"Hans Franke" <franke(a)sbs.de> wrote: > Yes, but as a rule, mainframes and most minis catch undefined > opcodes in a special exeption.

That's only a 'rule' for recent machines. Most of the early ones didn't.

When IBM was developing the 1401 emulation for the 360/30, they discovered that many customers were in fact using and dependent on undefined behaviors of the 1401. They had to emulate a lot of things that were never documented.

References: _The Mythical Man Month_ by Brooks, _Computer Architecture: Concepts and Evolution_ by Brooks and Blaauw, _IBM's 360 and Early 370 Systems_ by Pugh, Johnson, and Palmer