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).

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