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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.

[1] 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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