Debugging bootloader with gdb in qemu

问题

There seems to be a problem with the Freedos bootloader. (It appears that the bootcode can't find the kernel in certain circumstances.)

So I'm trying to debug the bootloader in qemu with gdb. Following the instructions found on several wiki and freely available online course materials, I run qemu like this

qemu-system-i386 -fda fdboot.img -boot a -s -S

And then connect gdb like this

$ gdb
(gdb) target remote localhost:1234

I can step through the first 10 - 12 instructions with si which I assume is the SeaBIOS.

But past that, when I try to step into bootloader code, it continues execution without breaking, all the way up to the FreeDos menu prompt. This totally skips the bootloader code which I would like to examine step by step as it is executed.

What do I need to do so that I can step though the bootloader?

[You can download the freedos floppy images from the project website if you want to try yourself.]

回答1:

Works fine here using qemu 1.3 and gdb 7.3.50.20111117 (you didn't say what versions you used). I was able to single step tons of instructions until I got bored and placed a breakpoint to catch the bootloader:

(gdb) br *0x7c00
Breakpoint 1 at 0x7c00
(gdb) c
Continuing.

Breakpoint 1, 0x00007c00 in ?? ()
(gdb) x/i $eip
=> 0x7c00:      jmp    0x7c3e

Note that I have set gdb to 16 bit mode first using set architecture i8086.

来源：https://stackoverflow.com/questions/14242958/debugging-bootloader-with-gdb-in-qemu