How to do source level debugging of x86 code with GDB inside QEMU?

Question

I wrote a x86 assembly program for MBR section. I compile it as follows:

nasm hellombr.asm -f bin -o hellombr.img

Then I run it in qemu:

Answer 1

You should let nasm create the debugging symbols in an ELF file and then dump this to a flat binary to be used in the MBR. You can then instruct GDB to read the necessary symbols from the ELF file.

The complete procedure would then become something like this:

$ nasm hellombr.asm -f elf -g -o hellombr.elf
$ objcopy -O binary hellombr.elf hellombr.img
$ qemu -s -S -fda hellombr.img -boot a
$ gdb
(gdb) symbol-file hellombr.elf
(gdb) target remote localhost:1234

For an explanation of the flags I pass to qemu see this answer.

Answer 2

Instead of using qemu, use bochs. It is completely compatible, albeit slower. It is also an emulator but if you make it from sources, using these flags and build it like this:

./configure --enable-debugger --enable-disasm --disable-docbook
make
make install

you can place breakpoints in your code, step through it, view GDT, IDT and everything you needed to know.

Answer 3

A really good (and simple) way is to use IDA with bochs, you find an excellent blog post on it here, along with some other hints/suggestions for bootloader development.