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.
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; } |
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!
qemu-aarch64.
Our first AArch64 assembler program
Let’s write a very simple program that just returns an error code of two.
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.
This work by Roger Ferrer Ibáñez is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International
A tiny GCC front end – Part 11 How (not) to write a C++ front end – Part 1
Nice to see there are other people who are into the “deeper” layers and ARM as well.
good to know! Thanks.
this is not my field of expertise so I’m afraid I cannot give you any useful pointers or resources for an AArch64-based OS.
I’ll let you know if I learn of any project like this.
Kind regards,
Roger
Some components haven’t been ported yet, and some don’t have full functionality. But through your chapter 3, at least, the code compiles with gcc and runs, and gdb works (though without full debug info). Though “perf” works, it doesn’t have all the counters implemented — just gives timings, at least on the openSUSE I’m running.
It appears that Fedora 26 will also have an aarch64 Raspberry Pi-3 implementation; last I saw, it was tentatively expected in June, 2017.
Looking forward better support of 64-bit software in the Raspberry Pi 3.
this is now true but it wasn’t when I wrote that post. I will amend it, though.
Kind regards,