It appears that Dan Cross <crossd(a)gmail.com> said:
My short list included PL/1, Algol/W, Fortran, and
Pascal. Fortran was
already mentioned. I don't think PL/1 (or PL/I) could have fit on
those machines. ...
There were a lot of PL/I subsets or variants used for system
programming. Intel had PL/M, used to write CP/M. IBM had PL/S, used to
write some parts of OS/360, and reimplemented at RAND in the early
1970s. XPL was writeen at Stanford in the late 1960s, intended for
writing compilers with a small one-pass compiler written in itself.
PL/360 was sort of PL/I-ish although it was really an IBM 360 assembler with
Algol like syntax, used to write Algol W.
One of these could have been a reasonable basis for a system language, but I
don't think the result would have been any better than C.
R's,
John