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.

Notes: 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.
   DOS and some other programs return values in the flags register, so
     any TSR which chains by calling the previous handler rather than
     jumping to it should ensure that the returned flags are preserved
     and passed back to the original caller

See Also: INT 2D