Call To ARMs
My 4th semester involves ARM programming. And proprietary tooling (Keil C). But we don’t do that here.
Building
Assembling and linking ARM binaries on non-ARM architecture devices
is fairly trivial. I went along with the GNU cross bare metal toolchain
binutils, which provides arm-as
and arm-ld
(among a bunch of other utils that I don’t care about for now).
Assemble .s
files with:
arm-none-eabi-as main.s -g -march=armv8.1-a -o main.out
The -g
flag generates extra debugging information that
gdb
picks up. The -march
option establishes
target architecture.
Link .o
files with:
arm-none-eabi-ld main.out -o main
Running (and Debugging)
Things get interesting here. gdb
on your x86 machine
cannot read nor execute binaries compiled for ARM. So, we simulate an
ARM processor using qemu
. Now qemu allows you to run
gdbserver
on startup. Connecting our local gdb
instance to gdbserver
gives us a view into the program’s
execution. Easy!
Run qemu
, with gdbserver
on port
1234
, with our ARM binary, main
:
qemu-arm -singlestep -g 1234 main
Start up gdb
on your machine, and connect to
qemu
’s gdbserver
:
(gdb) set architecture armv8-a
(gdb) target remote localhost:1234
(gdb) file main
Reading symbols from main... # yay!
GDB Enhanced
gdb
is cool, but it’s not nearly as comfortable as well
fleshed out emulators/IDEs like Keil. Watching registers, CPSR and
memory chunks update is pretty fun.
I came across gdb
’s TUI mode (hit C-x C-a
or type tui enable
at the prompt). TUI mode is a godsend.
It highlights the current line of execution, shows you disassembly
outputs, updated registers, active breakpoints and more.
But, it is an absolute eyesore.
Say hello to GEF! “GDB Enhanced Features” teaches our old dog some cool new tricks. Here are some additions that made my ARM debugging experience loads better:
- Memory watches
- Register watches, with up to 7 levels of deref (overkill, I agree)
- Stack tracing
And it’s pretty! See for yourself:
Editing
Vim, with syntax off
because it dosen’t handle GNU ARM
syntax too well.
I'm Akshay, programmer and pixel-artist.
I write open-source stuff to pass time. I also design fonts: scientifica, curie.
Send me a mail at nerdy@peppe.rs or a message at np@irc.rizon.net.