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