INT 2F - Multiplex - NOTES
        AH = identifier of program which is to handle the interrupt
           00h-7Fh reserved for DOS
           B8h-BFh reserved for networks
           C0h-FFh reserved for applications
        AL is the function code
   This is a general mechanism for verifying the presence of a TSR and
   communicating with it.  When searching for a free identifier code for AH
   using the installation check (AL=00h), the calling program should set
   BX/CX/DX to 0000h and must not depend on any registers other than CS:IP
   and SS:SP to be valid on return, since numerous programs now use additional
   registers on input and/or output for the installation check.
Note:   Since the multiplex chain is growing so long, and beginning to
          experience multiplex number collisions, I am proposing an alternate
          multiplex interrupt on INT 2D.  If you decide to use the alternate
          multiplex, please let me know.