Program and learn 64-bit ARM Assembly (AARCH64)

This is a short summary of Low Level Learning's video.

Hello World assembly:

# AARCH64 

.global _start
.section .text

_start:
  # Write to console
  mov x8, #64
  mov x0, #1
  ldr x1, =message
  mov x2, #13
  svc 0

  # exit savely
  mov x8, #0x5d
  mov x0, #0x41
  svc 0


.section .data
  message:
    .ascii "Hello World\n"

We can compile and execute this example with:

$ aarch64-linux-gnu-as -c example.S -o example.o
$ aarch64-linux-gnu-gcc example.o -o example.elf -nostdlib -static
$ qemu-aarch64 ./example.elf

We can also use our self compiled cross compiler for that task if we want.

Systemcalls in AARCH64

To invoke a syscall we search in the table for the systemcall and load the given values into the specific registers. In the example above to to exit savely we need to mov x8, #0x5d, this moves #0x5d to x8. According to the table #0x5d is the number of the system call to exit a programm. The value we load to x0 is the argument of the systemcall in this case it is the error code (we can load any value here). The other example is a bit more complex but basically the same, we just need to load (ldr x1, message) the message to the specified register and move the length of the string (13bytes) to x2.

AARCH64 syscall table

https://chromium.googlesource.com/chromiumos/docs/+/master/constants/syscalls.md#arm64-64_bit

Source

https://www.youtube.com/watch?v=8NdrdxkBP3U