I actually wrote that idea (two different signs) in the post but took it out :-) My thinking is that the hardware must support negation, so it would save space

I actually wrote that idea (two different signs) in the post but took it out :-) My thinking is that the hardware must support negation, so it would save space to have one constant and negate it instead of two constants with different signs. I'm still investigating the chip, so I hope to find the exponent ROM (or Boolean logic as you suggest) and solve this puzzle.