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Replace and Superseded Routines

Under certain conditions, it may be desirable to replace an installed driver. For example, a display card may use a temporary driver during system startup and then replace it with a better version from disk once the file system is running and initialized.

Replacing an installed driver is a two-step process. First, the driver to be replaced is requested to give up control of the device. Second, the new driver is installed and directed to take over management of the device. Two native driver commands are reserved for these tasks.

The kSupersededCommand selector tells the outgoing driver to begin the replacement process. The command contents are the same as with kFinalizeCommand. The outgoing driver should take the following actions:

• If it is a concurrent driver, it should wait for current I/O actions to finish.
• Place the device in a "quiet" state. The definition of this state is device specific, but it may involve such tasks as disabling device interrupts.
• Remove any installed interrupt handlers.
• Store the driver and the device state in the Name Registry as one or more properties attached to the device entry.
• Return noErr to indicate that the driver is ready to be replaced.

The kReplaceCommand selector tells the incoming driver to assume control of the device. The command contents are the same as those of kInitializeCommand. The incoming driver should take the following actions:

• Retrieve the state stored in the Name Registry and delete the properties stored by kSupersededCommand.
• Install interrupt handlers.
• Place the device in an active state.
• Return noErr to indicate that the driver is ready to be used.