< When IBM was developing the 1401 emulation for the 360/30, they discover < that many customers were in fact using and dependent on undefined behavi < of the 1401. They had to emulate a lot of things that were never docume This was also true for undocumented PDP-8 series microcoded OPR instructions. In the later 8E and 8A and cmons parts there would be support for some of them. Other cpus with undocumented instruction are 8085, z80 and I'll bet a slew more. Those however the undocumented instructions were also copied and assured to work by licensed and non-licensed copies of the parts. Allison