INT 15 - SYSTEM - WAIT ON EXTERNAL EVENT (CONVERTIBLE)
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


1541 - SYSTEM - WAIT ON EXTERNAL EVENT (CONVERTIBLE)