Burroughs/Unisys MCP, over the last 45+ years, is written in ALGOL or close
variants (ESPOL, NEWP, DCALGOL), with only a minuscule amount of lower-level
code achieved via bit-bashing machine-code into HLL variables. Some
networking subsystems are done in Pascal or C and other languages depending
on which part of the company was involved and other circumstances.
IBM's early OS/400 operating system for the CISC AS/400 (1988) was written
in mostly PL/MI (Programming Language / Machine Interface), which via
various steps derived from PL/I. Some layers of IO support was done in
Modula-2. Later, for the RISC versions of AS/400 parts of OS/400 (SLIC) were
migrated to C or C++ but much of OS/400 stayed with PL/MI or Modula-2. -- [
paraphrased from Dan Hicks in comp.sys.ibm.as400.misc ]