Thank you for sharing this. That was my first question, i.e. was the star background hardcoded (or at least statically generated)? That's super interesting that it was built-in at all. I suppose so many shooter games had starfield backgrounds that it made sense at the time to include it.
https://github.com/mamedev/mame/blob/master/src/mame/video/c...
edit: didn't see that "col" also contained an x-offset. So like (256*16)x256 but presumably "sparse", so I get more where the "star" idea comes from