'bootlib' is a tiny library written in x86 assembly which should provide you just enough functionality to let you make your own bootable program (in assembly) without too much trouble.
It is more or less a collection of the parts of x86502 which are useful for other programs as well.
If you have any questions about it, feel free to contact me.
The source code is hosted on GitHub: https://github.com/m-ou-se/bootlib
The full package can be downloaded as a tarball from: https://github.com/m-ou-se/bootlib/tarball/master