The Free On-line Dictionary of Computing (19 January 2023):

SPX

   1.  Sequenced Packet Exchange.

   2.  A graphics program for the Atari microcomputer.


(ftp://src.doc.ic.ac.uk/computing/systems/atari/umich/Graphics/spx_v18.lzh).

   3. Simplex.

   (1997-03-04)


The Free On-line Dictionary of Computing (19 January 2023):

spx

    The filename extension for Screen
   Peace eXtension files.

   (1995-03-14)