Service_USB 18 - USB system module pre-stop
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## Name

Service_USB 18 - USB system module pre-stop — Used to indicate to other applications that part of the USB system is stopping.

## On Entry

R0 | 18 (reason code)
R1 | &80C00 (USB Service call)
R2, R3, R4 | Same as Service_USB 16

## On Exit

All registers preserved to pass on, else

R0 | 0 to refuse access or 1 to cause a re-try later.
R1 | 0 to claim or preserved to pass on.
R2, R3, R4 | preserved

## Use

This service call is produced by all core USB system modules. It is sent by a module before it commences shutting down. User device drivers do not issue this service. Each type of USB component has a unique type identifier, if you write a module that you feel requires an allocation please contact Simtec Electronics for details.

This call is used to allow other modules in the USB system to perform an orderly shutdown. Regardless of this the service call must not be claimed.

This service call merely indicates an intent of the module to stop, if internal state does not allow the module to stop the shutdown will be aborted. The module will only have actually stopped upon receipt of a Service_USB 17 - USB system module stopped.

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Service_USB 17 - USB system module stopped | Home | Service_USB 19 - Resume halted shutdown