GCOS!

GCOS is the collective name for a group of mostly-unrelated operating systems from GE, Honeywell, and Bull/Atos. GCOSes run on systems ranging from the Level 61 small business minicomputer to large mainframes like the DPS 9000/TA300.

GCOS 61

Long-dead OS for the Level 61 system inherited from Bull. Primitive.

GCOS 62 (GCOS 4)

Minicomputer OS. EOL'd in Bull's markets in the late 1980s, but a variant is still sold by NEC (under emulation) as ACOS-2.

Level 6 (GCOS 6)

Minicomputer OS. Multics-like command shell. Sold for both technical/control workloads and business workloads. Killed circa 1991 and migrated to an AIX-based emulation environment, HVX.

Level 64 (GCOS 7)

Entry mainframe ("midframe") OS. 32-bit, EBCDIC, and has a robust UNIX environment (which also provides services for TCP/IP.) Large but declining customer base in Western Europe, almost none in the US. NEC sells a variant as ACOS-4, running on custom CISC CPUs; Bull's own systems were moved to emulation on x86 and Itanium systems around 2001.

Level 66 (GCOS 8)

The high end of the GCOS family. Originated at GE circa 1962, runs on 36-bit mainframes (until ~2006) and under emulation on Itanium. GCOS 8 still has customers in both America and Europe. NEC sold a variant as ACOS-6 after inheriting Toshiba's mainframe customer base, but mostly abandoned 36-bit systems through the 1990s and repositioned ACOS-4 as the top end of their product family.

A writeup I did on GCOS 8 history, hosted with permission at linkerror.com