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*Concepts you may want to Google beforehand: monolithic kernel, microkernel, debugger, gdb*
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**Goal: Pause and organize our code a little bit. Then learn how to debug the kernel with gdb**
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Maybe you didn't realize it, but you already have your own kernel
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running!
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However, it does very little, just print an 'X'. Now is the time to stop for
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a moment and organize the code into folders, create a scalable Makefile for future code,
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and think on a strategy.
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Take a look at the new folder structure. Most of the files have been symlinked
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from previous lessons, so if we have to change them at some point, it will be
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a better idea to remove the symlink and create a new file.
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Furthermore, since from now on we will use mostly C to code, we'll take advantage of qemu's
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ability to open a connection to gdb. First, let's install a cross-compiled `gdb` since
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OSX uses `lldb` which is not compatible with the ELF file format (neither is the `gdb` available
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on Homebrew's repos)
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```sh
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cd /tmp/src
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curl -O http://ftp.rediris.es/mirror/GNU/gdb/gdb-7.8.tar.gz
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tar xf gdb-7.8.tar.gz
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mkdir gdb-build
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cd gdb-build
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export PREFIX="/usr/local/i386elfgcc"
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export TARGET=i386-elf
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../gdb-7.8/configure --target="$TARGET" --prefix="$PREFIX" --program-prefix=i386-elf-
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make
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make install
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```
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Check out the Makefile target `make debug`. This target uses builds `kernel.elf`, which
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is an object file (not binary) with all the symbols we generated on the kernel, thanks to
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the `-g` flag on gcc. Please examine it with `xxd` and you'll see some strings. Actually,
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the correct way to examine the strings in an object file is by `strings kernel.elf`
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We can take advantage of this cool qemu feature. Type `make debug` and, on the gdb shell:
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- Set up a breakpoint in `kernel.c:main()`: `b main`
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- Run the OS: `continue`
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- Run two steps into the code: `next` then `next`. You will see that we are just about to set
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the 'X' on the screen, but it isn't there yet (check out the qemu screen)
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- Let's see what's in the video memory: `print *video_memory`. There is the 'L' from "Landed in
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32-bit Protected Mode"
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- Hmmm, let's make sure that `video_memory` points to the correct address: `print video_memory`
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- `next` to put there our 'X'
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- Let's make sure: `print *video_memory` and look at the qemu screen. It's definitely there.
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Now is a good time to read some tutorial on `gdb` and learn super useful things like `info registers`
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which will save us a lot of time in the future!
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You may notice that, since this is a tutorial, we haven't yet discussed which kind
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of kernel we will write. It will probably be a monolithic one since they are easier
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to design and implement, and after all this is our first OS. Maybe in the future
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we'll add a lesson "15-b" with a microkernel design. Who knows.
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