Electronic – What happens before main()

cembedded

I am working on embedded systems as beginner and have come across files like start.s or cstart files that run before main() function begins. What is the purpose of these or similar files? What information we are telling the system? I've heard of initialization but don't know exactly what that is.

Best Answer

It is completely dependent on the compiler and architecture, but generally that code initializes the most basic hardware required for the rest of the code to run. The code for example:

  • Defines the reset vectors

  • Defines the layout of data in memory (many systems use a linker script instead)

  • Defines the addresses of interrupt service routines in a big table (the interrupt vector table)

  • Initializes CPU registers, e.g. the stack pointer

  • Configures the core clock

In addition, that section also serves the runtime needs of the programming language used. It:

  • Initializes whatever function parameter passing system used

  • Initializes global variables by e.g. copying flash contents to RAM and zero-initializing memory

  • If dynamic memory allocation is used, initializes the heap

  • If floating point math is enabled, initializes the FPU (if available) or initializes the floating point library

  • If exceptions are used, initializes exception handling.

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