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{{Rating | 1|}}{{Template: Kernel designs}}
This is an introductory tutorial for kernel development on the x86 architecture. It is a minimal example only, and does not show you how to structure a serious project. However, completing this project is a good first step for aspiring kernel developers.
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== Building A Cross-Compiler ==
The first thing we need to do is to build ourselves a cross-compiler toolchain to let us compile and link code for the x86. Until we have that, we can't do anything. A standard C compiler won't work: it doesn't understand how to generate the machine code we need for this project.
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First, we create '''start.s'''. To best understand the code, I recommend typing it out by hand such that you can understand each part in detail.
<
// We declare the 'kernel_main' label as being external to this file.
// That's because it's the name of the main C function in 'kernel.c'.
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// This section contains data initialised to zeroes when the kernel is loaded
.section .bss
// Our C code will need a stack to run. Here, we allocate
// We can expand this later if we want a larger stack. For now, it will be perfectly adequate.
.align 16
stack_bottom:
.skip
stack_top:
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hlt // Halt the CPU
jmp hang // If that didn't work, loop around and try again.
</syntaxhighlight>
=== kernel.c ===
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'kernel.c' contains our main kernel code. Specifically, it contains code for displaying text on the screen using the VGA textmode buffer.
<
// GCC provides these header files automatically
// They give us access to useful things like fixed-width types
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term_print("Welcome to the kernel.\n");
}
</syntaxhighlight>
=== linker.ld ===
This file is a '''link script'''. It is used to define how bits of the final kernel executable will be stitched together. It allows us to specify alignment, address offset, and other such useful properties of parts of the code that we need when our kernel gets linked together into an executable. You don't normally do this when you're writing C code - the compiler will do this for you. But this an operating system kernel, so we need to explicitly define a lot of these things.
<
/* The bootloader will start execution at the symbol designated as the entry point. In this case, that's 'start' (defined in start.s) */
ENTRY(start)
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}
</syntaxhighlight>
== Compiling And Linking ==
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=== grub.cfg ===
<
menuentry "My Kernel" {
multiboot /boot/mykernel.elf
boot
}
</syntaxhighlight>
Once you have done this, copy your final kernel '''.elf''' executable that we linked previously into the 'isoroot/boot' directory.
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</pre>
Congratulations! If all went to plan, you've just produced your first
Note: The x86 VGA textmode device we rely on in this tutorial is dependent on the existence of the BIOS. This ISO can only be booted properly from a UEFI system if it is in a BIOS compatibility mode (often referred to as 'legacy mode').
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=== Create A Build System For Your Kernel ===
At the moment, we compile and link our kernel by manually running several commands. As your kernel grows, this will no longer be a realistic way of doing things. You'll need to design and implement a build system that can build your kernel for you. Common kernel build systems use 'make', 'CMake', 'autoconf', 'SCons' or 'Tup'.
=== Call Global Constructors ===
In C and particularly C++, the compiler will often generate code that is meant to be run when the program starts. Currently, we don't have any means to run such code. As our kernel grows, we'll need to add support for this.
=== Going Further On x86 ===
There is a whole world of interesting things to explore through operating system development. To improve our kernel further, we will need to better understand the x86 and what hardware we have available.
[[:Category:Bare bones tutorials]]
[[:Category:C]]
[[:Category:C++]]
[[Category:User drafts]]
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