delay function for STM32F

Dear All,
please provide header file for delay function

I mean I need constant delay even processor frequency changed

Ex;

delay_ms(500);

Currently I am using following delay function which is varying when change the system clock

void Delay(__IO uint32_t nCount)
{
  while(nCount--)
  {
  }
}

please also advice what is the deferment between flowing veritable types

__IO uint32_t
uint32_t

Thanks in advnace