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INT 28 - DOS 2+ internal - KEYBOARD BUSY LOOP
   This interrupt is called from inside the "get input from keyboard" routine
   in DOS, if and only if it is safe to use INT 21 to access the disk at that
   time.  It is used primarily by the PRINT.COM routines and TSR programs, but
   any number of other routines could be chained to it by saving the original
   vector, and calling it with a FAR call (or just JMPing to it) at the end of
   the new routine.

   The INT 28h handler may invoke any INT 21h function except functions 00h
   through 0Ch (and 50h/51h under DOS 2.xx unless DOS CriticalErr flag is set).
   Calls to functions 3Fh and 40h may not use a handle which refers to CON.

   Until some program installs its own routine, this interrupt vector simply
   points to an IRET opcode.

Note:   supported in OS/2 compatibility box
SeeAlso: INT 2A/AH=84h