Stack alignment in x64 assembly

Question

how is the value of 28h (decimal 40) that is subtracted from rsp calculated in the following:

    option casemap:none

    includel         


        

Answer 1

I believe it's because before main is called, the stack is aligned. Then after the call, the act of the call was to push an 8-byte pointer (address of the caller) onto the stack. So at the beginning of main, it's 8 bytes off of the 16-byte alignment. Therefore, instead of 20h you need 28h, bringing the actual total to 28h + 8h (from the call) or 30h. Alignment. :)