latexman
Thinks s/he gets paid by the post
It wasn't off a mainframe, but I remember playing a Star Trek game from a friend's Atari home system circa 1979-1980. It was written in Basic, and my friend changed a few lines of code to make the game more difficult. I think it got loaded from a cassette tape, a common method when floppy disk drives were not yet standard.
The main (and fun) part of the game was when the Enterprise entered a grid (a sector in a galaxy) with multiple Klingon ships to battle against. Am I describing the game you mentioned?
Yes, sounds similar, but that was a lonnnnng time ago.