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

Portable Scheme Interpreter
PSI

   (PSI) A portable scheme interpreter by Ozan Yigit
   , David Keldsen and Pontus Hedman that
   includes a simple DAG compiler and a virtual machine.  It
   can be used as an integrated extension interpreter in other
   systems and allows easy addition of new primitives.  There are
   some unique debugging and tracing facilities.  Acceptable
   performance results from a fairly straight-forward
   implementation.  Continuations are fully and portably
   supported and perform well.  PSI is based on the simple
   compilers and virtual machine in Kent Dbyvig's thesis.

   The pre-release version conforms to R4RS with a number of
   useful extensions.

   (1993-02-19)