Exploring ARM Assembly on the Raspberry Pi: A Hello World Journey

Recently, I embarked on a journey to create a "Hello World" program on a Raspberry Pi 4 using ARM assembly language, and the experience was both enlightening and rewarding.

The Raspberry Pi is a versatile single-board computer that has captured the attention of hobbyists, educators, and professionals and others alike. Its low cost, small form factor, and GPIO pins make it an ideal platform for tinkering.

At the heart of Raspberry Pi's CPU is Quad-core ARM Cortex-A72 clocked at either 1.5 GHz or 1.8 GHz. I had the Pi lying around in dust for some time, so finally took pity and powered it up.

Below is the code breakdown that I tried to get to display Hello World on the raspi terminal (after many tutorials and man page searches):

.text

.global _start                 // Provide program starting address to linker 
_start:
        mov     x0, #1         // Prepare syscall number to write to stdout
        ldr     x1, =msg       // Load address of the message
        mov     x2, 13         // Set the length of the message
        mov     x8, #64      // Linux write to syscall
        svc     0                  // Invoke syscall to write

        mov     x0, #0        // Prepare syscall exit status
        mov     x8, #93     // Prepare syscall number for terminate program
        svc     0                 // Invoke syscall to exit

.data

msg:
        .ascii "Hello World\n" // Define the message        

Let's dissect what's happening here:

1. We begin by defining the text section where our code resides ( .text).
2. _start serves as the entry point of our program, denoted by .global _start.
3. We load the syscall number for writing to stdout into x0, the address of the message into x1, the length of the message into x2, and the syscall number for syscall into x8. Then, we invoke the syscall using svc 0 to write the message to the standard output.
4. Next, we load the exit status value into x0, prepare the syscall number in x8, and invoke the syscall using svc 0 to gracefully exit the program.
5. The .data section is used to define our message, "Hello World".

I ran the above by saving the code snippet in helloworld.s and running the below steps:

> as -o hello.o helloworld.s
> ld -o hello hello.o
> ./hello
Hello World        

This was a simple program to demonstrate the fundamentals of ARM assembly language programming on the Raspberry Pi, that may also get your foot into the door (and also a good way to occupy my Sunday)