🧠 What is a Kernel?
The Kernel is the core component of an Operating System.
It acts as a bridge between applications and the physical hardware.
It manages everything: CPU, memory, processes, devices, and system calls — all from deep inside the OS.
🔗 Role of the Kernel
When a program wants to:
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Read a file,
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Use the network,
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Allocate memory,
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Spawn a thread,
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Talk to the GPU…
It doesn’t do it directly — it asks the kernel via system calls.
🔧 Key Responsibilities of the Kernel
| Responsibility | Description |
|---|---|
| 🧵 Process Management | Creates, schedules, kills processes (e.g., fork(), exec()), manages CPU time |
| 🧠 Memory Management | Allocates and frees RAM, handles virtual memory and paging |
| 🧱 File System Management | Controls access to files and directories (e.g., open(), read()) |
| 📡 Device Management | Talks to hardware via device drivers (e.g., disk, keyboard, network) |
| 🔐 Security & Access Control | Manages permissions, users, and isolation |
| 🔄 System Call Interface | Provides APIs for user programs to request kernel services |
🧱 Types of Kernels
| Type | Description | Example |
|---|---|---|
| Monolithic Kernel | All services run in kernel space — fast but riskier | Linux, Unix |
| Microkernel | Minimal core kernel; rest in user space — more secure/modular | Minix, QNX |
| Hybrid Kernel | Mix of both (some services in kernel space) | Windows NT, macOS |
| Exokernel | Gives apps direct control of hardware — minimal abstraction | Experimental |
📦 Kernel Space vs User Space
| Space | Access Level | Description |
|---|---|---|
| Kernel Space | 🔐 Full access | OS core runs here; handles critical resources |
| User Space | 🔒 Restricted | Apps run here; must use system calls to access kernel |
🔁 Context switch happens when a user app makes a system call → control enters kernel space → returns back.
⚙️ Kernel Lifecycle (High-Level)
[Bootloader (e.g., GRUB)]
↓
[Loads Kernel into memory]
↓
[Kernel initializes hardware, devices, memory]
↓
[Starts first process: init/systemd]
↓
[User space begins → login shell, desktop, etc.]
🧪 Real-World Examples
| OS | Kernel Used |
|---|---|
| Linux | Monolithic Linux Kernel |
| Windows | Hybrid (Windows NT Kernel) |
| macOS | Hybrid (XNU Kernel) |
| Android | Modified Linux Kernel |
| Embedded OS (e.g., RTOS) | Often Microkernel-based |
🧠 Interview-Ready Definition:
A Kernel is the core component of an operating system that directly manages system resources such as CPU, memory, file systems, and devices. It acts as a bridge between hardware and software, providing a controlled interface (via system calls) for user programs to safely access underlying hardware.
✅ Bonus Insight
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If the kernel crashes → whole system crashes (kernel panic)
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If a user app crashes → only that app is affected