Add support for sub-registers #22
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Currently, we only address whole registers, with no sub-indices. Therefore headers are always one byte long, and `make_header` could just return that byte. I'm currently working on support for the sub-registers that can only be addressed using sub-indices. For that I need to make `make_header` more flexible.
This is the only of the three access methods that needs to care about header length. Both `write` and `modify` use buffers that are provided by the `R`/`W` types and rely on those types to not mess up the buffer management.
I'm about to add more information to the register definition, which adds more characters to the line. I don't have space for those, if the comments stay where they were before this commit (comments would have spilled into the next line). So yeah, my dream of keeping those comments lined up forever has officially died.
Currently, the sub-index is `0x00` for all defined registers, and the code only supports this value. The next steps will make the code more flexible, until registers with a real sub-index can be added.
I think this leaves only `init_header` as the last piece of code that assumes an hard-coded header length of `1`.
This is the last change required to fully support sub-indices.
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Adds support for sub-registers (registers that can only be accessed with a sub-index) by extending the register definition macro syntax and updating all code that touches headers to support any header size.
This pull request only contains changes to infrastructure, but I have local code (coming soon) that adds support for sub-registers and tests that all of this actually work.