Int 2F Fn 1A43  - Avatar Serial Dispatcher - Install IRQ4 Handler          [S]

   AX = 1A43h
   BX = 4156h ('AV')
   ES:DI -> FAR handler for serial port using IRQ4
   DS = data segment needed by handler

Return: AX = 1A43h if ASD not installed
      = 0000h if no more room
      else handle to use when uninstalling

Notes: (see AX=1A42h)

See Also: AX=1A42h,AX=1A63h