How much diferrent and compatible is DragonFlyBSD to FreeBSD?

[rempas at tutanota.com]

When it comes to the core of the operating systems themselves (mostly the kernel), how much difference these two operating systems have? I know that DragonFlyBSD was a fork of FreeBSD so I suppose that they have most things in common. But what are different? For example, has DragonFlyBSD changed the numbers (or even the functionality) for the basic system calls? Or maybe changed some other stuff? Hence I wonder if there is a lot of works that has to been done to port FreeBSD binaries to DragonFlyBSD. Sorry if this question sounds too n00by but I want to know how much compatibility there is with FreeBSD.