(There’s a Twitter URL for it, but I’ve lost it, sorry.)
Emulating a simple bootloader

Generally speaking, emulating a bootloader is simpler than it is for regular binaries, because they lack external libraries and usually have direct access to memory and hardware. In this case, the bootloader is a binary for x86 architecture which runs in 16-bits real mode using BIOS calls to perform its loading duties and textual input/output. The idea here is to emulate Cropta1 crackme using radare2 ESIL emulation, providing the needed BIOS via a trivial quick & dirty python implementation of just what it’s needed to run the crackme code. There are several ways to do it, I tried two of them and here is the story. […]

http://radare.today/posts/emulating-simple-bootloader/