I'm also making space invaders emulator and still have some problems which prevents playing the game.
At the moment everything looks like it should until to the part where invaders should come to the screen and then the screen wipes black and resets the program. Same thing happens if I insert coins and press start.
I think the shift register works like it should but I don't know why does this happen.
Other thing is that I got bored playing with the space invaders and tried out the balloon bomber and lunar rescue roms also as they should work with the same hardware I think. But I don't get it how can they work if the rom size is bigger than space invaders 8KB < 12KB. Then they try to set the stack pointer to 0x2400 like space invaders. Isn't that rom area? Anyway as result I get messed graphics in the title screen because it is writing over the program area