The Free On-line Dictionary of Computing (30 December 2018):

as31

    An 8031/8051 assembler by Ken
   Stauffer  and Theo Deraadt which
   produces a variety of object code output formats.  The
   distribution includes an assembler, yacc parser, and
   documentation.  as31 runs on Sun-3, Sun-4, SunOS 4.0,
   Tandy 6000, and Xenix.

   as31 Home (http://pjrc.com/tech/8051/#as31_assembler).

   (2002-05-07)