AIX
Quote from IBM:
"The AIX® operating system is designed to deliver outstanding scalability, reliability, and manageability. Best of all, the AIX operating system comes from IBM, the world's leading technology company.
IBM has broad experience in providing solutions to businesses of every size, in every industry, in every corner of the world. IBM has an excellent reputation for service and support. Whether you’re looking for planning services, integration and installation, tuning, migration, or everyday support, IBM provides service and support to help keep your business running.
AIX is an open, standards-based operating system that conforms to The Open Group's Single UNIX Specification Version 3. It provides fully integrated support for 32- and 64-bit applications. The AIX operating system provides binary compatible support for the entire IBM UNIX product line including IBM Power™ Systems, System p™, System i™, pSeries®, iSeries™ servers as well as the BladeCenter® blade servers. AIX also supports qualified systems offered by hardware vendors participating in the AIX Multiple Vendor Program. So, as you move to newer versions of the AIX operating system, its excellent history of binary compatibility provides confidence that your critical applications will continue to run."
