MPE/iX is a proprietary mainframe OS for HP3000 hardware.

When I worked as a DBA at Hewlett Packard, my teammates all used MPE/iX,
and I was "the UNIX guy". It's pretty simple really, but it has all its
own quirky tools (like JCL, a batch scripting language) that are unlike
things that UNIX folks are used to.

I was thankful that the only interaction I had with MPE/iX was FTP'ing
files back and forth; for that purpose it might as well have been UNIX.

Also, MPE/iX has its own HP-supplied SQL RDBMS product, Allbase. I used
it on UNIX while I was there, but I didn't know database stuff well
enough at the time to make a judgement about how good it is. The only
thing I remember is that you had to pre-allocate large files for data
and indexes.

Read all about MPE/iX if you have nothing better to do with your time:
http://www.docs.hp.com/mpeix/

Regards,
Bill Karwin