This commit adds our first kernel written in C.
You might want to write your whole kernel in C but even then you can’t really avoid writing a little bit of assembly. Our entry point is this file. The first thing to do is declare your symbols (start and kmain) in our case. Secondly, initialize the stack pointer esp to some reserved stack space. Next, call the main function in the kernel kmain.
It is important to note that all this is done in the .text section. The reserved space for stack is allocated in .bss (Block Started by Symbol) section.
We also add ‘multiboot’ info to this file for the bootloaders like grub as well as our CPU emulator qemu.
Our first basic kernel simply does what our bootloader did before we wrote a kernel - it prints a message on the screen using the VGA text mode. The kernel.c file is self-sufficient if someone wants to know what it does.
Since we’ll be calling the kmain function in kernel.c from kernel.asm, we need to link the binaries. For this we will use the linker program ld. We configure ld using a simple linker script.
nasm -f elf32 kernal.asm -o kernel_asm.o
gcc -m32 -c kernel.c -o kernel_c.o
ld -m elf_i386 -T linker.ld kernel_asm.o kernel_c.o -o kernel.bin
# or use the makefile
make
To run, use:
qemu-system-x86_64 -kernel kernel.bin
# or use the makefile
make qemu_kernel