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.
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.
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.
Now test you can run a "Hello world" written in C. Create a
hello.c file with the following contents.
Now compile it with the cross-compiler for AArch64 that we have installed earlier (the
-static flag is important).
Check it is a AArch64 binary.
Trying to run it should fail with some confusing error.
But we can run it using the QEMU for AArch64 that we installed earlier.
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.
And now link it, for convenience we will use
Run it and check the return.
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
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
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.