There's something magical about the moment when you power on a computer and watch it come to life. Behind that simple boot sequence lies one of the most complex orchestrations in modern technology: the operating system taking control of raw hardware and transforming it into a usable computing environment.
Building an operating system from scratch is perhaps the ultimate exercise in systems programming. It requires understanding everything from assembly language and hardware interfaces to memory management and process scheduling. This is the story of creating OhneBS—a bare-metal operating system for the Raspberry Pi.
The Bootstrap Problem
Every operating system faces the fundamental bootstrap problem: how do you start a complex software system when you have no software infrastructure to build upon? When the Raspberry Pi powers on, you have raw silicon, uninitialized memory, and no guarantees about the state of any hardware peripherals.
"The bootstrap process is like pulling yourself up by your own bootstraps—except in this case, it actually works."
The solution lies in a carefully orchestrated sequence of increasingly sophisticated initialization steps. Each stage builds upon the previous one, gradually transforming the raw hardware into a functioning computing environment.
From Silicon to C
The journey begins in assembly language—the lowest level of abstraction above raw machine code. Before we can write a single line of C, we must establish the fundamental prerequisites that the C runtime expects: a valid stack, initialized memory regions, and proper processor state.
Figure 1: The multi-stage boot process from hardware reset to kernel main
The first few hundred bytes of code are critical. They must verify that we're running on the correct CPU core, set up a stack pointer, clear uninitialized memory regions, and finally jump to the kernel's main function written in C.
void kernel_main(void) {
// Initialize UART for serial communication
uart_init();
uart_puts("OhneBS v1.0 - Booting...
");
// Initialize memory management
memory_init();
uart_puts("Memory management initialized
");
// Set up interrupt handlers
interrupts_init();
uart_puts("Interrupt system ready
");
// Initialize GPIO for hardware control
gpio_init();
uart_puts("GPIO driver loaded
");
// Start the interactive shell
uart_puts("Starting shell...
");
shell_main();
}Hardware Abstraction
Once we have a C runtime environment, the next challenge is creating abstractions for the underlying hardware. The Raspberry Pi exposes its functionality through memory-mapped registers—specific memory addresses that, when written to or read from, control hardware behavior.
Building device drivers means understanding the hardware at an intimate level. Every GPIO pin, every UART configuration register, every mailbox communication protocol must be implemented from the ground up. There are no libraries to import, no APIs to call—just you, the hardware documentation, and raw memory addresses.
The Interactive Shell
The culmination of all this low-level work is something deceptively simple: a command prompt. But that blinking cursor represents the successful orchestration of bootloaders, memory management, interrupt handling, and device drivers all working in harmony.
The shell provides a tangible interface to all the underlying systems. When you type a command and see a response, you're witnessing the entire stack in action—from keyboard input processing through command parsing to output display, all running on an operating system you built from scratch.
Lessons from the Metal
Building an operating system teaches you to think in terms of layers and abstractions. Each level builds upon the previous one, hiding complexity while exposing just enough functionality for the next layer to build upon. This is the fundamental pattern that underlies all of computing.
Perhaps most importantly, it gives you a deep appreciation for the incredible complexity hidden behind every simple operation. That innocent-looking printf() statement represents the coordinated effort of bootloaders, memory managers, device drivers, and interrupt handlers—all working together to put characters on a screen.
Every modern convenience we take for granted in high-level programming languages represents sophisticated engineering solutions to fundamental computing challenges. Understanding these foundations makes you a better programmer at every level of the stack.
