Modern operating systems often feel like black boxes. We interact with processes, files, and network sockets without thinking about what actually happens underneath.
To better understand how low-level systems work, I built a small experimental kernel that runs on x86 architecture, implementing several core subsystems including interrupt handling, paging, device drivers, and a simple shell.
This article explains the architecture of that kernel and the main design decisions behind it.
Why Build a Kernel?
Operating systems are one of the most complex pieces of software ever created. But the fundamental ideas behind them are surprisingly understandable when explored step by step.
The motivation behind this project was to understand:
- How CPUs transition from bootloader to kernel
- How interrupts allow hardware to communicate with software
- How paging enables virtual memory
- How device drivers interact with hardware
- How kernel services can be exposed through system calls
Rather than studying these concepts in isolation, the goal was to implement them together inside a working kernel.
System Overview
The kernel runs on 32-bit x86 architecture and boots through a Multiboot2-compatible bootloader such as GRUB.
At a high level the architecture looks like this:
Bootloader (GRUB)
↓
Kernel Entry (Assembly)
↓
Interrupt Descriptor Table
↓
Memory Management (Paging)
↓
Hardware Drivers
↓
Kernel Shell
Each subsystem interacts with the others to form a minimal operating system environment.
Boot Process
The boot process begins when the bootloader loads the kernel into memory and transfers control to its entry point.
The kernel performs several early initialization steps:
- Initialize descriptor tables
- Set up memory paging
- Configure interrupt handling
- Initialize hardware drivers
- Start the kernel shell
This early boot stage is critical because the system is still running without many safety guarantees.
Interrupt Handling
Hardware interrupts are one of the most fundamental mechanisms in operating systems.
Devices such as keyboards, timers, and disks signal events to the CPU using interrupts.
The kernel implements:
- Interrupt Descriptor Table (IDT)
- Exception handlers
- IRQ handlers
The IDT maps interrupt numbers to handler functions, allowing the CPU to transfer control to the correct kernel routine.
Example flow:
Keyboard Key Press
↓
Keyboard Controller Interrupt
↓
CPU triggers interrupt handler
↓
Kernel keyboard driver processes input
Without interrupts, the CPU would have to constantly poll devices, which would waste enormous amounts of processing time.
Memory Management
Memory management is implemented using paging.
Paging allows the operating system to map virtual memory addresses to physical memory addresses.
The kernel initializes:
- Page directory
- Page tables
- Paging control registers
Once paging is enabled, memory can be organized into virtual address spaces.
Benefits of paging include:
- memory protection
- flexible address mapping
- foundation for user processes
In this kernel the paging system establishes a simple flat memory model for kernel execution.
Kernel Heap and Dynamic Memory
Operating systems cannot rely on standard libraries like malloc.
Instead, the kernel implements its own heap allocator.
This allows kernel subsystems to dynamically allocate memory for structures such as:
- device buffers
- process data
- driver state
Although simple, this allocator demonstrates the fundamental idea behind dynamic memory management in kernel space.
Device Drivers
Hardware interaction happens through device drivers.
Two drivers are implemented in the kernel:
VGA Display Driver
The kernel writes directly to VGA memory at:
0xB8000
This allows text to be displayed on screen without relying on BIOS services.
The VGA driver implements:
- character output
- cursor control
- color support
PS/2 Keyboard Driver
The keyboard driver handles hardware interrupts generated by the PS/2 controller.
The driver:
- reads scancodes from the keyboard port
- translates them into characters
- forwards input to the kernel shell
This demonstrates how hardware input flows through the interrupt system into software.
Timer Support
The kernel configures the Programmable Interval Timer (PIT) to generate periodic interrupts.
Timers are essential for:
- scheduling
- time measurement
- system responsiveness
Even in a simple kernel, timer interrupts provide the foundation for multitasking systems.
System Call Interface
A minimal system call interface is implemented to expose kernel services.
System calls act as the boundary between:
user programs
↕
kernel services
Although this kernel does not yet implement full user-mode processes, the syscall interface lays the groundwork for future expansion.
Interactive Kernel Shell
To make the system usable, the kernel includes a small command-line shell.
The shell allows interaction with kernel services and debugging of internal behavior.
Typical commands might include:
- memory inspection
- system information
- device testing
This interface helps verify that the kernel subsystems are functioning correctly.
Limitations
This kernel is intentionally minimal and designed primarily as a learning platform.
Some important limitations include:
- single-core execution
- no filesystem
- no process isolation
- no SMP support
- no preemptive multitasking
Despite these limitations, the project demonstrates how core operating system components interact in a real execution environment.
What This Project Taught Me
Building even a small kernel provides deep insights into how computers work.
Key lessons from this project include:
- hardware interrupts are fundamental to system responsiveness
- memory management is the foundation of modern OS design
- driver design requires careful interaction with hardware
- observability and debugging are essential for low-level development
Most importantly, operating systems are not magic, they are carefully structured layers of software interacting directly with hardware.
Conclusion
Building an operating system kernel is one of the most educational experiences for understanding systems engineering.
This project demonstrates how several essential subsystems, interrupts, memory management, device drivers, and system calls, combine to form the foundation of an operating system.
Future work could include:
- implementing user processes
- adding a filesystem
- supporting multitasking
- expanding hardware driver support
Even a small kernel reveals just how fascinating low-level systems development can be.
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