• Kernel: The core, central component of an OS that has complete control over everything in the system. It manages system initialization, memory, and device communication, operating in a highly privileged supervisor mode.
• Shell: The outer layer of the OS that acts as an interface between the user and the kernel. It interprets user commands and can be Command Line (CLI) like Bash/cmd or Graphical (GUI) like Windows Desktop.
• Process Management: Responsible for creating, scheduling, synchronizing, and deleting active processes. It utilizes CPU scheduling algorithms to maximize processor utilization.
• Memory Management: Tracks every byte of primary memory (RAM) and manages allocation and deallocation of memory blocks to active programs to prevent conflicts.
• Device Management: Manages device communication through respective drivers. It uses buffering, caching, and spooling to handle input/output operations smoothly.

Operating systems are classified based on how they process tasks, manage users, and respond to external events.

• Batch Processing OS: Users do not interact with the computer directly. Similar jobs are grouped into 'batches' and submitted together to offline devices to optimize CPU idle time.
• Real-Time OS (RTOS): Designed for applications where response time is critical. Divided into Hard RTOS (strict deadlines, e.g., missile guidance) and Soft RTOS (some delay acceptable, e.g., media streaming).
• Time-Sharing OS: Enables multiple users located at various terminals to use a single computer system concurrently. CPU time is shared among users using rapid context switching.

Efficient memory management ensures that multiple processes can run simultaneously without interfering with each other's address spaces.

• Paging: A memory management scheme that eliminates the need for contiguous allocation of physical memory. It divides physical memory into fixed-size blocks called 'Frames' and logical memory into 'Pages'.
• Segmentation: A memory management technique where memory is divided into logical segments of variable lengths based on the user's view of the program (e.g., functions, objects, main program).
• Virtual Memory: An OS technique that allows execution of processes that are larger than the physical RAM. It maps logical memory addresses to physical storage (Hard Disk/SSD) using page tables.
• Thrashing: A highly undesirable state where the operating system spends more time swapping pages in and out of virtual memory than executing actual instructions, leading to system freeze.
• Fragmentation: Internal fragmentation occurs when allocated memory is slightly larger than requested memory. External fragmentation occurs when total free memory is enough to satisfy a request, but it is non-contiguous.

Process scheduling determines which process gets CPU time, while deadlock management prevents system lockups.

• Process States: A process transitions through various states during execution: New (created), Ready (waiting for CPU), Running (instructions executing), Waiting/Blocked (waiting for an event), and Terminated (finished).
• Scheduling Algorithms: Algorithms used by the OS to allocate CPU: First-Come First-Served (FCFS), Shortest Job First (SJF), Round Robin (RR - uses time slices for time-sharing), and Priority Scheduling.
• Deadlock: A situation where a set of processes are blocked because each process is holding a resource and waiting for another resource held by some other process in the set.
• Coffman Conditions: Four conditions must hold simultaneously for a deadlock to occur: Mutual Exclusion, Hold and Wait, No Preemption, and Circular Wait.
• Banker's Algorithm: A deadlock avoidance algorithm developed by Edsger Dijkstra. It tests for safety by simulating the allocation of predetermined maximum possible amounts of all resources.

File systems define how data is stored and retrieved, while command line interfaces allow direct administrative control.

• FAT vs NTFS: FAT (File Allocation Table) is older, lacks security, and has file size limits (4GB for FAT32). NTFS (New Technology File System) supports file-level security, journaling, compression, and massive file sizes.
• MS-DOS Internal Commands: Commands built directly into the command interpreter (command.com) and loaded into RAM during boot, such as DIR (list directory), COPY (copy files), DEL (delete), and CLS (clear screen).
• MS-DOS External Commands: Commands stored as separate executable files on disk (not loaded into RAM automatically), such as FORMAT.EXE, CHKDSK.EXE, and DISKCOPY.COM.
• Linux Commands: Essential commands for system navigation: 'ls' (list directory contents), 'cd' (change directory), 'chmod' (change file permissions), and 'grep' (search text patterns using regular expressions).