Exploring AArch64 assembler – Chapter 1

AArch64 is a new 64 bit mode that is part of the ARMv8 architecture presented in 2011 by ARM. It has been progressively been deployed in smartphones and servers. So I think it is a good moment to learn a bit more about the assembler of this architecture.

Hardware availability

Single board computers with ARMv6/ARMv7 architecture are easily available nowadays. One of the most popular choices is the Raspberry Pi.

In contrast, single-board computers that support the 64-bit mode of ARMv8 are less common but they are slowly becoming more popular these days. For instance the Pine64, the ODROID-C2, the Dragonboard 410c, etc. Any of them will do and in general they differ on the specific System on Chip being used.

Note: the Raspberry Pi 3 has a CPU (Cortex-A53) that implements the 64-bit mode of the ARMv8 architecture and technically could run a 64-bit system. But the software system provided by the Raspberry Foundation (Raspbian) is only for 32-bit and there are no official plans for a 64-bit system.

Software alternative

Does this mean that without hardware it is not possible to play with AArch64? No! We can still do many things using a cross-toolchain and QEMU in user mode.

Example for Ubuntu 16.04

Just install QEMU and a cross-toolchain for AArch64.

$ sudo apt-get install qemu-user gcc-aarch64-linux-gnu

Now test you can run a "Hello world" written in C. Create a hello.c file with the following contents.

#include <stdio.h>

int main(int argc, char *argv[])
{
  printf("Hello AArch64!\n");
  return 0;
}

Now compile it with the cross-compiler for AArch64 that we have installed earlier (the -static flag is important).

$ aarch64-linux-gnu-gcc -static -o hello hello.c

Check it is a AArch64 binary.

$ file hello
hello: ELF 64-bit LSB executable, ARM aarch64, version 1 (SYSV), statically linked, for GNU/Linux 3.7.0, BuildID[sha1]=97c2bc66dbe4393aab9e4885df8e223a6baa235a, not stripped

Trying to run it should fail with some confusing error.

$ ./hello 
-bash: ./hello: No such file or directory

But we can run it using the QEMU for AArch64 that we installed earlier.

$ qemu-aarch64 ./hello
Hello AArch64!

Yay!

Note: If you use this option, remember always to run your programs using qemu-aarch64.

Our first AArch64 assembler program

Let's write a very simple program that just returns an error code of two.

1
2
3
4
5
6
7
8
// first.s
.text

.globl main

main:
     mov w0, #2
     ret

Let's assemble it.

$ aarch64-linux-gnu-as -c first.s

And now link it, for convenience we will use gcc.

$ aarch64-linux-gnu-gcc -static -o first first.o

Run it and check the return.

$ ./first             # or use qemu-aarch64 ./first
$ echo $?
2

Yay!

Let's go through each line of the code above.

1
2
// first.s
.text

Line 1 is just a comment with the name of the file used in this example. Any text in a line that follows a // is a comment and it is ignored. Line 2 is an assembler directive that means "now come instructions of the program". This is because we can also express data in an assembler file (data goes after a .data directive).

4
.globl main

This is another assembler directive that means main is going to be a global symbol. This means that when constructing the final program, this file will have the global main symbol that is needed by the C library to start a program.

6
7
8
main:
     mov w0, #2 // w0 ← 2
     ret        // return

This is the entry point of our program. Line 6 itself is just a label for the symbol main (that we mentioned above it was a global symbol). Lines 7 and 8 are two instructions. The first one just sets the register w0 to be 2 (we will see what registers are in the next chapter). The second one returns from the main, effectively finishing our program.

When finishing a program, the contents of the register w0 are used to determine the error code of the program. This is the reason why echo $? above prints 2.

Reference documentation

Documentation for the AArch64 instruction set can be found in the ARM® Architecture Reference Manual ARMv8, for ARMv8-A architecture profile

That's all for today.