ChibiOS Free Embedded RTOS

• Root functions cannot be called by any other function.
• No register preserved through the call in order to save stack space.
• Can be called by assembler code only.
• The main() function is implicitly a root function.
• RTOS tasks can be declared as root functions saving RAM.
• IRQ handlers can be declared as root functions saving RAM if called by an assembler veneer.

  root void foo(void) {
    ...
  }

• Functions only accessing its own parameters, not even const global data.
• Can take pointers as parameters but cannot dereference.
• Const functions can only call const functions.

  const double sqrt(double x) {
    ...
  }

• Functions only accessing its own parameters and const global data.
• Can take pointers as parameters but cannot dereference for write.
• Pure functions can only call pure or const functions.

  pure uint32 lookup(const uint32 *table, int index) {
    return table[index];
  }

• Asynchronous function do not do any special processing required by ISRs, for example saving/restoring registers, it is a pure attribute to a normal function. Interrupt-related attributes are eventually added using __attribute__ because can be architecture-dependent.
• Asynchronous functions can preempt other asynchronous functions.
• Asynchronous functions can only call async (of the same class), pure or const functions.
• Asynchronous functions can only access const global variables or automatic variables.
• Asynchronous functions can be called by other async functions of the same class.
• Access to global variables can be enabled by the use of exclusive compound statements in order to enforce arbitration.

async <<class>> <return type> <function name> (<parameters list>|void )

/* Defines and asynchronous function of class "ISR".*/
async <ISR> void my_isr2(void) {
}