cfenollosa_os-tutorial/14-checkpoint

Concepts you may want to Google beforehand: monolithic kernel, microkernel, debugger, gdb

Goal: Pause and organize our code a little bit. Then learn how to debug the kernel with gdb

Maybe you didn't realize it, but you already have your own kernel running!

However, it does very little, just print an 'X'. Now is the time to stop for a moment and organize the code into folders, create a scalable Makefile for future code, and think on a strategy.

Take a look at the new folder structure. Most of the files have been symlinked from previous lessons, so if we have to change them at some point, it will be a better idea to remove the symlink and create a new file.

Furthermore, since from now on we will use mostly C to code, we'll take advantage of qemu's ability to open a connection to gdb. First, let's install a cross-compiled gdb since OSX uses lldb which is not compatible with the ELF file format (neither is the gdb available on Homebrew's repos)

cd /tmp/src
curl -O http://ftp.rediris.es/mirror/GNU/gnu/gdb/gdb-7.8.tar.gz
tar xf gdb-7.8.tar.gz
mkdir gdb-build
cd gdb-build
export PREFIX="/usr/local/i386elfgcc"
export TARGET=i386-elf
../gdb-7.8/configure --target="$TARGET" --prefix="$PREFIX" --program-prefix=i386-elf-
make
make install

Check out the Makefile target make debug. This target uses builds kernel.elf, which is an object file (not binary) with all the symbols we generated on the kernel, thanks to the -g flag on gcc. Please examine it with xxd and you'll see some strings. Actually, the correct way to examine the strings in an object file is by strings kernel.elf

We can take advantage of this cool qemu feature. Type make debug and, on the gdb shell:

• Set up a breakpoint in kernel.c:main(): b main
• Run the OS: continue
• Run two steps into the code: next then next. You will see that we are just about to set the 'X' on the screen, but it isn't there yet (chech out the qemu screen)
• Let's see what's in the video memory: print *video_memory. There is the 'L' from "Landed in 32-bit Protected Mode"
• Hmmm, let's make sure that video_memory points to the correct address: print video_memory
• next to put there our 'X'
• Let's make sure: print *video_memory and look at the qemu screen. It's definitely there.

Now is a good time to read some tutorial on gdb and learn super useful things like info registers which will save us a lot of time in the future!

You may notice that, since this is a tutorial, we haven't yet discussed which kind of kernel we will write. It will probably be a monolithic one since they are easier to design and implement, and after all this is our first OS. Maybe in the future we'll add a lesson "15-b" with a microkernel design. Who knows.