Sidenote: The Mainframe Six
Created 23 September 2022 - Updated 24 September 2022
There are six companies remaining in the mainframe industry - Hitachi, NEC, Fujitsu, Atos (formerly Bull), Unisys, and IBM. Three of those - Fujitsu, NEC, and IBM - still develop their own CPUs, though Fujitsu is likely to do so for only one more generation. If I had to speculate about where each of them is used, this is more or less what I'd end up with.
- IBM is global and has, as best I can tell, between 3000 and 7000 Z customers. Many of these are on very large systems; IBM scales to far higher performance levels and core counts than other vendors.
- Unisys is global, with higher concentrations in Latin America (especially for MCP) and East Asia (especially for OS 2200.) MCP is more common in banking and telecom, OS 2200 in airline and government. Both exited custom CPUs in the early 2010s but have fast emulators. I'd guess there's 800-1200 MCP sites and a much smaller number of OS 2200 sites - which is a little sad because OS 2200 is awesome. MCP uses an impressive high-level-language-oriented descriptor ISA, while OS 2200 uses a 36-bit ones' complement ISA with some unusual characteristics.
- Fujitsu is mostly global, with the notable exception of North America, and has several distinct mainframe families. The ex-Siemens BS2000 lineup and the semi-IBM-compatible GS21 family use different software but on the same 390-based custom CPUs. The former ICL family, the 29-series descriptor systems, lives on in Britain in finance and government but is slowly declining and has been primarily run emulated for over 20 years. BS2000 is concentrated in Germany, the GS21 userbase in Japan. I'd guess there's a total of 1000-1500 Fujitsu mainframe customers, with a majority being in Japan, but I'm not super confident in that.
- Hitachi, though they once had a thriving global business including North America, markets their systems exclusively in Japan. Until approximately 2020, they built custom CPUs, but in the latest generation - the AP10000 - they rebadge IBM Z running their own MVS-derived VOS3 operating system. I'd guess they have 200-300 sites, almost all in Japan.
- Atos, which recently finished devouring the French computer company Groupe Bull, owns Bull's GCOS 7 and GCOS 8 operating systems. Both are unique ISAs, incompatible with each other and running completely different software; GCOS 7 is 32-bit, EBCDIC, vaguely MVS-like, and even includes a POSIX subsystem (which it uses for TCP/IP support.) GCOS 8 is older, ASCII, and 36-bit word-oriented. GCOS 7 is emulated on x86, GCOS 8 (as far as I'm aware) is still emulated on Itanium; I would guess the combined total of customer sites for G7 and G8 is less than 100, mostly in Western Europe but with a handful in North America and probably Africa.
- NEC's ACOS-4 operating system is a distant cousin of Bull's GCOS 7, and NEC still designs its own processors to run it. These are fairly big systems by the standards of non-IBM mainframes - up to 48 custom-designed cores and 256GB RAM - and while the customer base is almost exclusively Japanese, they've historically sold a few ACOS systems elsewhere in Asia and even North America. I would guess that at this point there are 200-400 sites remaining, almost all in Japan.
 Bull is the exception; GCOS 7 and GCOS 8 are at least as incompatible as MCP and OS 2200 are. One could probably make a case for Fujitsu, but generally the markets for GS, BS, and VME have little overlap (except in Britain.)
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