Int 2F Fn 1A42  - Avatar Serial Dispatcher - Install IRQ3 Handler          [S]

   AX = 1A42h
   BX = 4156h ('AV')
   ES:DI -> FAR handler for serial port using IRQ3
   DS = data segment needed by handler

Return: AX = 1A42h if ASD not installed
      = 0000h if no more room
      else handle to use when uninstalling

Notes: the handler need not save/restore registers or signal EOI to the
     interrupt controller
   the handler should return AX=0000h if the interrupt was meant for it,
     and either leave AX unchanged or return a non-zero value otherwise
   the most recently installed handler will be called first, continuing
     to earlier handlers until one returns AX=0000h

See Also: AX=1A43h,AX=1A62h