Common Tasks | Building your first embedded image

Write and compile "Hello World!"

Let's start with a simple C program, and use the armclang and armlink tools to compile and generate an executable image.

In your command-line terminal, use your favorite editor, for example, vi, to create a new file called hello_world.c with the following contents:
```
#include <stdio.h>

int main(void) {
  printf("Hello World\n");
  return 0;
}
```
Compile the C code to object code with armclang:
```
$ armclang -c -g --target=aarch64-arm-none-eabi hello_world.c
```
This command tells the armclang compiler to compile hello_world.c for the Armv8-A architecture and generate an ELF object file hello_world.o.

The options used in this command are:
- -c tells the compiler to stop after compiling to object code. We will perform the link step to create the final executable in the next step.
- -g tells the compiler to include debug information in the image.
- --target=aarch64-arm-none-eabi tells the compiler to target the Armv8-A AArch64 ABI.
Create an executable image by linking the object using armlink. This generates an ELF image file named __image.axf:
```
$ armlink hello_world.o
```

Because we have not specified an entry point, when you run this image the entry point defaults to __main() in the Arm libraries. These libraries perform a number of setup activities, including:

Copying all the code and data from the image into memory.
Setting up an area of memory for the application stack and heap.
Branching to the main() function to run the application.

This diagram illustrates the code startup sequence that shows how control passes from the C library to your code:

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