What is an Operating System?#

An OS is the software layer between hardware and applications. It manages resources (CPU, memory, disk, I/O) and provides abstractions so programs don’t need to interact with hardware directly.

flowchart TD
    Apps["Applications (browser, editor, server)"]
    Apps --> OS["Operating System"]
    OS --> HW["Hardware (CPU, RAM, Disk, Network)"]

Core Responsibilities#

Process Management#

What is a Process?#

A process is a running program — it has its own memory space, file descriptors, and execution state.

stateDiagram-v2
    [*] --> New: Created
    New --> Ready: Admitted
    Ready --> Running: Scheduled
    Running --> Ready: Preempted
    Running --> Waiting: I/O request
    Waiting --> Ready: I/O complete
    Running --> Terminated: Exit

Process vs Thread#

Scheduling#

The OS decides which process gets CPU time:

Context Switch#

When the OS switches from one process to another:

1. Save current process state (registers, program counter)
2. Load next process state
3. Resume execution

Context switches are expensive (~1-10μs) — too many = poor performance.

Memory Management#

Virtual Memory#

Each process sees its own private address space — the OS maps virtual addresses to physical RAM:

flowchart LR
    subgraph Process["Process View"]
        Stack["Stack (grows down)"]
        Heap["Heap (grows up)"]
        Data["Data (globals)"]
        Code["Code (text)"]
    end
    
    Process -->|"Page Table"| RAM["Physical RAM"]
    Process -->|"Page Table"| Swap["Swap (disk)"]

Key Concepts#

Why Virtual Memory?#

• Isolation — processes can’t access each other’s memory
• Simplicity — every process thinks it has all memory to itself
• Overcommit — total virtual memory can exceed physical RAM

Kernel vs User Space#

flowchart TD
    subgraph User["User Space"]
        App1["Application 1"]
        App2["Application 2"]
        Lib["Libraries (libc)"]
    end
    
    User -->|"System Calls"| Kernel["Kernel Space"]
    
    subgraph Kernel
        Sched["Scheduler"]
        MM["Memory Manager"]
        FS["File System"]
        Drivers["Device Drivers"]
    end
    
    Kernel --> HW["Hardware"]

System calls are the interface between user space and kernel (e.g., open(), read(), write(), fork(), exec()).

Types of Operating Systems#

Linux Dominance in Servers#

~96% of cloud servers run Linux because:

• Free and open source
• Stable and secure
• Lightweight (no GUI needed)
• Excellent tooling and automation
• Container-native (cgroups, namespaces)

Key Takeaways#

1. The OS manages resources — CPU (scheduling), memory (virtual memory), I/O (drivers)
2. Processes are isolated — each has its own memory space
3. Threads share memory — faster but less safe than processes
4. Virtual memory provides isolation and allows overcommit
5. System calls are how programs request OS services
6. Linux dominates servers — understanding it is essential for backend/cloud work