Int 21H, AH=44H, AL=03H

2 and higher

WRITE TO CHARACTER DEVICE CONTROL CHANNEL

AX = 4403h
BX = file handle referencing character device
CX = number of bytes to write
DS:DX -> data to write

CF clear if successful

    AX = number of bytes actually written
CF set on error
    AX = error code (01h,05h,06h,0Dh) (see #01680 at AH=59h/BX=0000h)

format of data is driver-specific
  (also see separate entries below for some specific cases)
if the file handle refers to "4DOSSTAK", the 4DOS (v2.x-3.03)
  KEYSTACK.SYS driver will push the specified characters on the
  keyboard stack; similarly for "NDOSSTAK", the NDOS KEYSTACK.SYS
  driver will push the characters onto the keyboard stack
this function was not supported by Digital Research's DOS Plus

AX=4400h,AX=4402h“DOS 2+”,AX=4405h“DOS”,INT 2F/AX=122Bh, INT 2F/AX=D44Dh,INT 2F/AX=D44Fh

Text based on Ralf Brown Interrupt List Release 61