What a troll. Seriously. There is not just one manufacturer that makes mainframes.
IBM mainframes dominate the mainframe market at well over 90% market share. Unisys manufactures ClearPath mainframes, based on earlier Sperry and Burroughs product lines. In 2002, Hitachi co-developed the zSeries z800 with IBM to share expenses, but subsequently the two companies have not collaborated on new Hitachi models. Hewlett-Packard sells its unique NonStop systems, which it acquired with Tandem Computers and which some analysts classify as mainframes. Groupe Bull's DPS, Fujitsu (formerly Siemens) BS2000, and Fujitsu-ICL VME mainframes are still available in Europe. Fujitsu, Hitachi, and NEC (the "JCMs") still maintain nominal mainframe hardware businesses in their home Japanese market, although they have been slow to introduce new hardware models in recent years.
The amount of vendor investment in mainframe development varies with marketshare. Unisys, HP, Groupe Bull, Fujitsu, Hitachi, and NEC now rely primarily on commodity Intel CPUs rather than custom processors in order to reduce their development expenses, and they have also cut back their mainframe software development. (However, Unisys still maintains its own unique CMOS processor design development for certain high-end ClearPath models but contracts chip manufacturing to IBM.) In stark contrast, IBM continues to pursue a different business strategy of mainframe investment and growth. IBM has its own large research and development organization designing new, homegrown CPUs, including mainframe processors such as 2008's 4.4 GHz quad-core z10 mainframe microprocessor. IBM is rapidly expanding its software business, including its mainframe software portfolio, to seek additional revenue and profits.
Quit being a hypocrite and go after every mainframe manufacturer. And yes, I am calling you a hypocrite and a troll.
Nor does IBM force you to use a specific operating system.
From the IBM web site of all places.
Linux on System z
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Increasing economics for server consolidation
Linux and z/VM benefit from the enormous improvements of the IBM zEnterprise 196 server capabilities in the areas of consolidation, security, reliability and disaster recovery. You can "do even more with less".
10 Years Linux on IBM System z
For the last decade, clients around the world have benefited from the strengths of Linux on System z.
Learn more about Linux on System z
I don't see companies like Apple advertising the fact, and also offering to help install other operating systems on their hardware.
Show me in witting, on the IBM web site, where their software can not be run on mainframes built by other manufacturers. I don't want your blog, I want IBM official restrictions. Otherwise, your a troll. Nothing more, nothing less.