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aarch64 Assembly

Baby Steps

Alt Text

Introduction

This project is long overdue. I've been developing applications for devices that contain ARMv8 processors for a long time, and I have wanted to break into the assembly language. Here we go!

Raspberry Pi 3

For this project, I chose to use a Raspberry Pi 3 for development. I figured I could cross-compile and then use qemu or something else, but I found that I was happier with just spinning up a RPI, connecting it to the network, and accessing the device via a secure shell (ssh).

With all this in mind, I'm running Fedora Minimal 37 on the Rasperry Pi 3, and will outline several of the packages I selected to assemble, link, and test this project, as well as debug it with GCC.

Assembling

as -g -o <filename>.o <filename>.s -march=armv8-a

Linking

ld -o <filename> <filename>.o

Debugging with GDB

gdb <filename>

b _start

r

info registers

stepi or nexti (step into or step over)

Note, if you would like to see the effect of your code, use stepi. If you would like to skip a line, use nexti.

Here are some useful GDB commands:

  • start: start the program and pause before executing the first instruction.
  • run: start the program and continue to the next breakpoint, if any.
  • break: set a breakpoint at a line or address where you want to pause the program.
  • delete: delete a breakpoint.
  • info break: show all breakpoints.
  • continue: continue the program until the next breakpoint.
  • next: execute the next line of source code (step over function calls).
  • step: execute the next line of source code (step into function calls).
  • finish: continue execution until the current function returns.
  • print: display the value of a variable or expression.
  • display: display the value of a variable or expression after each step or continue.
  • x/nfu addr: examine the memory at a given address, with the following options:
    • n: number of units to display (default is 1).
    • f: format (e.g. x for hexadecimal).
    • u: unit size (e.g. b for byte, w for word, g for giant).
  • info registers: display the contents of all registers.
  • info registers register: display the contents of a specific register (e.g. info registers x0).
  • layout asm: show the source code and assembly code side-by-side.
  • layout reg: show the source code and register contents side-by-side.
  • layout split: show the source code, assembly code, and register contents in separate windows.
  • quit: exit gdb.

Note on ARMv7 vs ARMv8