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.

See also: 2D

2F - Multiplex - NOTES