Re: Upwards and downwards compatible, Computer architects leaving Intel...

According to Thomas Koenig  <tkoenig@netcologne.de>:

I have seen plenty of undefined behaviour in ISA's over the years.  (A
very common case is that instruction encodings that are not specified
are left as UB so that later extensions to the ISA can use them.)

>
A much better idea is to raise an exception, that way you can
be sure that nobody uses it for nefarious purposes.

That was one of the innovations of S/360.  Every unused opcode traps
with an operation exception.  This both avoided games with undocmented
features, and let them provide limited upward compatbility in software.
The 360/91 did not have the decimal instructions, but a module in OS/360
caught the traps and simulated them so programs would run, albeit slowly.

I also understand that they did a lot of development of early S/370
software on a version of CP/67 that trapped and simulated the new
instructions before the S/370 hardware was ready.

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