🚀 go-pugleaf

> Leonard, I think you are splitting hairs here, that kind of assist
> "instruction" is really just a subroutine of the operating system that IBM
> has decided to make faster by implementing in microcode.  They know a lot
> about the operating system in question and as you noted, they can modify
> control blocks of that operating system.  However, to call them "instructions"
> seems to be going a bit far.

Ok, now that we have nitpicked the terminology to death, let's think
about what started this -- a request for a new instruction which would
switch two pages in the page tables.  Since this can clearly be done
as an MVS service via an SVC, it can also be done as an assist
instruction.

One big problem that I have with exotic instructions is that they
aren't on all 370s/390s/whatever.  I have yet to find a machine that
has MVCIN.  Has anybody worked on a machine that had the math assists
(SQRT and trig functions)?  I'm still reluctant to write code using
BAS and BASR, since they don't exist on non-XA machines.  Suppose I
use an MVCIN instruction?  Either:
(1)  You can only run it on a machine that has MVCIN, if you can find
     one, or
(2)  I have to write dual path code so that it works on a machine
     without MVCIN.  At which point I won't bother to write the code
     in the first place which uses MVCIN.

Be careful what you ask for.  You may get it.
(However, asking for an instruction to be added to the standard
instruction set and therefore always installed on all models is
not likely to happen.)


/Leonard