I was able to verify few operations on basic arguments and compare them to real Sinclair Scientific for match. Following problems remain:

- LED blurs at higher frequency. Calculation can be done at 12.5MHz but LED is blurry above 40kHz or so

- breakpoint and single step logic needs to be laid out more logically on the switches, and dual clock unit avoided

- VGA tracer still drops some character from display.

Note that none of the above impacts calculation. I also briefly tested TI mode, seems no regression was introduced there.