Concepts you may want to Google beforehand: kernel, ELF format, makefile
Goal: Create a simple kernel and a bootsector capable of booting it
Our C kernel will just print an 'X' on the top left corner of the screen. Go ahead
You will notice a dummy function that does nothing. That function will force us
to create a kernel entry routine which does not point to byte 0x0 in our kernel, but
to an actual label which we know that launches it. In our case, function
i386-elf-gcc -ffreestanding -c kernel.c -o kernel.o
That routine is coded on
kernel_entry.asm. Read it and you will learn how to
[extern] declarations in assembly. To compile this file, instead of generating
a binary, we will generate an
elf format file which will be linked with
nasm kernel_entry.asm -f elf -o kernel_entry.o
A linker is a very powerful tool and we only started to benefit from it.
To link both object files into a single binary kernel and resolve label references, run:
i386-elf-ld -o kernel.bin -Ttext 0x1000 kernel_entry.o kernel.o --oformat binary
Notice how our kernel will be placed not at
0x0 in memory, but at
bootsector will need to know this address too.
It is very similar to the one in lesson 10. Open
bootsect.asm and examine the code.
Actually, if you remove all the lines used to print messages on the screen, it accounts
to a couple dozen lines.
Compile it with
nasm bootsect.asm -f bin -o bootsect.bin
Putting it all together
Now what? We have two separate files for the bootsector and the kernel?
Can't we just "link" them together into a single file? Yes, we can, and it's easy, just concatenate them:
cat bootsect.bin kernel.bin > os-image.bin
You can now run
os-image.bin with qemu.
Remember that if you find disk load errors you may need to play with the disk numbers
or qemu parameters (floppy =
0x0, hdd =
0x80). I usually use
qemu-system-i386 -fda os-image.bin
You will see four messages:
- "Started in 16-bit Real Mode"
- "Loading kernel into memory"
- (Top left) "Landed in 32-bit Protected Mode"
- (Top left, overwriting previous message) "X"
As a last step, we will tidy up the compilation process with a Makefile. Open the
script and examine its contents. If you don't know what a Makefile is, now is a good time
to Google and learn it, as this will save us a lot of time in the future.