#+date: <2015-02-20 00:00>
#+title: GDB Registers

This will serve as a brief guide on setting up GDB to step through
assembly code and witness register changes on the fly.

** Compile the program to debug
:PROPERTIES:
:CUSTOM_ID: h-adac1e41-906a-43a7-9f03-1237daaabf01
:END:

I'm assuming here that you have the source of the C file that you wish
to step through(this post assumes you are trying to learn about
registers for educational purposes).

Create a Makefile for your architecture(s) of choice. Here is an example
of a Makefile for 3 of them:

#+begin_src makefile
all: 32bit 64bit 64bitInLine

32bit: MyProgram.c gcc -g -O1 -fno-inline -fno-omit-frame-pointer -m32
MyProgram.c -o MyProgram

64bit: MyProgram.c gcc -O1 -fno-inline MyProgram.c -o MyProgram\_64bit

64bitInLine: MyProgram.c gcc -O1 MyProgram.c -o MyProgram\_64bit\_inline
#+end_src

This should result in 3 output files with different assembly associated
with each.

** GDB
:PROPERTIES:
:CUSTOM_ID: h-5fc9a60c-09e9-422a-8f37-df6437afc10c
:END:

The GNU Project Debugger(GDB) is a fantastic tool that I am still
learning about. If you have no idea what it is, it is a fairly powerful
utility for debugging primarily c/c++ as I currently understand it. I
recommend this [[http://beej.us/guide/bggdb/][crash course guide]] for a
quick intro. One of it's many features is the ability to step through
and view registers as they change(the purpose of this post).

So, to run GDB on a program made for this purpose you can use: {%
highlight bash linenos %} $gdb MyProgram
#+end_src

once at the GDB prompt, set a breakpoint at the main() function and then
run the program - this will cause the breakpoint to instantly be hit.

#+begin_src bash
(gdb)break main
(gdb)run
#+end_src

The command we can use from here to step through the program by assembly
line is 'si'(step instruction). However, we would not be able to see the
values of registers as they change on the fly - this is accomplished
with a tui view. You can activate the register tui layout with the
command =layout reg=, which will result in something like this:

#+CAPTION: GDB
[[http://i.imgur.com/SZkGALi.png]]

And now you can step through each instruction using 'si'. As registers
change value, they will be highlighted in the top window. Have fun!