Ha. I have a love-hate relationship with the MC6805. On the one hand, I built a lot of devices based on the 68HC05 series. On the other, having to implement a 16-bit division routine on one was what made me swear off assembly language for good.
it would be reductive to chalk up the sound of the 80s to any one synth. Especially when the Fairlight was responsible for a number of iconic songs:
http://ghservices.com/gregh/fairligh/topsong/
https://www.righto.com/2021/11/reverse-engineering-yamaha-dx...
Well I found the schematics: it uses a 6805 and a 6303!
http://www.midimanuals.com/manuals/yamaha/dx7/schematics/yam...