xx_initcall ( ) in kernel

Dear friends, I want to know the basics xx_initcall ( ) in linux kernel. How this function works ? Observed in kernel code, many places late_initcall ( ), early_initcall ( ), arch_initcall ( ), core_initcall ( ) gets called. When did all these functions gets call during kernel initialization ? I am more interested in what is the sequence of these functions gets called one after another. Thank you. Regards, Sumeet