alt Hacker NewsNot only was it microcoded, but it was sufficiently divorced from the assumptions of the 68000 instruction set that IBM were able to have Motorola make custom "68000-based" chips that ran S/370 code directly.
Want a different architecture? Sure, just draw it with a different ROM. Simple (if you've got IBM money to throw around).
I read (30 years ago) the book "Microprocessor Design" by Nick Tredennick. He was the architect and wrote the microcode for both the 68K and the S/370. The S/370 was based on his recent design experience with the 68K, but it wasn't just a microcode swap. In the book he describes his process where he would write the ucode for each instruction on a 3"x5" card (or was a 4"x7"). At times he'd find sequences that were too clunky and then go back to the circuit design folks and ask for them to add some extra logic for that corner case to make the ucode more efficient.
The book also had a glossary section in the back and a number of the entries were funny. One I recall was his definition for "methodology", which was something like "A word people use when 99% of the time they mean 'method'."