Re: Microcode is hard
In 1966 the RCA Spectra 70/45 mainframe had microcode that could be software selected for either IBM 360 compatible mode - or second generation RCA 501 (English Electric KDF8) mode.
The ICL 2903 mini-computer range was programmed at microcode level.
The IBM 370 compatible mainframes like the Fujitsu/ICL Atlas range in the 1980s had microcode that could be loaded from floppy disc. This offered various processing speed upgrades as and when the customer required them - without major disruption.