INT 15h,  90h (138)      Device Busy                                     many

This service is called to notify the system that a device is waiting
to be serviced.

On entry:      AH         90h
AL         Device type (see below)

Returns:       Carry      0 - Wait not satisfied
1 - Minimum wait time satisfied

                                                                          

Notes:         This service is not available for the PC, PCjr, and
XT dated 11/08/82.

The device types are classified into three generalized groups. These
groups are:

00h - 7Fh     Serially reusable devices.
80h - BFh     Reentrant devices. ES:BX is used to differentiate calls.
C0h - FFh     Devices that are time-out only with no posting.

The defined device types are listed below:

      Device Type (AL)    Device                                     Time-out
00h           Fixed disk                                    Yes
01h           Diskette                                      Yes
02h           Keyboard                                      No
03h           Pointing Device                               Yes
80h           Network                                        No
(ES:BX, pointer to Network Control Block)
FDh           Diskette drive motor start                    Yes
FEh           Printer                                       Yes

When a device is needed by a process, but access to
it is delayed, the system can perform another task
to achieve maximum performance until the device is
freed up for the process to use. In a multi-tasking
environment, that is the purpose for this service.
The operating system can keep track of which process
is waiting for which device, and perform another
task while the device is busy. See Service 91h for a
service that signals that a device is ready for a
process.

See also: INT 15h, 91h

INT 15h, 90h (138) Device Busy many