[cctalk] Re: IBM 360 and 1400 series emulation

Nearly all the 360s were microcoded, so adding a bit more microcode let them emulate 1400/7000 series computers as a standard optional feature. (well the model 44 emulated the 1620, and probably the 95/195 could not emulate anything since they were hard wired). I do not recall that was documented in the Principles of Operation. Does anybody here know how emulation was turned on (and off)? was there some special bit or bits in the PSW, or an undocumented instruction? --Carey