using System;

public class Program

{

    public struct ULARGE_INTEGER

    {

        public UInt32 LowPart;

        public UInt32 HighPart;

    }

​

    public static void Main()

    {

        ULARGE_INTEGER a;

        a.HighPart = 0x80123456;

        a.LowPart = 0x789ABCDE;

        UInt32 m = 585741;

        //Calculate 0x80123456789ABCDE % 585741

        UInt32 r = Program.Mod64(a, m);

        Console.WriteLine("0x{0:x8}{1:x8} mod {2} = {3}",

                a.HighPart, a.LowPart, m, r);

    }

    public static UInt32 Mod64(ULARGE_INTEGER a, UInt32 m)

    {

        /*

            Calculates

            r = a mod m

​

                a.hiPart: High 32-bits of UInt32 a

                a.loPart: Low  32-bits of UInt32 a

                 m: modulus

            In other words:

                r = Ah||Al mod m

        */

        //I have no idea if this overflows some intermediate calculations

        UInt32 Al = a.LowPart; 

        UInt32 Ah = a.HighPart;

​

        UInt32 remainder = (((Ah % m) * ((0xFFFFFFFF - m) % m)) + (Al % m)) % m;

​

        return remainder;

    }

}