[AT]
Expects: AH    90H
AL    device type:  00H fixed disk time-out
01H diskette time-out
02H keyboard
03H pointed device time-out
21H waiting for keyboard input
80H network
fcH fixed-disk reset time-out
fdH diskette drive motor start
feH pointing device time-out
ES:BX addr of network control block (if AL = 80H or higher)
CF    must be NC (0) before calling
                                                                  
Returns: AH    00h wait time not satisfied
CF    NC (0) wait time not satisfied; use your own wait rtn
CY (1) minimum wait time satisfied (wait performed)
                                                                  
Info: This service is designed to be intercepted by a multitasking
operating system.  It could be used by BIOS disk handlers or
other hardware to perform programmed wait-for-I/O-completion,
which would allow other tasks to be dispatched while I/O is in
progress.

When unintercepted, the BIOS simply sets AH to 0 and clears CF.

See Also: INT 15H (extended AT services)
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INT 15H 90H: Device Busy