INT 14 - X00 FOSSIL - STUFF RECEIVE BUFFER
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, ^C/^K) is performed on the character
fully re-entrant

See also: 1420

1421 - X00 FOSSIL - STUFF RECEIVE BUFFER