I had the same experience when doing an exercise implementing `mmap` for `xv6` -- that was the last lab. There was no specification except for a test file. Passing that test file is relatively easy and I could game it. I consulted the manpage of `mmap` but it is pretty far from a specification, so eventually I had to write a lot of tests in Linux to figure out what it can do and what it can't do (what happens when I over-mmap? what happens when I write back pass EOF? etc.), and write the same tests for `xv6` so that I could test my implementation. Not sure about hardware, but it is really hard to get a clear specification for software.