Many older games from the dawn of computer based ('solid state') pinball have some pretty basic rules, and I've often pondered hacking the roms to improve them, but reverse engineering assembly from scratch isn't really my idea of fun so I never really got into it. Driving home one night though, I got to thinking: the hardware for these machines is pretty simple, the schematics are available (and even have memory addresses!), would it really be that hard to just write a brand new rom from scratch? Sure, I didn't know assembly or anything, but it couldn't be that hard!
So I sat down with a copy of the Motorola 6800 reference manual, some data sheets, the schematics, and a copy of PinMAME to debug my code in, found where the entry point/address was, and started coding.
Long story short, it actually was pretty easy! The williams hardware was designed for simplicity from a programming standpoint (though that ended up making the boards overly complicated and unreliable), all the I/O was easy to work with and within a few days I had simple drivers written for all the different peripherals and a basic 'OS' for a game that could control lights, display scores, and jump into a callback table when a switch was hit by the ball.
Programming a game itself using this would prove a bit more complicated of course, due mostly to timing/threading issues (of course, I just had to write a custom threading system for this 1MHz processor), but nothing extreme. A few months later, and I had a brand new rom for my Hot Tip, with many fancy new features which I showed off in a video here
I didn't document the whole process very much, but if you'd like to know more, or would like to try to write your own new game rom using this as a basis, drop me a line!
The full source code and roms are available at http://github.com/zacaj/williams-sys-3-6