One of the main technical nuances about the new Eclipse box mentioned in the book was the management edict for "no mode bit" in the machine architecture/CPU.
The book never explains how the team solved this restriction.
One of the tricks was that exceptions/traps/interrupts were handled in a neat way.
The microcode looked at the first instruction of the handler. If it was an old instruction, a backwards compatible stack frame was created. If it was a new instruction, a new-style stack frame was created.
Data General published a couple of papers about their new 32-bit architecture.
The book never explains how the team solved this restriction.
Someone asked that exact question and Carl Alsing himself (head of the Microkids) answers the question, see https://cs.stackexchange.com/questions/44915/data-general-mv...