So I've been continuing to play around with AVR assembly and struggling to get things working. With my first full-time job underway, I've had limited time to devote to this hobby, and I don't have all of the tools that I should have to properly test my prototype systems. However, I still have had the nagging suspicion that I was approaching the development process the wrong way. I finally realized what was missing while listening to an episode of Elicia White's Making Embedded Systems podcast that discusses test-driven development (TDD) for embedded systems. At first, I simply refused to believe that TDD and bare-metal programs were compatible, but the more I thought about it, the more I understood that the process of modularizing your code and running each module one at a time in a specially-configured environment was almost exactly the same.
That podcast, though, only discussed the semantics of testing C programs, which are easily compiled for almost any platform out there (as long as there aren't any platform-specific elements in your program, which should be discouraged). Assembly programs, though, are the least portable of all. A stable hardware environment with debugging support or an architectural simulator is required. Fortunately, Simulavr is available for AVR programs (though it only supports a limited subset of all devices). In addition, its developers have implemented a nice Tcl interface for it, which makes the process of writing unit tests much easier.
I have started a simple project to implement and test the above concepts. The files can be found on GitHub. Downloading the files (as well as installing Tcl, Simulavr, and all of their prerequisites) and running 'make test' launches the testing process. As of this writing, it is still not completely functional due in part to the lack of documentation for the simulator interface, but some more experimentation should produce a system that is able to test individual assembly routines by providing arguments and testing outputs via core registers. If it becomes robust enough, I can hopefully merge it into my other projects to help with their development.