Int 15 Fn 41  - System - Wait On External Event (convertible And Some Oth  [B]

   AH = 41h
   AL = condition type
       bits 0-2: condition to wait for
            0 any external event
            1 compare and return if equal
            2 compare and return if not equal
            3 test and return if not zero
            4 test and return if zero
       bit 3:    reserved
       bit 4:    1=port address, 0=user byte
       bits 5-7: reserved
   BH = condition compare or mask value
   BL = timeout value times 55 milliseconds
       00h means no timeout
   DX = I/O port address if AL bit 4 set
   ES:DI -> user byte if AL bit 4 clear

Note:  call AH=C0h and examine bit 3 of feature byte 1 to determine whether
     this function is supported

See Also: AH=83h,AH=86h,AH=C0h