Int 14 Fn 21  - X00 Fossil - Stuff Receive Buffer                          [S]

   AH = 21h
   AL = character
   DX = port number

Notes: the given character is inserted at the end of the receive buffer as if
     it had just arrived from the serial port; all normal receive
     processing (XON/XOFF,  K) is performed on the character
   fully re-entrant

See Also: AH=20h"X00"