In this lesson, you will learn how to orchestrate the final hand-off from your low-level assembly startup code to your high-level C main function.
The Final Bridge
Up to this point, our startup sequence has been focused on preparation: we defined the vector table, set the stack pointer, copied initialized data from Flash to RAM, and cleared the .bss (zero-initialized) segment. At this stage, the processor’s environment finally matches the expectations of a C program. The memory is ready, the stack is positioned, and the global variables are set to their required starting values.
The final step is to "jump" from the architectural environment into the C application logic. Because the C language standard assumes the existence of a main() function as the entry point for the user’s code, we must explicitly transfer control to it.
Executing the Branch
In most architectures (like ARM Cortex-M), this is accomplished using a branch instruction. Since we are moving from assembly (the Reset Handler) to C, we must ensure we invoke main() in a way that the compiler recognizes as the start of the application logic.
Consider this snippet of a Reset_Handler:
// ... (previous steps: stack setup, data copy, bss zeroing)
// Branch to the main function
bl main
// Fallback if main() ever returns (it shouldn't!)
loop:
b loop
Why a Loop?
You might notice the loop: label after the branch to main. In embedded systems, main() is intended to be an infinite loop (e.g., while(1) { ... }). If, for any reason, your code reaches the line after bl main, it means the application has unexpectedly exited. Since a microcontroller has no "Operating System" to return to, we place the processor into an infinite branch-to-self loop to prevent it from executing random memory contents (garbage data) that follow the function call.
By successfully branching to main, you have completed the bridge between raw silicon and your C-based firmware, effectively "booting" your application.
Key Takeaway: Linking to main is the final hand-off where the startup sequence finishes environment preparation and transfers execution control to the C-defined entry point, ensuring the processor enters the application's infinite execution loop.